Adding quark spin effects to PYTHIA string fragmentation

TL;DR

This paper enhances the PYTHIA 8 event generator by incorporating quark spin effects into string fragmentation, enabling more accurate simulations of polarized semi-inclusive deep inelastic scattering and related asymmetries.

Contribution

The paper introduces a major update to StringSpinner, allowing the production and polarized decay of vector mesons in PYTHIA 8, improving the simulation of spin-dependent phenomena.

Findings

Validated the new implementation against experimental data

Simulated transverse-spin asymmetries in SIDIS

Provided predictions for rho meson asymmetries

Abstract

Quark spin effects in hadronization have been recently included in the PYTHIA 8 event generator for the simulation of the deep inelastic scattering (DIS) process off a polarized nucleon target via the external StringSpinner package. The spin effects can be simulated for the production of pseudoscalar mesons using the string+${}^3P_0$ model of polarized quark fragmentation and parametrizations of the transversity PDFs. We present here a major development of StringSpinner which includes the production of vector mesons in the polarized PYTHIA 8 string fragmentation as well as their polarized decays. The package is validated and used to simulate the transverse-spin asymmetries in semi-inclusive DIS (SIDIS). The simulation results on the Collins and dihadron asymmetries for the final state mesons are compared with the asymmetries measured in SIDIS off transversely polarized protons. Also shown are the predictions for the Collins asymmetries for $\rho$ mesons produced in SIDIS off transversely polarized protons.