Quark spin effects in $e^+e^-$ annihilation: a Monte Carlo event generator study

TL;DR

This paper introduces a novel implementation of quark spin effects in the PYTHIA Monte Carlo generator, enabling detailed simulation of spin-dependent phenomena in $e^+e^-$ annihilation and comparison with experimental data.

Contribution

It is the first to incorporate quark spin effects into PYTHIA using the string+${}^3P_0$ model, allowing for realistic simulations of spin asymmetries in meson production.

Findings

Simulated Collins asymmetries agree with BELLE and BABAR data.

The model's analyzing power aligns with phenomenological extractions.

Implementation enhances PYTHIA's capability for spin physics studies.

Abstract

Quark spin effects in $e^+e^-$ annihilation to pseudoscalar and vector mesons are implemented for the first time in the PYTHIA Monte Carlo event generator. The spin-dependent fragmentation of the string stretched between the produced quark-antiquark pair with correlated spin states is described by the string+${}^3P_0$ model implemented in the string fragmentation routine of PYTHIA by using the StringSpinner package. The simulated events are used to study the model predictions for the Collins asymmetries of mesons produced back-to-back in the $e^+e^-$ center of mass system by using both the thrust axis method and the hadronic plane method. The obtained asymmetries are compared to the available data from the BELLE and BABAR experiments and the underlying Collins analysing power from the string+${}^3P_0$ model is compared with phenomenological extractions.