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QCS2026

→ Asia/Tokyo
JAEA Tokai Mirai Base

JAEA Tokai Mirai Base

7-25, Funaishikawa Eki Higashi 3, Tokai, Ibaraki, Japan
Ahmad Jafar Arifi (JAEA), Akira Dohi (RIKEN), Atsushi Hosaka (RCNP), Bikai Gao (RCNP), Daisuke Fujii (JAEA), Hajime Sotani (Kochi University), Hajime Togashi (RIKEN), Kei Suzuki (JAEA), Ken'ichiro Nakazato (Kyushu University), Masayasu Harada (Nagoya Ubiversity), Nobutoshi Yasutake (Chiba Institute of Technology), Philipp Gubler (JAEA), Takumi Muto (Chiba Institute Technology), Tetsuo Hyodo (Tokyo Metropolitan University), Toshiki Maruyama (JAEA), Tsuneo Noda (Kurume Institute of Technology)
Description

Overview

We are pleased to announce that international workshop (QCS 2026) will take place at Tokai, Ibaraki, Japan from 29 Apr to 2 May 2026.This workshop is one of the series of workshops among China, Japan, and Korea, to promote international cooperation and exchanges of scientific achievements and perspective in the field of high-density matter and compact stars.

Novel hadronic and/or quark phases are expected to be realized in high-density hadronic matter. Mechanisms of phase transitions and phase equilibrium between these phases should be clarified to obtain the QCD phase diagram. Toward understanding high-density matter in neutron stars (NSs) and formation processes of black holes (BHs) etc., the equation of state (EOS), describing the pressure-density relation, has a key role. In another aspect, dynamical properties of proto-neutron stars are described by the use of hot and dense supernova matter EOS with trapped neutrinos together with transport equations. These issues should be explored based on realistic interaction models that describe the hadron interactions including baryon-baryon and meson-baryon interactions for hadronic phases and quark-hadron and quark-quark interactions for hadron-quark transitions and quark phases. In addition, strangeness degrees of freedom such as hyperons, kaons, and strange quarks may have important effects on the phase structure of high-density matter. Furthermore, these model predictions should be confronted with information from not only lattice QCD results but also heavy-ion experiments and astrophysical observations. High-density matter that may be produced during heavy-ion collisions is one of emphasized topics of the workshop. In fact, heavy-ion experiment, at bombarding energies lower than the existing experiments, is discussed as one of the future projects of J-PARC facility.

Both young and senior researchers from nuclear and hadron physics, particle physics, astrophysics, condensed matter physics, and from these interdisciplinary fields are welcome to join us.

Topics

  • Equation of state for compact stars, supernovae, and black hole formation

stiffening high-density EOS by three-baryon repulsions, effects of partial chiral symmetry restoration, reaction rates, etc.

  • Various phases in hadronic matter and quark matter

baryon superfluidity, hyperon-mixed matter, meson condensation, inhomogeneous chiral density waves, quarkyonic matter, color superconductivity, etc.

  • QCD phase diagram

hadron-quark phase transition, hadron-quark crossover, chiral phase transition, etc.

  • Theoretical approaches for high-density matter

relativistic mean-field models, chiral perturbation theory, chiral effective models for hadrons, chiral quark models, lattice QCD, etc.

  • Heavy-ion collision experiments and high-density matter

hyperon(Y)-nucleon (N), YY interactions, density-dependence of symmetry energy and slope, hadron correlations by femtoscopy, etc.

  • Observations and properties of compact stars

NS mass and radius by NICER observations, gravitational waves and multi-messenger observations, neutrinos from supernovae, glitches, etc.

  • High-energy astrophysics

NS-NS mergers, supernovae, cooling history and heating of compact stars, low-mass X-ray bursts, etc.

  • Other related topics

Conference Photo

Photos during talks are attached (Participants can download).

Registration and Registration Fee

Registration is now open, and the deadline is 15 Apr. 2026. 

Registration fee (includes Banquet and 4 Lunchboxes): We accept credit cards (MasterCard, Visa, Diners, Amex, and JCB)

  Until 15 Mar. 2026 Until 15 Apr. 2026
Non-students 25,000 JPY 30,000 JPY
Students 25,000 JPY 25,000 JPY

* About Cancellation: Since April 16 JST 24:00, 10,000 JPY is required to be paid for registrars who cancel.

Previous QCS conference info.

1st Quarks and Compact Stars (QCS 2014), Oct. 20-22, 2014, KIAA, Peking University, China

2nd Quarks and Compact Stars (QCS 2017), Feb. 20-22, 2017, Yukawa Institute, Kyoto University, Japan

3rd Quarks and Compact Stars (QCS 2019), Sep. 26-28, 2019, Haeundae, Busan, Korea
https://old.apctp.org/plan.php/qcs2019

4th Quarks and Compact Stars (QCS 2023), Sep. 22-26, 2023, CGS at Yangzhou University, Yangzhou, China
https://psr.pku.edu.cn/conference/qcs/qcs2023/index.html

* This workshop is supported by

  • Advanced Science Research Center, JAEA
  • KMI for the Origin of Particles and the Universe
  • RCNP, Osaka University
  • RIKEN iTHEMS Working Group GWX-EOS
FirstCircular_Oct_29.pdf
map-japan.pdf
map-katsuta.pdf
map-mito.pdf
map-tokai-venue.pdf
QCS2026poster-6.pptx
QCS2026poster.pdf
QCSPhoto.jpg
Second circular_Feb_2.pdf
Photo
Contact
  • Wednesday 29 April
    • 1
      Opening Address

      Atsushi Hosaka

    • Session 1
      Convener: Hajime Sotani (Kochi University)
      • 2
        Close-in pulsar planets as a potential probe for the interior of compact stars ----(online)----

        strong textStrange-quark matter (SQM) may be the true ground state of hadronic matter, indicating that the observed pulsars may actually be strange stars (SSs), but not neutron stars. According to the SQM hypothesis, the existence of a hydrostatically stable sequence of SQM stars has been predicted, ranging from 1 to 2 solar mass SSs, to smaller strange dwarfs and even strange planets. While gravitational wave (GW) astronomy is expected to open a new window to the universe, it will shed light on the search for SQM stars. Here we show that due to their extreme compactness, strange planets can spiral very close to their host SSs without being tidally disrupted. Like inspiraling neutron stars or black holes, these systems would serve as new sources of GW bursts, producing strong GWs at the final stage. The events occurring in our local universe can be detected by upcoming GW detectors, such as Advanced LIGO and the Einstein Telescope. This effect provides a unique probe to SQM objects and is hopefully a powerful tool for testing the SQM hypothesis.

        Speaker: Yongfeng Huang (Nanjing University)
        StrangeQuarkStarsYongFengHuang35min.pdf
      • 3
        Constraining the Maximum Mass of Neutron Stars through Multimessenger Gravitational-Wave and Electromagnetic Observations ----(online)----

        The increasing number of compact binary coalescences observed by the LIGO-Virgo-KAGRA (LVK) Collaboration, highlighted by the landmark multimessenger detection of the binary neutron star merger GW170817, has opened new frontiers in nuclear astrophysics. This report presents a methodology to constrain the maximum mass of neutron stars, and consequently the neutron star equation of state, by combining gravitational-wave and electromagnetic observations. Specifically, gravitational-wave data provide a precise measurement of the total mass of the binary system. By determining, through electromagnetic signatures, whether the post-merger remnant promptly collapses to a black hole or forms a stable neutron star, we can establish observational upper and lower bounds on the maximum mass. The details of this multimessenger approach and its implications for understanding ultra-dense matter will be discussed.

        Speaker: He Gao
      • 4
        Measure the moment of inertia and ellipticity of neutron stars with gravitational waves----(online)----

        We identify a previously unrecognized spin–orbit resonance that can naturally arise in neutron star binaries. This resonance provides a new and direct probe of neutron star ellipticity, enabled by finite stellar quadrupole moments such as those produced by strong internal magnetic fields. We show that the resonance produces a distinctive and measurable gravitational-wave phase shift, allowing precise measurement of the neutron star’s ellipticity and moment of inertia.
        We further conduct the first ellipticity search across the entire gravitational-wave catalog up to O4a, finding no detections but establishing the framework for future constraints. We demonstrate that detecting this resonance would have significant implications for both astrophysics and fundamental physics, including the internal structure of neutron stars, the prevalence of magnetars in binaries, and tests of strong-field gravity.

        Speaker: Zhiqiang Miao (Tsung-Dao Lee Institute)
    • 11:10
      Break
    • Session 1
      • 5
        Bayesian Cross-Validation of Neutron and Quark Star Models with NICER and GW Constraints -----(Online) ------ (Award candidate)

        The internal composition of compact stars remains largely unresolved in spite of the latest mass–radius measurements by NICER and tidal deformability constraints from gravitational-wave observations. A central challenge is to identify the distinction in the macroscopic observable structure of nucleonic neutron stars and self-bound quark stars. We employ a leave-one-out cross-validation approach in a Bayesian analysis. We quantify the agreement of individual stellar observations with nucleonic and quark equation of state (EoS) models satisfying established theoretical and experimental constraints using posterior pointwise predictive density. Our analysis shows that heavier pulsars, such as PSR J0740+6620, and low-mass objects like HESS J1731–347, are better described by quark-matter EoSs, while stars near the canonical 1.4 M$_\odot$ mass exhibit ambiguous behavior. PSR J0030+0451 shows marginal preference for nucleonic matter, whereas others lean toward quark models. These results highlight the sensitivity of model selection to individual observations and the need for more precise measurements. The methodology is readily extendable to hybrid and hyperonic star models, among other exotic possibilities, offering a systematic framework for probing the preferences of the observational data. Unlike global EoS inference, our approach isolates how each observation individually prefers or challenges competing models.

