18-22 October 2021
Matsue, Shimane Prefecture, Japan
Asia/Tokyo timezone


20 Oct 2021, 17:06
Room 601 (Kunibiki Messe)

Room 601

Kunibiki Messe

Parallel Session Presentation Transverse momentum structure (TMD) Transverse Momentum Structure (TMD)


Xavier Artru (Institut de Physique des deux Infinis de Lyon (IP2I))


The Lund String Fragmentation Model, widely used in Monte Carlo generators of jets, is extended to include the quark spin as a full quantum-mechanical degree of freedom, described by Pauli spinors. Such a model is needed to describe consistently the azimuthal asymmetries in jets from polarized quarks, like the Collins effect, di-hadron asymmetry and jet handedness.
The model is formulated in terms of quark propagators and quark-meson-quark vertices which are combinations of Pauli matrices. The basic assumption is that, when a string breaks, the created q-qbar pair is in the 3P0 state. This is implemented by a quark propagator proportional to mu+sigma_z sigma.k_T, where mu is a complex mass parameter. The model has been recently translated in a recursive Monte Carlo code for pseudoscalar meson production.
The introduction of vector mesons needed to complete the model requires a special recipe accounting for the entanglement between the spins of the meson and of the left-over quark. The longitudinal and transverse mesons are coupled to the quark line with two independent complex, constants G_L and G_T.
The expected properties are chiefly:
- Collins asymmetries of the same size but opposite signs for pi+ and pi-,
- di-hadron asymmetry linked to the Collins one,
- generation of jet handedness,
- vector mesons have Collins asymmetries whose signs depend on their linear polarizations,
- vector mesons can be generated with oblique (i.e., between transverse and longitudinal) polarizations. This is a new source of di-hadron asymmetry.
The main theoretical aspects of the model are presented in this talk, while the results of MC simulations and their comparison with existing data are the subject of a different dedicated talk (A. Kerbizi).

Primary author

Xavier Artru (Institut de Physique des deux Infinis de Lyon (IP2I))

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