Workshop on Heavy-Quark Exotics

19 Feb 2026, 13:20 → 21 Feb 2026, 22:25 Asia/Tokyo

Etsuko Itou (iTHEMS, RIKEN), Takumi DOI (RIKEN)

This workshop is a meeting focused on research related to 
the KAKENHI Grant-in-Aid for Scientific Research (S), 
“Unraveling the Mysteries of Heavy-Quark Exotic Hadrons from QCD” (PI: Takumi Doi).
We aim to foster intensive discussions on lattice QCD methods and effective field theories, to explore future research directions, and promote potential collaborations.

Venue:
Toba seaside hotel (鳥羽シーサイドホテル)
https://www.tobaseasidehotel.co.jp/

Please note that accommodation will be arranged collectively by the organizers.
If you wish to stay at the venue hotel, please indicate your request in the registration form.

Dates:
From the afternoon of February 19 to around noon on February 21, 2026

Deadline of Registration:
12 Dec. 2025

Deadline of abstruct submission:
31 Jan. 2026

Supported by
JSPS KAKENHI 基盤(S) 
No. 23H05439 「QCDから解き明かす重クォークエキゾチック粒子の謎」
(研究代表者：土井 琢身)

Thursday 19 February

13:20 → 17:40 Day 1 afternoon: Day1 afternoon

13:20 Opening

Opening of the workshop

Speaker: Takumi DOI (RIKEN)

13:30 Overview of Lattice QCD

I give an overview on the progress in Lattice QCD.

Speaker: Takumi DOI (RIKEN)

14:10 Reconsideration of two-pion exchange potential

The HAL computations suggest the ``two-pion exchange potential'' at (semi-)long range regime. In this talk I introduce its theoretical development and explain my ideas.

Speaker: Daiki Suenaga (KMI/Nagoya U.)

ExoticToba_Feb.pdf

15:20 Charmonia scatterings from lattice QCD

Recent discoveries in the $J/\psi$-$J/\psi$ spectrum have sparked significant interests in understanding charmonium-charmonium interactions. In this talk, we will show some preliminary results on charmonia scatterings from lattice QCD, with a special focus on interaction mechanism, coupled channel effects, near-threshold properties. Implications to the experimental results will be also mentioned.

Speaker: Yan Lyu

16:00 Di-Jpsi scattering states with the quark Pauli-blocking effects

The scattering states of the double-charmonium, such as $J/\psi J/\psi$, $\eta_c J/\psi$, and $\eta_c\eta_c$, are investigated by a simplified quark cluster model focusing on the Pauli principle over the quarks.
We found that its effects on the $c\bar cc\bar c$ quark system can be expressed by a two-meson potential, which has a long-range repulsion with a short-range attraction.
This Pauli-blocking effect gives rise to a node in the scattering phase shift at a specific energy, which is also seen in the cross sections.
We will also demonstrate how the correlation function from heavy ion collisions is affected by this Pauli-blocking effect and also discuss possible coupling to the relative $P$-wave $\chi_{cJ}J/\psi$ channel.

Speaker: Sachiko Takeuchi (RCNP, RIKEN Nishina Center)

toba2026_STakeuchi.pdf

17:00 Heavy hadronic molecules and their partners

Recent accelerator experiments have reported unexpected states known as exotic hadrons, whose properties cannot be explained by the conventional picture. In the heavy quark sector, $XYZ$, $T_{cc}$, $P_c$ etc. have been reported since the discovery of $X(3872)$. Many exotic hadrons are located near the hadron threshold, and thus hadronic molecular structures have been considered. In this talk, we study hadronic molecules of heavy hadrons. In the heavy hadron sector, the heavy quark symmetry is important, which induces channel coupling effects and also predicts partner states. We study bound state properties of hadronic molecules and also discuss partner states of these exotics, predicted by symmetries.

Speaker: Yasuhiro Yamaguchi (Nagoya University)

19:00 → 21:00 Dinner (個室宴会場 汀（みぎわ）館５階 吉月)

Friday 20 February

07:00 → 09:00 Breakfast (Seaside Hall, 汀館6階)

09:30 → 12:40 Day 2 morning

09:30 Lattice QCD studies on (anti)kaon-nucleon interactions and exotic states

Elucidating the nature of exotic hadrons is one of the central challenges in hadron physics. In recent years, numerical simulations based on lattice QCD have become a powerful tool for investigating these hadrons from first principles. The HAL QCD method, which explores hadron interactions in lattice QCD through interaction potentials, has demonstrated particular advantages for systems including baryons.
In this talk, I will present our recent studies using the HAL QCD method on kaon-nucleon and antikaon-nucleon interactions, which are closely related to the $\Theta^+$ pentaquark and the $\Lambda(1405)$ excited baryon, respectively.