        Speaker: Anagh Venneti (Department of Physics, Birla Institute of Technology and Sciences Pilani, Hyderabad campus)
        JapanPresentation.pdf
      • 6
        Empirical Relation for the Neutron Star Maximum Mass within Relativistic Mean-Field Theories (Award candidate)

        We obtain a empirical relation for the neutron star maximum mass arising from a particular
        combination of the saturation density (n0), the effective mass (m∗), and (when present) the vector
        meson self-coupling constant (ζ) within the relativistic mean-field model framework. Observations
        of massive neutron stars heavier than ∼ 2M⊙ have eliminated the softest equation of state from
        consideration and impose strong constraints on nuclear interactions used to model dense nuclear
        matter. To date, numerous efforts have been made to refine relativistic mean-field models through
        the inclusion of additional mesons, such as the δ meson, as well as through the introduction of
        additional couplings. We show that current RMF models, including our own constructions, exhibit
        a maximum neutron star mass that is primarily determined by the combination of the saturation
        density, the effective mass at saturation, and the vector meson self-coupling constant. When con-
        straining the pure neutron matter equation of state using chiral effective field theory (ChEFT) at
        low densities, 250 parameter sets were generated to derive an empirical formula for the maximum
        mass of neutron stars and apply the formula with the present relativistic mean field models.

        Speaker: Gihwan Nam (Yonsei)
        260429_QCS.pdf
      • 7
        From nucleons to nuclei and neutron stars

        We discuss the properties of the static neutron stars and strangeness-mixed stars, based on the equations of state derived from a pion mean-field approach. The model parameters are fixed according to the phenomenological observation related to the pion-nucleus scattering and the bulk properties of nuclear matter near the saturation point. We examine the energy and pressure behavior inside a neutron star. We show that in the framework of the present approach the central densities in various neutron stars vary within the range of 3 to 4 normal nuclear matter densities. The mass-radius relations are also obtained and discussed. The slope parameter dependence of the radii of the neutron stars with their fixed masses are discussed. We also study the strangeness-mixed stars or the hyperon stars using the same sets of parameters. As the strangeness content of strange matter increases, the binding energy per nucleon is saturated and the corresponding equation of state becomes softer.

        Speaker: Ulugbek Yakhshiev (Inha University)
    • 12:40
      Lunch
    • Session 2
      Convener: Hajime Togashi (RIKEN)
      • 8
        Nuclear masses and structures from the DRHBc framework and their astrophysical implications

        Understanding dense and hot QCD matter in compact stars, supernovae, and neutron-star mergers requires reliable microscopic nuclear physics inputs far from the valley of stability. Nuclear mass tables provide essential quantities such as binding energies, neutron separation energies, β-decay Q-values, and nuclear deformation, which directly affect the equation of state, r-process nucleosynthesis, and the composition of dense astrophysical matter.

        In this work, we present recent results from the Deformed Relativistic Hartree–Bogoliubov theory in continuum (DRHBc) and discuss their relevance to the topics discussed at this conference. The DRHBc framework, based on covariant density functional theory, incorporates deformation, pairing correlation and continuum effects that are crucial for describing extremely neutron-rich nuclei. We analyze neutron separation energy systematics, the neutron drip line, shell evolution, shape transitions, and odd–even staggering, and discuss how these nuclear structure features are related to r-process paths, neutron-star crust composition, and dense-matter modeling under β-equilibrium

        Speaker: Myeong-Hwan Mun (Kyungpook National University)
      • 9
        Inferring the trace anomaly of dense matter from neutron star observations

        The trace anomaly $\Delta$ is an important quantity that measures the broken conformal symmetry in neutron star matter. In this talk, we will introduce quasi-universal relations that connect the stellar profile of $\Delta$ to the compactness, moment of inertia, and tidal deformability of neutron stars. We will also discuss how the behavior of $\Delta$ inside neutron stars can be inferred from neutron star observations through both electromagnetic and gravitational-wave channels.

        Speaker: Lap-Ming Lin (The Chinese University of Hong Kong)
        OCS2026.pdf
    • 14:50
      Break
    • Session 2
      Convener: Hajime Togashi (RIKEN)
      • 10
        A study of neutron star property based on the Parity Doublet Models

        I will briefly summarize our works in which we studied neutron star property based on Parity Doublet Models. In the low-density region, we construct the EoS using hadronic models based on the parity doublet structure with the chiral invariant mass of nucleons. In the high density region, the EoS is obtained in an NJL-type quark model. By connecting two EoSs with assuming the quark-hadron crossover or first order phase transition, we construct unified EoSs for dense matter. We then derive the M-R relation of neutron stars from the unified EoSs and compare the result with the observational constraints to obtain an allowed range for the chiral invariant mass.

        Mainly based on
        [1] Y. Motohiro, Y.Kim, M.Harada, Phys. Rev. C 92, 025201 (2015);Erratum: Phys. Rev. C 95, 059903 (2017).
        [2] T. Minamikawa, T. Kojo and M. Harada, Phys. Rev. C 103, 045205 (2021).
        [3] T. Minamikawa, B. Gao, T. Kojo and M. Harada, Symmetry 15, 745 (2023).
        [4] B. Gao, Y. Yan and M. Harada, Phys. Rev. C 109, 065807 (2024).
        [5] B. Gao, W.L.Yuan, M. Harada and Y.L. Ma, Phys. Rev. C 110, 045802 (2024)

        Speaker: Masayasu Harada (KMI, Nagoya University)
        2604-QCS2026-Harada.pdf
      • 11
        Chiral symmetry restoration and hyperon suppression in neutron stars

        The ``hyperon puzzle'' remains a fundamental challenge in nuclear astrophysics. We investigate hyperon emergence in neutron star matter using the $SU(3)$ parity doublet model with chiral representation $(3,\bar{3}) + (\bar{3},3)$. This framework naturally incorporates chiral symmetry restoration and provides a systematic description of baryon masses in dense matter through the interplay between the chiral condensate and the chiral invariant mass $m_0$. We find that the hyperon onset density exhibits strong sensitivity to $m_0$: for $m_0 = 500$ MeV, hyperons first appear at $1.9n_0$ while for $m_0 \gtrsim 750$ MeV, hyperons emerge only above $5n_0$. This delayed onset arises from the weakened density dependence of baryon masses at larger $m_0$ values. When the hyperon onset density exceeds the expected quark-hadron transition range ($2$--$5n_0$), matter undergoes deconfinement before hyperons populate, avoiding the EoS softening while maintaining consistency with massive neutron star observations. Our results demonstrate that chiral dynamics provides a natural resolution to the hyperon puzzle without requiring ad hoc repulsive hyperon interactions.

        Speaker: Bikai Gao (RCNP, Osaka University)
      • 12
        Chiral effective Lagrangian at finite density and temperature with broken scale invariance (Award candidate)

        We present an equation of state for strongly interacting matter applicable over a broad range of temperatures and baryon densities. This is based on an effective Lagrangian with explicitly broken chiral symmetry, where scale-invariance breaking is regulated by a dilaton field that mimics the dynamics of the gluon condensate in quantum chromodynamics (QCD). The model includes baryons and mesons ($\sigma, \pi, \omega, \rho$) and incorporates thermal field fluctuations beyond mean-field approximation. Mapping the QCD phase diagram, we find a crossover associated with chiral symmetry restoration at high temperatures, consistent with lattice QCD, which turns into a first-order phase transition at high baryon densities.

        We apply this EOS to the structure of neutron stars and compare the results with two distinct frameworks: an extension of the effective Lagrangian to the $SU(3)_f$ sector, and a Bayesian uncertainty quantification based on relativistic mean-field models, involving the exchange of $\sigma$, $\omega$, and $\rho$ mesons, as well as nonlinear nucleon–$\sigma$ couplings and density-dependent $\rho$ coupling. This comparison shows that the dilaton-based EOS is fully compatible with the tidal deformability inferred from GW170817 while simultaneously supporting the existence of massive neutron stars.

        Speaker: Luca Passarella (Politecnico di Torino and INFN)
    • Poster Session
      • 13
        Bayesian Inference of nuclear equation of state in RMF models (Award candidate)

        Abstract
        Bayesian analysis methodologies provide a robust framework for systematically integrating prior knowledge to refine the understanding of parameter spaces in theoretical models. Consequently, this approach has emerged as a pivotal tool in research domains such as nuclear physics and nuclear astrophysics. Leveraging experimental data from finite nuclei, we employ Bayesian analysis to constrain the coupling constants and additional parameters within the Relativistic Mean Field (RMF) model. These constraints are subsequently extrapolated to uniform nuclear matter and integrated with bounds derived from complementary theoretical approaches, including chiral effective field theory, heavy-ion collision experiments, and astronomical observations. This synthesis yields a relatively comprehensive constraint on the nuclear matter equation of state. By self-consistently integrating experimental data on binding energy, charge radii, and neutron skin thickness through Bayesian likelihood estimation, the methodology generates probability distributions for model parameters and elucidates inter-parameter correlations. These results furnish critical theoretical benchmarks for advancing research in nuclear physics and nuclear astrophysics.

