Transverse Single-Spin Asymmetry for Inclusive and Diffractive Electromagnetic Jets at Forward Rapidity in $p^{\uparrow}$+p Collisions at $\sqrt{s} = 200$ GeV and $510$ GeV at STAR

TL;DR

This paper investigates the transverse single-spin asymmetry (A_N) in electromagnetic jets at forward rapidity in polarized proton-proton collisions at STAR, revealing a negative A_N in diffractive processes and projecting improved measurements at higher energy.

Contribution

It provides the first detailed measurement of A_N for electromagnetic jets in diffractive processes and offers statistical projections for future high-precision data at 510 GeV.

Findings

Negative A_N observed in diffractive electromagnetic jets

Different sign of A_N in inclusive vs. diffractive processes

Enhanced statistical precision projected for 510 GeV data

Abstract

There have been numerous attempts, in the last decades, to understand the origin of the unexpectedly large transverse single-spin asymmetry ($A_{N}$) observed in inclusive hadron productions at forward rapidities in transversely polarized $p^{\uparrow}$+$p$ collisions at different center-of-mass energies ($\sqrt{s}$). The current theoretical frameworks aimed at explaining this puzzle include the twist-3 contributions in the collinear factorization framework, as well as the transverse-momentum-dependent contributions from the initial-state quark and gluon Sivers functions, and/or final-state Collins fragmentation functions. Besides, there are indications that the diffractive processes may contribute to the large $A_{N}$. We present the detailed investigations into the $A_{N}$ for electromagnetic jets (EM-jets) produced in inclusive processes using the Forward Meson Spectrometer with transversely polarized $p^{\uparrow}$+ $p$ data at $\sqrt{s} =$ 200 GeV collected in 2015 at STAR. We observe a negative value for the $A_{N}$ of EM-jets in diffractive processes. This finding shows a different sign for $A_{N}$ in inclusive processes and needs further theoretical input in order to be understood. Finally, we present the statistical projections of the $A_{N}$ for inclusive and diffractive EM-jets utilizing $p^{\uparrow}$+ $p$ data at $\sqrt{s} =$ 510 GeV collected in 2017 at STAR. This dataset allows for a substantial enhancement in statistical precision.