Speaker: Kotaro Murakami (Institute of Science Tokyo)

slide_HQE_KbarN_KN.pdf

10:10 Insights into the Kbar N interaction and Lambda(1405) through model analysis of HAL QCD results

The HAL QCD method has been established as a reliable method to study hadron-hadron interactions. However, in the $\bar{K} N$ potential, which dynamically generates the $\Lambda (1405)$ as the bound state, singular behavior around the origin has been observed. In order to clarify the cause of such behavior in the HAL QCD method, we calculate the R-correlators and local potentials in an effective model of hadron-hadron interactions and compare it with the $\bar{K} N$ potential in the HAL QCD method.

Speaker: Takayasu Sekihara (Kyoto Prefectural University)

260220_Heavy_Sekihara.pdf

11:20 Study of Tcc based on a model including compact and molecule states, and a diquark model.

I will summarize main points of the following two works.
[1] S. Ampuku, Y. Yamaguchi and M. Harada, [arXiv:2511.13003 [hep-ph]].
We develop a model that incorporates mixing between hadronic-molecular and compact multiquark components. We then apply this framework to the specific case of the Tcc and analyze the peak structure in the D0-D0-pi+ invariant-mass spectrum reported by LHCb. We find that the model admits three solutions of comparable fit quality that can account for the Tcc. One solution corresponds to a predominantly compact tetraquark configuration. The other two solutions imply a molecular Tcc: (1) the Tcc is a D+ - D0 molecule and an additional D0 - D+ molecular state appears; (2) the Tcc is a D0 - D+ molecule and an additional D+ - D0 molecular state is found below the D0-D0-pi+ threshold. These molecular states are not simple I = 0 states, but mixtures of I = 0 and I = 1 states. We show that all three scenarios are also consistent with the experimentally observed near-threshold $D^0D^0$ and $D^0D^+$ invariant-mass distributions.
[2] M. Weber, D. Suenaga and M. Harada, in preparation.
We investigate the Tcc tetraquark, treating it as a bound state of a heavy diquark and a light anti-diquark. Using the Silvestre–Brac potential and solving the Schrödinger equation via the Gaussian Expansion Method, we find that the excitation energy between the heavy-diquark and light anti-diquark is unexpectedly larger than that between the two light anti-quarks within the anti-diquark -- contrary to the naive expectation where the former is smaller than the latter. We trace this inversion of the mass hierarchy to the centrifugal force acting on the light degree of freedom. Applying the same framework to other systems (Tbb, Lambda_b, Lambda_c) yields qualitatively identical behavior, demonstrating the robustness of the mechanism. These results provide new insights into diquark dynamics and the mass structure of exotic hadrons.

Speaker: Masayasu Harada (KMI, Nagoya University)

2602-KibanS-Harada.pdf

12:00 Compositeness of near-threshold states in charged few-body systems

In recent years, an increasing number of exotic hadrons have been observed, many of which appear near scattering thresholds. In this work, we focus on near-threshold states in systems governed by both short-range and Coulomb interactions, and calculate the compositeness to quantify their internal structure. When the Bohr radius $a_B$ is larger than the magnitude of the effective range $|r_e|$, the universal behavior driven by the short-range interaction emerges before the Coulomb interaction becomes dominant. In this case, the compositeness increases near the threshold as a remnant of the short-range universality. On the other hand, if $a_B < |r_e|$, the compositeness does not increase even near the threshold, since the Coulomb interaction dominates and the system no longer exhibits universal behavior. As an application of this framework, we discuss the internal structure of exotic hadrons and nuclear systems.

Speaker: Tomona Kinugawa (RIKEN)

WS_exotic_Kinugawa.pdf

12:40 → 13:40 Lunch

13:40 → 17:20 Day 2 afternoon

13:40 Anti-D meson Nucleon interaction from Lattice QCD

The $\bar{D}N$ system is one of the most fundamental systems involving charmed mesons and nucleons. However, experimental data are scarce, and effective models do not constrain the low-energy scattering parameters well. I will report the results of my analysis on the "HAL-conf-2023" LQCD data concerning the $\bar{D}N$ channel.

