Predictions for Sivers single spin asymmetries in one- and two-hadron electroproduction at CLAS12 and EIC

TL;DR

This paper models the Sivers effect in semi-inclusive deep inelastic scattering using a modified PYTHIA generator to estimate single spin asymmetries at CLAS12 and EIC, highlighting the importance of parton showers.

Contribution

It introduces a modified PYTHIA Monte Carlo that includes the Sivers effect and assesses its impact on predicted asymmetries, revealing potential underestimations in leading order approximations.

Findings

Parton showers significantly affect SSA predictions.

Modified PYTHIA provides more accurate SSA estimates.

Leading order approximations may underestimate Sivers PDFs.

Abstract

The study of the Sivers effect, describing correlations between the transverse polarization of the nucleon and its constituent (unpolarized) parton's transverse momentum, has been the topic of a great deal of experimental, phenomenological and theoretical effort in recent years. Semi-inclusive deep inelastic scattering measurements of the corresponding single spin asymmetries (SSA) at the upcoming CLAS12 experiment at JLab and the proposed Electron-Ion Collider will help to pinpoint the flavor structure and the momentum dependence of the Sivers parton distribution function describing this effect. Here we describe a modified version of the $\tt{PYTHIA}$ Monte Carlo event generator that includes the Sivers effect. Then we use it to estimate the size of these SSAs, in the kinematics of these experiments, for both one and two hadron final states of pions and kaons. For this purpose we utilize the existing Sivers parton distribution function (PDF) parametrization extracted from HERMES and COMPASS experiments. Using this modified version of $\tt{PYTHIA}$, we also show that the the leading order approximation commonly used in such extractions may provide significantly underestimated values of Sivers PDFs, as in our Monte Carlo simulations the omitted parton showers and non-DIS processes play an important role in these SSAs, for example in the COMPASS kinematics.