        Speaker: Jing- Jing Geng (Yangzhou University)
      • 14
        Compact star cooling with a 2SC+<dd> phase

        Neutron stars are compact objects that can be likened to a single gigantic nucleus. Their central density exceeds nuclear density, allowing diverse states to appear that are not seen in ordinary nuclei, such as hyperon mixing, quark deconfinement and nucleon superfluidity. These states strongly affect the neutrino emission dominating neutron star cooling, thereby determining their thermal evolution. Comparisons between observed neutron star temperatures and theoretical calculations enable exploration of their internal states.
        We have investigated the thermal evolution of neutron stars, taking into account the effects of colour-superconducting quark matter. Due to the degrees of freedom of quark colour and flavour, several pairing possibilities are conceivable in the colour superconducting state. We assume that the CFL (Colour Flavour Locked) or 2SC (Two-flavour superconductivity) phase appears. We also consider the effects of the 2SC+<$dd$> phase. In this phase, the ${}^3P_2$ superfluidity of neutrons in the hadron phase is succeeded by unpaired $d$ quarks in the 2SC phase. We found that incorporating the 2SC+<$dd$> phase can explain the observational results of cold neutron stars.

        Speaker: Tsuneo NODA (Kurume Institute of Technology)
        QCS2026-A0Poster-NODA.pdf
      • 15
        Role of the a0(980) Meson in Asymmetric Matter and Constraints on the Chiral Invariant Mass of the Nucleon in an Extended Parity Doublet Model

        The properties of dense, neutron-rich matter are strongly influenced by the isovector structure of the nuclear interaction. In particular, the isovector scalar meson a0(980) (also known as the delta meson) modifies the neutron–proton effective mass splitting and significantly affects the behavior of isospin-asymmetric matter. In this talk, we construct an extended parity doublet model including the a0 meson to investigate its role in asymmetric matter and explore its implications for dense baryonic systems. Finite nuclei and neutron stars provide complementary probes of low- and high-density matter, respectively.

        We construct a hadron–quark crossover equation of state including the a0 meson and analyze neutron-star properties. We examine how the a0 meson modifies neutron-star structure, the symmetry energy, and higher-order symmetry-energy parameters. Using neutron-star observational data, we constrain the chiral invariant mass of the nucleon, m0. In particular, motivated by the recent observation of an unusually light and compact neutron-star candidate in HESS J1731−347, we show that the 1 sigma data from this source impose a remarkably narrow constraint on the allowed values of m0 and L0 within the crossover framework: 830 MeV < m0 < 900 MeV for L0 = 40 MeV, and 850 MeV < m0 < 890 MeV for L0 = 45 MeV.

        We also investigate the role of the a0 meson in finite nuclei. By fitting the model to experimental data, including binding energies and charge radii, we further constrain m0 and examine how the isovector scalar channel influences finite-nucleus observables such as neutron-skin thickness.

        Taken together, these results demonstrate how the isovector scalar interaction shapes asymmetric matter across a wide density range and determine how neutron-star and finite-nucleus data constrain the chiral invariant mass in dense baryonic matter.

        Speaker: Yuk-Kei Kong (Nagoya University)
      • 16
        Phase Transition to Hyperon Matter in Neutron Stars using an Effective Model with Chiral Invariant Masses (Award candidate)

        The baryon mass originates from spontaneous chiral symmetry breaking and a chiral invariant mass. The chiral invariant mass of hyperons has not been sufficiently considered. In this study, we investigate the dependence of the density at which hyperons appear on their chiral invariant masses in neutron star matter. We employ SU(2)L×SU(2)R chiral symmetry and introduce the chiral invariant masses of hyperons independently of the chiral invariant mass of nucleons. We find that the Lambda baryon can emerge in neutron star when the chiral invariant mass is as large as the vacuum mass. Neutron star observations may therefore constrain the chiral invariant mass of Lambda baryon.

        Speaker: Masayuki Kanazawa (Nagoya University)
      • 17
        Nucleon chiral-invariant mass from gravitational form factors and neutron star constraints

        The nucleon mass consists of a chiral-variant part and a chiral-invariant part. This mass decomposition is essential for understanding the properties of dense nuclear matter, especially in the high-density region across the chiral phase transition. While the value of the chiral-invariant mass significantly influences the equation of state of neutron stars, its magnitude remains poorly constrained.

        In this talk, we present a novel framework to quantitatively determine the chiral-invariant mass by focusing on the nucleon gravitational form factors (GFFs). Assuming the dominance of the lightest sigma meson, we establish a direct connection between the GFFs and the nucleon mass decomposition. Our results are consistent with recent lattice QCD data and suggest a sizable chiral-invariant mass. Furthermore, the obtained value aligns well with neutron star constraints.

        Speaker: Mamiya Kawaguchi (Anhui University of Science and Technology)
      • 18
        Exploring the Roles of Nucleon Effective Mass and Multineutron States in Supernova Matter (Award candidate)

        This study focuses on constructing equations of state based on relativistic mean-field (RMF) theory to investigate the impact of remaining uncertainties in nuclear matter properties. We examine two key factors: the nucleon effective mass and the presence of multineutron states in non-uniform nuclear matter.
        Our results demonstrate that a larger nucleon effective mass within the RMF enhances the abundance of heavy nuclei in neutron-rich environments, primarily due to variations in the density dependence of the symmetry energy. Regarding multineutron states, we evaluate the effects of incorporating dineutron and tetraneutron states. In highly neutron-rich environments, the fraction of the multineutron states becomes dominant at high densities, significantly reducing the fraction of unbound neutrons.
        Furthermore, the formation of dineutrons and tetraneutrons increases the fractions of unbound protons and heavy nuclei with larger mass and proton numbers.
        Our findings suggest that an increased effective mass in RMF formalism and the presence of multineutron states lead to stronger neutrino trapping and extend the duration of neutrino emission from proto-neutron stars. In particular, the increase in free protons can enhance neutrino emission rates, potentially improving the efficiency of neutrino heating.

        Speaker: Tatsuya Matsuki (Tokyo University of Science)
      • 19
        Impact of Hexaquark H Particle on Compact Star Properties

        Title: Impact of Hexaquark H Particle on Compact Star Properties

        Abstract： We study the possible existence of the hexaquark H particle (uuddss) in neutron stars. Within the Chromomagnetic Interaction framework, the flavor-singlet H exhibits a mass of 2212.7 MeV. Using the relativistic mean-field model, we explore how H particle couplings influence neutron star structure. Our results indicate that H particles could persist as stable constituents, reducing the maximum mass and providing new insights into the dense matter equation of state.

        Speaker: Xuhao Wu
        Properties of H particle-admixed compact star （EPJC2026）
        QCS2026-WU.pdf
      • 20
        Three-flavor color superconducting quark matter and quark stars

        We investigate the stability of beta-equilibrated three-flavor color-superconducting quark matter within the Nambu–Jona–Lasinio model under conditions relevant to compact stars. Owing to the attractive interaction between quarks, color superconductivity is expected to occur, with the color–flavor-locked phase favored at high densities. Motivated by the observations of possible low-mass self-bound compact stars, we systematically explore the model parameter space and identify the conditions under which three-flavor color-superconducting quark matter becomes absolutely stable at zero pressure. Our findings suggest the existence of a physically viable region of parameter space in which this phase constitutes the true ground state of strongly interacting matter, thereby supporting the scenario of self-bound color-superconducting quark stars.

        Speaker: Wen-Li Yuan (Peking University)
        poster_WenliYuan_QCS2026.pdf
      • 21
        Observational probes of the neutron star equation of state with hyperons, bosonic dark matter, and quark matter

        The possible presence of dark matter in neutron star interiors may significantly affect their structure and observable properties. Among various candidates, the hypothetical sexaquark has been proposed as a bosonic dark matter particle that could form under the extreme conditions of neutron star cores. In this work, we investigate whether a hybrid neutron star model including hyperons, bosonic dark matter in the form of sexaquarks, and deconfined quark matter can satisfy current observational constraints. The hadronic phase is described using the DD2Y-T model with hyperons, while the quark phase is modeled with a nonlocal Nambu–Jona-Lasinio framework. The phase transition is implemented as a smooth crossover using the replacement interpolation construction. Using Bayesian analysis and incorporating current observational data, including mass–radius measurements and tidal deformability constraints, we find that the presence of sexaquarks softens the equation of state while remaining consistent with observations. The analysis favors a sexaquark mass in the range 1885-1935 MeV, suggesting that neutron star observations can place meaningful constraints on this exotic dark matter candidate.

        Speaker: Mahboubeh Shahrbaf Motlagh (University of Wroclaw)
        QCS2026_Tokai.pdf
      • 22
        Neutron Stars Admixed with Bosonic Dark Matter: Multi-Messenger Constraints and Pulse-Profile Signatures (Award candidate)

        In this talk, I will discuss the effects of self-interacting bosonic dark matter on neutron stars in light of recent observations from NICER and LIGO/Virgo. In this framework, sub-GeV bosonic dark matter can be present either as a dense core inside the star or as an extended halo around it, modifying key observable properties such as the mass, radius, and tidal deformability. By varying the dark-matter particle mass, self-coupling strength, and dark-matter fraction, we determine the regions of parameter space consistent with current astrophysical constraints.