Speaker: Dr Wren Yamada (RIKEN)

14:20 Femtoscopic studies for charm hadron interaction

The femtoscopic study using the momentum correlation function from the high-energy collisions provides new insights for the hadron interactions. The correlation function is known to be sensitive to the low-energy interaction and the particle emission source. In addition, the femtoscopy approach is advantageous for the charm sector which is not easily studied by the traditional scattering experiments. In this talk, I will discuss the recent results in the charm femtoscopic experimental data and the theoretical model analyses.

Speaker: Yuki Kamiya (Yukawa Institute for Theoretical Physics, Kyoto University)

Toba.pdf

15:30 Femtoscopy of heavy-flavor hadrons

We study the D\bar{D}^, DD^, and B\bar{B}^* correlation functions and discuss the structure of the associated exotic hadrons.

Speaker: Makoto Takizawa (Showa Pharmaceutical University)

Heavy-Quark-Exotics _20260220.pdf

16:10 Accessing higher partial waves through femtoscopy

Recently, femtoscopy has gathered attention as a new approach to
access hadron--hadron interactions through momentum correlation functions.  In particular, the $s$-wave interaction has been extensively discussed in the field.  In this talk, we discuss the effect of interactions in higher partial waves on the femtoscopic correlation function.  We first attempt to extend existing $s$-wave formulae of the two-particle correlation function to the general case with higher partial waves.  To avoid an unphysical behavior caused by a naive assumption of the traditional Lednicky-Lyuboshitz formula, we introduce regularized Lednicky-Lyuboshitz formulae and validate their performances. Based on the obtained formula, we demonstrate the nature of the higher-partial-wave contributions to the correlation function in the $(q', a'_l)$ plane, with $q'$ and $a'_l$ being the relative momentum and scattering length, respectively, in units of the source size $R$.  We also discuss future extension with the non-spherical source.

Speaker: Koichi Murase (RCNP, the University of Osaka)

20260220-murase-RegularizedLL.pdf

16:50 Free discussions

19:15 → 20:45 dinner (Seaside Hall, 汀館6階)

20:45 → 22:00 Free discussions

Saturday 21 February

07:00 → 09:00 Breakfast (Restaurant LARGE, 汀館6階)

09:00 → 11:40 Day 3 morning

09:00 Scalar mesons and gllueballs from recent perspectives

It has been discussed for a long time that light flavors, i.e., up, down and strangeness, give a variety of exotic hadrons. One of the most important examples are scalar mesons and glueballs. In this talk, we discuss these better-known light exotic particles from new perspectives. Firstly, we discuss a new framework for inner-structures of scalar mesons, and propose to study them by experiments in relativistic heavy ion collisions. Secondly, we present the Hopfion picture for glueball candidates in scalar mesons, and discuss their mass spectra in terms of the Skyrme-Faddeev model. Those studies will serve a new point of view for immemorial exotic hadrons.

Speaker: Prof. Shigehiro Yasui (Nishogakusha Univ.)

09:40 Preliminary results on Tbb

I will show some preliminary results on doubly bottom tetra quark states.

Speaker: Parikshit Junnarkar (RCNP, Osaka)

HQE_Workshop.pdf

10:50 Mass spectrum of hadrons in dense two-color QCD

In the vacuum of ordinary QCD, the lightest hadron is the pion, which is an isotriplet pseudoscalar meson. This can be shown using QCD inequalities, if we assume that the theory possesses gamma_5 Hermiticity and that disconnected diagrams do not contribute. However, in the finite-density regime, gamma_5 Hermiticity is lost, and this argument no longer holds. In contrast, in two-color QCD, an analogous argument based on the pseudo-reality of fermions shows that the isosinglet scalar meson becomes the lightest hadron. This naturally leads to the question: what happens to the masses of other hadrons?
In this talk, I will present results for the hadron mass spectrum obtained from first-principles calculations in two-color QCD.

Speaker: Etsuko Itou (iTHEMS, RIKEN)

2026-02-KibanS-workshop.pdf

11:30 Closing

Speaker: Takumi DOI (RIKEN)