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

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.