        Furthermore, I will present a novel aspect of this study: the impact of dark-matter halos on the X-ray pulse profiles of rotating neutron stars. Because dark matter changes the external spacetime geometry, it influences photon propagation and gravitational light bending, which can leave measurable imprints on the observed light curve. I will show that the minimum flux of the pulse profile can be significantly affected by the presence of a dark-matter halo, opening a new possibility for probing dark matter in neutron stars through pulse-profile modeling.

        Speaker: Davood Rafiei Karkevandi
      • 23
        Spin Molecullar Dynamics with spin-orbit interaction

        We formulate molecular dynamics for spin systems by time-dependent SU(2) spinor parametrized with two real angles. With central interaction between particles, the total angular momentum and the total spin are conserved as well as the total energy and momentum.But with inclusion of spin-orbit interaction the total angular momentum and total spin become time-dependent quantity. They convert to each others in time. We show this behavior in the simulation of an isolated system of several number of particles. It will be interesting to compare the experimentally measured spin polarization in the participant region of non-central heavy-ion collisions with this calculation results to see the effects of spin-orbit interaction on the nuclear collision processes.

        Speaker: Toshiki Maruyama (JAEA)
        poster-1-maruyama.pdf
      • 24
        Dynamical response of kaon condensation to neutron-star oscillations

        We consider a response of kaon-condensed phase in hyperon-mixed matter [(Y+K) phase] to neutron-star oscillations. We obtain the sound velocity propagating in the (Y+K) phase out of chemical equilibrium with respect to the relevant weak reactions, searching for signals unique to kaon condensation associated with typical gravitational wave modes. The dissipation of vibrational energy including the (Y+K) phase and its damping time scale are also investigated.

        Speaker: Takumi Muto (千葉工業大学)
        Muto_QCS2026.pdf
      • 25
        Neutrino Emission and Early Crust Formation in Protoneutron Stars

        The cooling of protoneutron stars (PNSs) via neutrino emission and the onset of crust formation are both governed by the nuclear equation of state (EOS). In particular, properties of dense matter affect not only the neutrino opacity and thermal evolution, but also the characteristics of heavy nuclei emerging in the outer layers. This suggests a nontrivial interplay between neutrino cooling and crust formation.

        In this work, we investigate this connection by means of numerical simulations using several nuclear EOS models. By systematically varying the density slope of the symmetry energy, $L$, we analyze how the thermal evolution and the timing of crust formation are correlated.

        We find that smaller values of $L$ lead to slower cooling of the PNS. Nevertheless, the Coulomb coupling parameter of heavy nuclei increases under such conditions, resulting in an earlier onset of crust formation despite the prolonged cooling timescale. These results highlight the important role of the symmetry energy in shaping both the neutrino emission history and the emergence of the crust in PNSs.

        Speaker: Ken'ichiro Nakazato
        QCS2026Nakazato.pdf
      • 26
        Neutron star mass and radius of FRB 20240114A by identification of crustal oscillations

        By identifying quasi-periodic oscillations reported in FRB 20240114A (from the Five-hundred-meter Aperture Spherical Telescope) with neutron star crustal torsional oscillations, together with experimental constraints on the nuclear saturation parameter, the incompressibility, we constrain the mass and radius of an extragalactic neutron star at redshift z=0.13. Identifying the low-order QPO frequencies as fundamental oscillations, and frequencies of 567.7 Hz or 655.5 Hz (rest frame) as first overtone candidates implies neutron star mass ranges of 1.00-1.55 solar mass or 1.17-1.76 solar mass, respectively. Additionally, accounting for the nuclear parameter dependence of low-mass neutron star structure yields an estimated radius of 12.5-13.5 km. Simultaneously, we also constrain the nuclear saturation parameter, namely the density dependence of the nuclear symmetry energy, L, and determine it to be L=59.5-96.8 MeV, which is broadly consistent with the previous constraints on L obtained from the experiments and astronomical observations. Thus, a mapping of FRB QPOs to crustal torsional modes seems reasonable. This can be investigated with upcoming FRB surveys over a broad range of redshifts and more elaborate data analyses.

        Speaker: Hajime SOTANI (Kochi University)
      • 27
        Core-collapse supernova explosions hindered by dark photons

        Recent studies on the dark photon (DP) production in collapsing stars argue that the cooling effect induced by DPs can hinder supernova explosions and lead to a ``failing supernova" constraint on the photon-DP mixing parameter $\epsilon$. In order to verify the idea, we perform two-dimensional neutrino-radiation hydrodynamic simulations coupled with the DP production with the masses of 0.3 and 0.45 MeV. We find that the shock revival does not happen until the end of the simulations when $\epsilon\gtrsim3\times10^{-9}$. The photon-DP mixing parameter above this value can be excluded by the failing supernova argument. Interestingly, our constraint roughly coincides with the one reported by the previous studies which adopted the post-processing framework. This result motivates one to investigate a wider parameter range of DPs with self-consistent simulations and evaluate uncertainties in the constraint.

        Speaker: Kanji Mori (Keio University)
        qcs.pdf
  • Thursday 30 April
    • Session 3
      Convener: Ken'ichiro Nakazato (Kyushu University)
      • 28
        Dense matter and neutrinos in core-collapse supernovae and hot neutron stars

        I will provide an overview of the role of hot, dense matter in core-collapse supernovae and neutron stars, with a particular focus on the crucial roles of neutrinos. Supernovae, which result from the gravitational collapse of massive stars, produce bright optical displays, compact objects (such as neutron stars or black holes), and intense bursts of neutrinos. The explosion mechanism remains a central problem in nuclear astrophysics. The equation of state of dense matter is one of the key uncertainties and may ultimately determine the outcome of the explosion. During stellar collapse, core bounce, and the formation of compact remnants, matter experiences a wide range of extreme conditions. I will describe recent progress in astrophysical simulations and nuclear physics. I will also discuss the impact of the equation of state (thermodynamic quantities and composition) on supernova dynamics, the final fate of compact objects, and neutrino emissions.

        Speaker: Kohsuke Sumiyoshi (National Institute of Technology, Numazu College)
        2026-04b_QCS2026.pdf
      • 29
        Modeling of binary neutron star mergers for multi-messenger inference of neutron star properties

        The gravitational waves emitted by a binary of two neutron stars tell us not only information about the mass of neutron stars, but also information about the matter properties of deep inside neutron stars. The electromagnetic waves emitted by matter expelled by several mechanisms after the merger reveal the binary's post-merger activity. By combining those two 'messengers', we can better understand the neutron star. In this talk, I will review such multi-messenger aspects of binary neutron star mergers. For the understanding of the electromagnetic emission, we have to understand how matter is expelled in the binary merger; thus, we need a detailed model of the phenomenon. I will mention the current status of the numerical modeling of the binary merger and mass ejection.

        Speaker: Sho Fujibayashi (Tohoku University)
        CQS2026_Fujibayashi_20260430.pdf
      • 30
        Binary Neutron Star Merger as a Probe of Hadron-Quark Transition

        The convergence of multi-messenger observations—NICER, GW170817, and theoretical calculations—has tightly constrained the neutron star equation of state, pointing to a non-monotonic sound speed that suggests possible quark matter in massive neutron star cores. However, the masquerade effect obscures the nature of this transition in static stars.

        I will show how binary neutron star mergers provide a dynamical probe of this transition. Using general-relativistic simulations with a quark-hadron crossover equation of state, we find the post-merger remnant is less compact than in purely hadronic models, producing a lower, more stable gravitational-wave frequency f2. This signature clearly differs from the rapid frequency evolution expected from a strong first-order phase transition and may be detectable by next-generation observatories.

        Our results show that post-merger gravitational-wave spectra can discriminate between hadron-quark transition mechanisms, complementing constraints from static neutron star properties.

        Speaker: Yongjia Huang (Purple Mountain Observatory / RIKEN)
        QCS2026_YongjiaHuang.pptx
    • 10:30
      Break
    • Session 3
      Convener: Ken'ichiro Nakazato (Kyushu University)
      • 31
        Impact of Density-Dependent Axial Coupling Quenching and Nucleon Effective Mass on Neutrino Transport in Core-Collapse Supernovae

        We investigate how density-dependent axial-vector coupling quenching and nucleon effective mass modify neutrino transport in core-collapse supernovae (CCSNe). We implement these in-medium effects in neutrino absorption, emission, and scattering opacities used by NULIB and perform simulations of the Woosley–Weaver 15M⊙ progenitor (s15s7b2) with the GR1D code. The effective-mass correction primarily alters charged-current absorption and nucleon–nucleon bremsstrahlung, producing non-monotonic, radius- and time-dependent changes in neutrino luminosities through a competition between reduced emissivity and a shifted decoupling region. In contrast, axial-coupling quenching suppresses all weak rates and imprints most clearly on the prompt νe burst, where transport is controlled by rapid opacity changes near shock breakout. These results demonstrate that consistent dense-matter microphysics can leave distinct, potentially observable signatures in CCSN neutrino signals through
        transport effects.

        Speaker: Myung-Ki Cheoun (Soongsil University)
        2026 0430 Supernova Luminosity_short.pdf
      • 32
        Density functional theory for the supernova simulation

        In this talk, we present recent developments of a new nuclear-matter equation of state (EoS) at finite density and temperature based on realistic nuclear interactions and density functional theory. It is argued that the nucleon effective mass plays an important role in determining the thermal stiffness of the EoS. The uncertainty associated with the in-medium effective mass is systematically controlled within the Korea-IBS-Daegu-SKKU (KIDS) energy-density-functional framework. To construct a unified EoS applicable to stellar core-collapse simulations, we build an energy-density functional whose saturation properties reproduce those of the EoS obtained from the variational method with realistic nuclear forces (Togashi EoS). This enables the thermodynamic quantities of the non-uniform phase described by the Togashi EoS and those of the uniform phase obtained with the KIDS model to be connected smoothly. The resulting unified Togashi–KIDS EoS (TKIDS) provides an EoS table suitable for simulations that require a consistent treatment from the crust to the core. Using the TKIDS model, we investigate how the nucleon effective mass impacts the thermodynamic properties of hot nuclear matter, the bulk structure of compact stars, and the dynamics of corecollapse supernovae.

        Speaker: Chang Ho Hyun (Daegu University)
    • 12:10
      Lunch
    • Session 4
      Convener: Philipp Gubler (JAEA)
      • 33
        Self-bound hybrid stars with strong phase transitions can relieve major compact star observation tensions ----(online)----

        Some recent pulsar observations cannot naturally fit into the conventional picture of neutron stars: the compact objects associated with HESS J1731-347 and XTE J1814-338 have too small radii in the low-mass regime, while the secondary component of GW190814 is too massive for neutron stars to be compatible with constraints from the GW170817 event. In this study, we demonstrate that all these anomalous observations and tensions, together with other conventional ones such as recent NICER observations of PSR J0740+6620, J0030+0451, and PSR J0437-4715, can be naturally explained simultaneously by a new general type of self-bound hybrid stars with large density discontinuities, and thus are radially stable in either the slow or rapid phase transition context. As a proof of concept, we use hybrid quark stars, inverted hybrid stars, and hybrid strangeon stars as benchmark examples to explicitly demonstrate the advantage and feasibility of self-bound hybrid stars with strong phase transitions in relieving all tensions related to compact stars' masses, radii, and tidal deformabilities.

        Speaker: Chen Zhang (Tongji University)
      • 34
        Understanding of Equations of States from Nuclear Transport Simulations.

        Nuclear transport simulations are essential theoretical tools for exploring the properties of high-density nuclear matter. Numerous heavy-ion collision facilities around the world, such as RAON, RIKEN Nishina Center, and FRIB, are conducting a wide range of experiments, leading to the accumulation of extensive experimental data. In this context, the importance of nuclear transport simulations—capable of numerically describing the entire dynamical evolution of heavy-ion collisions from the initial non-equilibrium stage, through the high-density compression phase, to the final freeze-out stage—has become increasingly significant

        In this talk, I will introduce the overall characteristics and theoretical framework of a nuclear transport model developed in Korea, along with its numerical implementation. I will also discuss various physical properties investigated using this model, including the equation of state of high-density nuclear matter, symmetry energy, collective flow, and particle production ratios.

        Speaker: Myungkuk Kim (CENS/IBS)
    • 14:20
      Break
    • Session 4
      Convener: Philipp Gubler (JAEA)
      • 35
        Experimental plans in proton-nucleus and nucleus-nucleus collisions at J-PARC

        J-PARC is one of the world’s highest-intensity proton accelerators for material and life sciences, neutrino physics, and hadron and nuclear physics in a few ten GeV. J-PARC-HI (J-PARC Heavy-Ion Project) aims to accelerate heavy-ion beams at J-PARC, where we will introduce a heavy-ion injector consisting of a heavy-ion linac and a booster ring, while the heavy-ion beams are accelerated in the existing 3-GeV synchrotron (RCS) and 30-GeV synchrotron (MR). The maximum beam rate is expected to reach the world's highest value of $10^{11}$ Hz, and the beam energy can range from 1 to 12 AGeV/c. We aim to unravel QCD phase structures such as the first-order phase boundary, the QCD critical point, and color superconducting phases, which are predicted in theoretical models in a high-baryon density regime in the QCD phase diagram. We will measure various observables such as event-by-event fluctuations, dileptons, collective flow, and two-particle correlations. We also search for various multi-strangeness particles/nuclei and study hadron-hadron interactions, including strangeness.
        In this talk, we will discuss physics goals and experimental plans with a low-intensity injector and an experiment at the existing J-PARC E16 spectrometer (Phase 1), and with a high-intensity injector and an experiment with a new large acceptance spectrometer (Phase 2). We will also show the status of the dilepton and hadron measurements in p+A collisions at J-PARC, aiming to study the chiral-symmetry restoration with vector mesons in nuclei, which serves as a baseline experiment for J-PARC-HI.

        Speaker: Hiroyuki Sako (JAEA)
        QCS2026-J-PARC-HI-submit.pdf
      • 36
        Status and Commissioning Results of the J-PARC E16 experiment

        The J-PARC E16 experiment aims to experimentally observe modifications of vector meson mass spectra in a nuclear medium, which are expected as a consequence of the partial restoration of chiral symmetry at finite density. In the experiment, vector mesons are produced by the 30 GeV proton beam at J-PARC on nuclear targets, and their di-electron decay is detected to obtain the invariant mass spectrum. Because the electrons do not suffer from final-state strong interactions with the nuclear medium, unlike hadronic decay modes, this enables a clean measurement of the invariant mass spectrum without distortion.
        The E16 experiment completed its final commissioning run in June 2024 and started physics data taking in November 2025. From the analysis of the commissioning data, we confirmed that the vector mesons, in particular the $\phi$ meson, can be successfully reconstructed, with the $e^+e^-$ pairs collected despite 100 times more hadronic background. Furthermore, the production cross section of the $\phi$ meson was estimated from the obtained $\phi$ yield, assuming the production kinematics suggested by the event generator JAM.
        In this presentation, after giving an overview and current status of the E16 experiment, we report the results from the 2024 commissioning run.

        Speaker: Satomi Nakasuga
        QCS2026_Nakasuga.pdf
      • 37
        Splitting of Transverse and Longitudinal Spectral Functions of the $\phi$ Meson at Finite Temperature and Density

        We investigate the spectral function of the $\phi$ meson in hot and dense nuclear matter created in low-energy heavy-ion collisions, where the existence of the medium's rest frame leads to a splitting between the transverse and longitudinal modes of vector mesons. We compute the transverse and longitudinal spectral functions within the framework of QCD sum rules and apply them to the $\phi$-meson yield measured in future experimental facilities such as FAIR, HIAF, and J-PARC.

        Speaker: Hidefumi Matsuda (Zhejiang University)
    • Banquet "MARUTOKU" in Oarai area.

      "MARUTOKU" in Oarai area.

      https://www.shiosai-marutoku.com We will go there by Bus from Mirai Base. Departure time 16:30.

      "MARUTOKU" in Oarai area.
      https://www.shiosai-marutoku.com
      We will go there by Bus from Mirai Base.
      Departure time 16:30.

  • Friday 1 May
    • Session 5
      Convener: Tetsuo Hyodo (Tokyo Metropolitan University)
      • 38
        Experimental studies on hyperons in nuclei and neutrons stars

        Fifteen years have passed since the “hyperon puzzle” in neutron stars was raised, but it remains unsolved yet. Various experimental efforts have been and will be made to investigate hyperon-nucleon interactions in dense nuclear matter. Ξ hypernuclei as well as Λ hypernuclei have been intensively studied at J-PARC, clarifying an attractive Ξ-N interaction. High-quality Σ-N scattering experiments have been also realized, being followed by high-quality Λ-N scattering experiments, both of which will play essential roles to construct realistic YN interaction models. Then, high-precision Λ hypernuclear spectroscopy experiments for a wide range of mass number will be conducted at JLab and then at J-PARC in order to extract the strength of ΛNN three-body interaction, key information to solve the hyperon puzzle.

        Speaker: Hirokazu Tamura (Tohoku University)
        QCS2026_Tamura.pdf
      • 39
        Hyperon single-particle potentials in nuclear matter by chiral forces up to N2LO

        The appearance of hyperons in dense neutron-star matter strongly affects the corresponding equation of state. The onset density of hyperons depends on their single-particle potentials. In this talk, I report the single-particle potentials [1] of the $\Lambda$ and $\Sigma$ hyperons, calculated using up-to-date baryon–baryon interactions derived within chiral effective field theory up to next-to-next-to-leading order [2]. It is found that the $\Sigma$ single-particle potential is more attractive than that obtained with previous chiral forces at next-to-leading order, reflecting constraints from the $\Sigma N$ differential cross sections measured in the recent J-PARC E40 experiment. Our recent progress will also be presented.

        [1] A. Jinno, J. Haidenbauer, and U.-G. Meißner, Phys. Rev. C 112, 065209 (2025).
        [2] J. Haidenbauer, U.-G. Meißner, A. Nogga, and H. Le, Eur. Phys. J. A 59, 63 (2023).

        Speaker: Asanosuke Jinno (Kyoto University)
      • 40
        Constraining the $\Lambda\Lambda$ interaction with terrestrial and astronomical data

        Terrestrial double-$\Lambda$ hypernuclear data and astronomical observations of neutron stars provide complementary constraints on the $\Lambda\Lambda$ interaction. In this work, we investigate the $\Lambda\Lambda$ interaction within a Skyrme energy density functional framework. We employ a Skyrme-type $\Lambda\Lambda$ interaction that includes the standard $s$- and $p$-wave terms, as well as a density-dependent term that effectively represents an $N\Lambda\Lambda$ three-body force. The $s$-wave terms are constrained using data on double-$\Lambda$ hypernuclei supplemented by pseudodata obtained from core + $2\Lambda$ three-body model calculations including heavier hypernuclei. We show that the data on heavier systems are essential to simultaneously constrain the two $s$-wave parameters. We further explore the impact of the $p$-wave and $N\Lambda\Lambda$ components on the neutron-star properties and find that appropriate repulsive contributions of these terms yields consistency with current neutron-star mass-radius observations. These results indicate that the present framework provides phenomenologically acceptable equations of state for dense $(N,\Lambda)$ matter over a wide range of densities and highlight the importance of future experimental data on heavier double-$\Lambda$ hypernuclei.

        Speaker: Yusuke Tanimura (Soongsil University)
        QCS2026_20260501.pdf
    • 10:30
      Break
    • Session 5
      Convener: Tetsuo Hyodo (Tokyo Metropolitan University)
      • 41
        Lattice QCD studies on S-wave kaon-nucleon interactions and $\Theta^+$ pentaquark

        Interactions between a kaon and nucleon provide an essential input for understanding the partial restoration of chiral symmetry and the properties of pentaquarks. Although kaon-nucleon scattering experiments at low momenta have been performed since the 1960s, our understanding of these interactions remains limited due to the scarcity of experimental data near the threshold, which hampers a precise elucidation of the underlying physics. On the other hand, recent advances in computational resources and lattice QCD techniques have made it possible to determine hadron interactions from first principles.
        In this study, we investigate the S-wave kaon-nucleon interactions in lattice QCD using the time-dependent HAL QCD method. The calculation is performed with $N_f=2+1$ quark flavors at the physical point, $m_{\pi}\approx 137$ MeV. The resulting interaction potential exhibits repulsive (weak attractive) behavior in the isospin $I=1$ ($I=0$) channel. The extracted phase shifts indicate the absence of bound or resonant states corresponding to the $\Theta^{+}$ pentaquark in this system.

        Speaker: Kotaro Murakami (Institute of Science Tokyo)
        slide_QCS2026_KN_Theta_wob.pdf
      • 42
        Structure of hadrons toward dense matter

        We discuss hadron structure at low energies based on effective models. Conventional approach such as the quark model needs some updates in view of recent discoveries of non-conventional hadrons especially in resonance regions. The relevance of the chiral structure of resonances and impact on dense matter will be discussed.

        Speaker: Atsushi Hosaka (RCNP, Osaka University)
        2026_0429_QCS_hosaka.pdf
      • 43
        Studying proton-nucleus collisions to extract the behavior of the phi meson in nuclear matter

        At present, there is no clear consensus regarding how the mass and width of
        the phi meson are modified in a dense environment such as nuclear matter,
        nor on the strength of its chiral mixing with the axial-vector chiral
        partner. Although a number of theoretical studies have addressed these
        questions, establishing a direct connection with experimental observables
        remains a significant challenge. This difficulty arises in part because the
        phi meson in nuclear matter is typically produced in proton–nucleus (pA)
        reactions, which are inherently non-equilibrium processes.
        In this presentation, I will review the current status of theoretical research on the
        in-medium properties of the phi meson, with a particular emphasis on ongoing
        transport simulations of pA reactions that produce phi mesons inside nuclei.
        These reactions are being explored experimentally at KEK E325 and in the
        J-PARC E16 and E88 experiments.

        Speaker: Philipp Gubler (JAEA)
        QCS_Gubler_2026.pptx
    • 12:30
      Lunch
    • Session 6
      Convener: Takumi Muto (Chiba Institute Technology)
      • 44
        A quarkyonic matter model for neutron star matter

        Recent observations of neutron stars, combined with causality, thermodynamic stability, and nuclear constraints, indicate a rapid stiffening of QCD matter at densities slightly above nuclear saturation density (n0 = 0.16fm-3). The evolution of the stiffening is faster than expected from purely nucleonic models with many-body repulsion. Taking into account the quark substructure of baryons, we argue that the saturation of quark states occurs at baryon density ~ 2-3n0, driving quark matter formation even before baryonic cores of radius ∼0.5 fm spatially overlap. We describe the continuous transitions from hadronic to quark matter within a quarkyonic matter model in which gluons are assumed to remain confining at densities of interest. To obtain analytic insight into the transient regime, we construct an ideal model of quarkyonic matter, the IdylliQ model, in which one can freely switch from baryonic to quark descriptions and vice versa.

        Speaker: Toru Kojo (KEK)
        2026_0501_QCS.pdf
      • 45
        Quarkyonic hadron-quark crossover from an ultracold atom perspective

        The hadron-quark crossover (HQC) is one of the most exciting topics in nuclear astrophysics. Recently, microscopic features of HQC have been discussed in terms of the quarkyonic matter picture. In this talk, we address this problem from an Interdisciplinary context, by drawing an analogy with pairing fluctuations in the Bardeen-Cooper-Schrieffer to the Bose-Einstein condensate crossover, which has been well established in ultracold atom experiments.

        Speaker: Hiroyuki Tajima (The University of Tokyo)
        QCS2026_tajima.pdf
      • 46
        Quarkyonic matter within a quark-meson coupling model

        We investigate dense nuclear matter within a unified framework that combines the quarkyonic picture with the quark-meson coupling (QMC) model, focusing on symmetric and isospin-asymmetric nuclear matter. Nucleons are described explicitly at the quark level using relativistic Gaussian quark wavefunctions, providing a microscopic realization of a dual baryon-quark description of dense matter. Nuclear interactions are incorporated self-consistently through scalar and vector mean fields acting directly on confined quarks, ensuring thermodynamic consistency and a smooth crossover from hadronic to quark-dominated regimes without invoking a sharp phase transition. We show that the onset of quark saturation is strongly correlated with the effective nucleon size; for a proton root-mean-square radius of 0.8 fm, quark saturation appears around 1.5 times the nuclear saturation density in symmetric nuclear matter. The resulting equation of state exhibits a characteristic soft-to-stiff behavior in the sound velocity and yields pressures for both symmetric and asymmetric nuclear matter that are consistent with empirical constraints from heavy-ion collision experiments.

        Speaker: Tsuyoshi Miyatsu (Soongsil University)
        QCS2026-Miyatsu.pdf
      • 47
        Color-Spin Molecular Dynamics and Multi-Quark Clustering in Neutron Stars

        We investigate the equation of state of neutron-star matter using color–spin molecular dynamics, explicitly incorporating the internal color and spin degrees of freedom of quarks and their dynamical evolution. Flavor conversion processes, including the emergence of strangeness, as well as beta equilibrium, are determined self-consistently through energy minimization. Our results show that, in the interior of stable neutron stars, quark–hadron deconfinement does not occur. Instead, quarks form bound structures, leading to the self-consistent emergence of multi-quark clustering. The resulting cluster-size distribution exhibits pronounced peaks at quark numbers that are integer multiples of three, corresponding to baryonic degrees of freedom.

        Speaker: Nobutoshi Yasutake (Chiba Institute of Technology)
    • 15:20
      Break
    • Session 7
      Convener: Masayasu Harada (Nagoya Ubiversity)
      • 48
        Exotic Hadronic Matter in Neutron Stars: Bayesian Inference of Antikaons and Δ-Resonances ----(online)----

        We present a unified Bayesian analysis of the dense-matter equation of state (EOS) of neutron stars by incorporating exotic hadronic degrees of freedom, namely antikaon condensation and Δ-resonances, within a density-dependent relativistic hadron framework. The model is constrained using nuclear saturation properties, chiral effective field theory ($\chi$EFT) calculations for symmetric and pure neutron matter up to $\sim 2\rho_0$, and multi-messenger astrophysical observations, including NICER mass–radius measurements of PSR J0030+0451 and PSR J0740+6620, tidal deformability constraints from GW170817, and the existence of $\sim 2~M_\odot$ neutron stars. For the antikaon sector, Bayesian inference yields an optical potential depth at nuclear saturation density of $U_{\bar{K}}(\rho_0)=-129.4_{-3.8}^{+12.5}$MeV ($68\%$ credible interval), indicating moderately strong attraction. We find that $K^⁻$ condensation is absent in canonical $1.4~M_\odot$ neutron stars but becomes energetically favourable in massive stars with $M\geq 2~M_\odot$, leading to significant EOS softening and a reduction of the squared speed of sound to $c_s^2\leq 0.3$ in the condensed phase. In parallel, the inclusion of Δ-resonances softens the EOS at intermediate densities $(\sim 1 - 3\rho_0)$ while preserving sufficient stiffness at higher densities to support maximum masses above $2~M_\odot$. Δ baryons are found to populate the outer core, contributing up to $\sim 20\%$ of the baryon fraction in the most massive configurations, and yielding radii $R_{1.4}\sim 12 - 13$ km and tidal deformabilities $\Lambda_{1.4}\sim 300 - 600$, consistent with GW170817. Furthermore, Δ-admixed EOSs predict fundamental f-mode frequencies $f_{1.4}=1.97_{-0.22}^{+0.17}$ kHz with damping times $τ_f\sim0.19_{-0.03}^{+0.05}$ s, highlighting strong correlations between compactness, tidal deformability, and asteroseismic observables. Our results demonstrate that antikaon condensation and Δ-resonances play complementary yet competing roles in neutron star interiors and establish a statistically consistent framework for probing dense QCD matter through current and future multi-messenger observations.

        Speaker: Vivek Baruah Thapa (Birangana Sati Sadhani Rajyik Vishwavidyalaya)
        Presentation_ID03_Thapa.pdf
      • 49
        Probing Dark Matter from Cold Neutron Stars to Proto-Neutron Stars Using Two-Fluid Modeling, Bayesian Evidence, and Machine-Learning Inference

        Abstract: Neutron stars provide a high-density laboratory to test dark matter (DM) through its gravitational imprint on stellar structure and early evolution. In this talk, I present a unified set of results based on two-fluid modeling of cold neutron stars and evolving proto-neutron stars (PNSs), together with Bayesian model selection, multi-messenger constraints, and inverse parameter inference. For fermionic DM with repulsive self-interactions, correlations between DM microphysics and global observables (mass, radius, and tidal deformability) can appear strong when the nuclear sector is fixed, but become significantly weaker once realistic hadronic equation-of-state (EoS) uncertainties are included, indicating that bulk properties alone may not uniquely identify a DM model. Using RMF-based scenarios (standard NL, a stiffened NL--$\sigma$ cut, and a DM-admixed neutron decay anamoly), Bayesian evidence quantifies how combined nuclear and astrophysical datasets rank the competing mechanisms and exposes where specific constraints introduce tension. For axion-like-particle (ALP) mediated DM, I construct a large ensemble of DM-extended EoSs across $(m_\chi, q_f)$ and confront it with multi-messenger data (radio/X-ray pulsars, GW170817, and HESS~J1731$-$347) using likelihood/KDE-based scoring to isolate the viable parameter region. A supervised surrogate model then enables the inverse mapping from reconstructed mass--radius information to DM parameters, showing that the mass--radius curve-shape indicator $R_{1.6}/R_{1.4}$ primarily constrains $m_\chi$, while $\Lambda_{1.4}$ is most sensitive to the DM Fermi momentum $q_f$. Finally, for PNS evolution with non-annihilating DM coupled only through gravity, DM cores can produce compressional heating whereas extended DM halos can cool the baryonic matter, yielding a qualitatively distinct thermal signature testable with supernova neutrino signals and young pulsar cooling curves.

        Speaker: Prashant Thakur (Yonsei University)
        QCS_2026_Prashant_Thakur.pdf
      • 50
        Sensitivity of neutron drip lines and neutron star properties to the symmetry energy

        We study how the nuclear symmetry energy and its density slope parameter affect the neutron dripline of finite nuclei and the macroscopic properties of neutron stars within a semiclassical liquid drop model (LDM). Our analysis employs energy density functionals constrained by chiral effective field theory. The symmetry energy at saturation density is fixed, while the surface tension parameter is calibrated by minimizing the root-mean-square deviation of total binding energies for 2208 experimentally known nuclei. Using this framework, we systematically explore correlations between symmetry energy parameters and key observables, including neutron driplines, crust–core transition densities, and the radii of 1.4 M⊙ neutron stars. Furthermore, we investigate the connection between macroscopic neutron star properties, such as the radius R₁.₄, and microscopic nuclear features, including the extent of isotopic chains and the last bound nucleus in the Z = 28 isotopic sequence. These results provide a unified perspective linking finite nuclei systematics with neutron star observables through the density dependence of the symmetry energy.

        Speaker: Yeunhwan Lim (Yonsei University)
      • 51
        Study of pion condensate in nuclear matter based on a parity doublet model (Award candidate)

        This work focuses on the existence of pion condensate in neutron star matter based on a parity doublet model. Parity doublet model is a model that explains the hadron mass as two parts: one part is generated by spontaneous breaking of chiral symmetry and the other is an invariant mass that does not depend on the breaking of chiral symmetry and is called a chiral invariant mass $m_0$. A pion serves as psedo-Numbu-Goldstone boson associated with spontaneous chiral symmetry breaking and has a small mass. According to Bose-Einstein condensate theory, in nuclear matter, pion condensate could occur when the isospin chemical potential is large enough. In this study, we determine the phase transition line from normal phase to pion condensate phase on the $\mu_B-\mu_I$ plane, where $\mu_B$ is the baryon number chemical potential and $\mu_I$ is the isospin chemical potential. The result shows that in low density neutron star matter, pion condensate is unlikely to occur.

        Speaker: LIU Xiang
      • 52
        Compact dwarfs and multibaryon states within the equivparticle model (Award candidate)

        Using an equivparticle model with mean-field approximation (MFA), we systematically investigate quark matter properties across scales, from microscopic multibaryon states to macroscopic compact dwarfs. First, we determine the mass spectra of color-singlet $N$-quark configurations ($N=3$ to $18$) based on SU(6) symmetry and Bayesian inference. Our results predict several bound states, including the H-dibaryon and $D_{03}$, providing a microphysical basis for pulsar-like objects.
        By incorporating linear confinement and perturbative interactions, we extend this framework to study strangelets and nonstrange quark matter ($ud$QM) nuggets. We examine the structures of compact dwarfs where these nuggets form body-centered cubic lattices in a uniform electron background. Despite the inherent stability of larger nuggets, these dwarfs remain stable against fusion due to the Coulomb barrier. Furthermore, we find that the radial oscillation frequencies of $ud$QM and strangelet dwarfs are typically higher than those of traditional white dwarfs. Even when covered by normal matter, these objects maintain their stability without exceeding the mass-radius limits of ordinary white dwarfs. This unified study establishes a consistent link between the microscopic mass spectra of multibaryon states and the dynamical stability of macroscopic compact stars.

        Speaker: Hao-Song You (Tsung-Dao Lee Institute, Shanghai Jiao Tong University)
        QCS2026-you(2).pptx
  • Saturday 2 May
    • Session 8
      Convener: Tsuneo Noda (Kurume Institute of Technology)
      • 53
        New Perspectives in the Study of Neutron Stars

        Neutron stars can be regarded as natural laboratories to explore physics of many-nucleon systems from sub- to supra-saturation densities. We aim to understand various macroscopic astrophysical phenomena of neutron stars based on microscopic theories. In this talk, I will review recent activities in our nuclear theory group at Science Tokyo (see, e.g., Refs. [1-8]) and discuss selected topics in some details.

        [1] K. Yoshimura and K. Sekizawa, Phys. Rev. C 109, 065804 (2024).
        [2] K. Yoshimura and K. Sekizawa, Phys. Rev C 112, 065804 (2025).
        [3] K. Yoshimura and K. Sekizawa, arXiv:2601.13636 [nucl-th]
        [4] H. Kwon and K. Sekizawa, arXiv:2505.20990 [nucl-th].
        [5] H. Kwon, K. Yoshimura, T. Miyatsu, K. Sekizawa, and M.-K. Cheoun, arXiv:2511.10996 [astro-ph.HE].
        [6] Y. Nam and K. Sekizawa, arXiv:2510.20353 [nucl-th].
        [7] Y. Nam and K. Sekizawa, arXiv:2511.13263 [astro-ph.HE].
        [8] T. Hattori and K. Sekizawa, arXiv:2512.22577 [nucl-th].

        Speaker: Kazuyuki Sekizawa (Institute of Science Tokyo)
        sekizawa.pdf
      • 54
        Superfluid hydrodynamics in neutron stars: thermal aspects and stability

        Born from gravitational-core collapse supernovae, with initial temperatures as high as $\sim10^{12}$K, neutron stars cool down to temperatures $10^9$K within a few days, providing a unique opportunity to explore matter under extreme conditions. In particular, neutron stars contain nuclear superfluids whose presence is supported by observations of pulsar frequency glitches, rapid decline in luminosity of the Cassiopeia A remnant, and crust cooling of neutron stars in low-mass X-ray binaries.

        Despite the importance of the superfluid dynamics in interpreting these astrophysical phenomena, most microscopic calculations of the nuclear pairing properties have been carried out so far for static situations. We have recently studied the dynamics of hot neutron-proton superfluid mixtures within the time-dependent nuclear energy-density functional theory [1,2]. The disappearance of superfluidity has also been investigated and reveals the presence of a dynamical "gapless" state in which nuclear superfluidity is not destroyed even though the energy spectrum of quasiparticle excitations exhibits no gap.

        This gapless state affects considerably the thermal properties of neutron stars and also raises questions about neutron vortex dynamics [3,4]. Implications for the crust cooling of neutron stars in low-mass X-ray binaries will be discussed, as well as the dynamical stability of neutron stars.

        [1] V. Allard & N. Chamel, Phys. Rev. C 103, 025804 (2021).
        [2] V. Allard & N. Chamel, Phys. Rev. C 108, 045801 (2023).
        [3] V. Allard & N. Chamel, Phys. Rev. Lett. 132, 181001 (2024).
        [4] V. Allard & N. Chamel, Eur. Phys. J. A 60, 116 (2024).

        Speaker: Valentin Allard (Yonsei University)
        Talk_JAEA_2026.pdf
      • 55
        Synchrotron Radiation of Ultra–High–Energy Protons in Strong Magnetic Fields

        The origin of ultra–high–energy cosmic rays (UHECRs), including the $2.44\times10^{20}$ eV event detected by AMATERAS, remains an open problem in astrophysics. Strong magnetic fields, such as those associated with magnetars, are expected to play an important role both in accelerating particles to ultra–high energies and in shaping the accompanying radiation. In such environments, synchrotron emission from UHE protons can produce high-energy $\gamma$-rays that provide valuable information on the nature of UHECR sources.
        We present a fully relativistic description of synchrotron emission from UHE protons in intense magnetic fields, treating both electromagnetic $\gamma$ emission and strong-interaction particle production processes involving pions and $\rho$ mesons. The formulation is based on an exact treatment of proton motion in a magnetic field, allows proton recoil effects to be consistently included, and provides a coherent theoretical framework for describing synchrotron emission in the ultra–high–energy regime.
        For UHE protons, the Landau quantum number can reach values as large as $ N \gtrsim 10^{15}$, rendering direct calculations impractical. We overcome this difficulty by identifying a generalized scaling rule for transition probabilities, which allows results for extremely large Landau quantum numbers to be inferred from calculations at much smaller values. Our results provide a reliable theoretical basis for interpreting high-energy radiation from UHECR acceleration sites in strong magnetic fields.
        [1] Telescope Array Collaboration, Science 382, 903 (2023).
        [2] T. Maruyama et al., Phys. Rev. D in press.

        Speaker: Tomoyuki Maruyama (College of Bioresource Sciences, Nihon University)
    • 10:30
      Break
    • Session 8
      Convener: Tsuneo Noda (Kurume Institute of Technology)
      • 56
        Thermal Evolution of Neutron Stars and Related Problems ----(online)----

        We will review recent developments in the problems of thermal evolution of neutron stars and other related objects. Since the first discovery of a neutron star, over years we have seen significant developments in the understanding of these stars, through advancement in both theory and observation. Consequently, we now have better hope for the understanding of high density matter beyond the nuclear density.

        Speaker: Sachiko Tsuruta (Montana State Univrsity)
        Hyperon_Cooling - 2026-04-25T104900.027.pdf
        ocs2026fffinal.ppt
      • 57
        Asteroseismology of neutron stars with both hyperons and $\Delta$ resonances (Award candidate)

        Employing relativistic energy density functionals PKDD, DD-ME2, and TW99 for nucleon-nucleon interactions, we investigate the impact of $\Delta$ resonances on the frequencies of non-radial $g$-mode oscillations in neutron stars adopting the universal coupling scheme. Our results show that when the proportion of $\Delta$ resonances in the neutron star interior reaches a certain threshold, the $g$-mode oscillation energy becomes concentrated within the region where $\Delta$ resonances appear, leading to a sharp increase in the $g$-mode frequency. In addition, we also examine the $f$-mode and $p$-mode oscillations and find that the impact of $\Delta$ resonances on these modes is less pronounced than on the $g$-mode. We also compare the oscillation frequencies calculated in the Cowling approximation with those from full general relativity, confirming that the Cowling approximation introduces an error of approximately 20\% for the $f$-mode and within 10\% for the $g$-mode. The notable effect of $\Delta$ resonances on neutron star $g$-mode frequencies holds important implications for probing the internal composition of neutron stars.

        Speaker: Hao Sun (扬州大学)
        Hao-Sun QCS.pptx
      • 58
        Neutron Star Cooling with Y+K phase

        The hot neutron star cools down due to the loss of neutrinos produced by the scattering of particles mainly inside the core, which enables us to probe the state of high-density matter through temperature observations. Recent X-ray observations have confirmed the existence of too cold
        isolated neutron stars beyond the standard cooling scenario without rapid cooling (i.e., minimal cooling scenario), which may imply the existence of exotic particles such as mesons, hyperons, and
        quarks. Among them, hyperons are thought to be a powerful candidate for the rapid cooling process known as the hyperon direct Urca process to explain cold neutron stars, in addition to another candidate, the nucleon direct Urca process. However, because of the large uncertainties of baryon superfluidity/superconductivity, whether such direct Urca cooling can work efficiently as a rapid cooling mechanism is controversial. For instance, strong proton superconductivity could suppress both of them. We utilize the latest equation of state with exotic matter hyperons and Kaon condensation (i.e., Y+K phase), and discuss the impact of the Y+K phase on cooling curves, focusing on
        the role of Kaon Urca process and proton superconductivity. We also discuss the possibility to "see" the signature of K condensation from temperature observations, i.e., to realize the situation where the Kaon Urca process is dominant.

        Speaker: Akira Dohi (RIKEN)
        qcs26_adohi.pdf
    • 12:15
      Lunch
    • Session 9
      Convener: Nobutoshi Yasutake (Chiba Institute of Technology)
      • 59
        Induced magnetic field by the chiral transitions

        Origin of the strong magnetic field observed in compact stars has been an interesting issue. If nuclear matter favors spin polarization, it gives a possible origin of the magnetic field. However, any realistic calculation has not given positive results. We discuss some microscopic origin of the magnetic field in dense quark matter. We consider here two types of chiral transitions, dual chiral density wave (DCDW) phase and tensor condensate, both of which lead to spontaneous magnetization. After presenting some specific features of both phases, we discuss their magnetic properties. Different from the non-relativistic matter, magnetization is not simply proportional to spin polarization in quark matter. We carefully evaluate the thermodynamic potential under the external magnetic field, from which we can derive magnetization and magnetic susceptibility. Finally, we can see such spontaneous magnetization produces a sufficiently strong magnetic field of $O(10^{16})$ G on the surface of compact stars, which may be comparable with the observation of magnetars.

        Speaker: Toshitaka Tatsumi (home)
        QCS2026-Tatsumi.pdf
      • 60
        Equation of state for dense stellar matter obtained by relativistic mean-field (RMF) theory (Award candidate)

        Data tables on the equation of state (EOS) and microscopic structures for dense stellar matter under both cold and finite-temperature conditions with proton fractions $Y_p =0.01$-$0.65$ and baryon number densities $n_\text{b}=10^{-8}$-$2 \ \mathrm{fm}^{-3}$ are obtained adopting two classes of relativistic density functionals, namely, those with nonlinear self-couplings and those with density-dependent couplings. The EOSs of dense stellar matter inside neutron stars with baryon number densities $n_\text{b}=7.6\times 10^{-11}$-$2 \ \mathrm{fm}^{-3}$ are obtained as well fulfilling $\beta$-stability condition. In general, the dense stellar matter exhibits droplet phase at $n_\mathrm{b}\lesssim 0.015\ \mathrm{fm}^{-3}$, while more exotic structures such as rods, slabs, tubes, and bubbles appear sequentially as density increases, but the structures gradually disappear with increasing temperature, and finally the system becomes a uniform phase. The critical proton fractions $Y_p^\mathrm{drip}$ ($\approx 0.26$-0.31) for neutron drip are obtained, where neutron gas emerges outside of nuclei at $Y_p< Y_p^\mathrm{drip}$. For dense stellar matter at small densities ($n_\text{b}\lesssim 10^{-5} \ \mathrm{fm}^{-3}$) or large proton fractions ($n_\text{b}\lesssim0.1 \ \mathrm{fm}^{-3}$ and $Y_p\gtrsim Y_p^\mathrm{drip}$), the EOSs and microscopic structures are generally insensitive to the adopted density functionals. With the onset of neutron drip at $Y_p\lesssim Y_p^\mathrm{drip}$, the uncertainties emerge and peak at $n_\text{b} \approx 0.02 \ \mathrm{fm}^{-3}$ within the range $10^{-5} \lesssim n_\text{b}\lesssim0.1 \ \mathrm{fm}^{-3}$. At $n_\text{b}\gtrsim0.1 \ \mathrm{fm}^{-3}$, the dense stellar matter becomes uniform and muons eventually appear, where the uncertainties in the EOSs grow significantly.

        Speaker: JiaXing Niu (Yangzhou university)
        NJX_QCS_RMF_EOS.pptx
      • 61
        Properties of magnetized dense stellar matter in white dwarfs and neutron star crusts

        The properties of magnetized dense stellar matter in white dwarfs and neutron star crusts are investigated in a fully three-dimensional geometry with periodic boundary condition. The Thomas-Fermi approximation is employed to fixed the electron density profiles, while the nonuniform magnetic fields are treated with equivalent magnetic charge method.

        Speaker: Cheng-Jun Xia (Yangzhou University)
        20260502_CJXia.pdf
    • Career Talk: Tanur Sinha (Elsevier)
      Convener: Atsushi Hosaka (RCNP, Osaka University)
    • 62
      Closing Address

      Toshiki Maruyama

      remarks2.pdf