Recent Cold QCD Results from STAR

TL;DR

Recent STAR measurements of azimuthal distributions of charged hadrons in jets from transversely polarized proton collisions provide new insights into the spin-momentum correlations and flavor-dependent spin dynamics within the nucleon, advancing the understanding of transverse spin phenomena.

Contribution

This work presents new precise measurements of azimuthal asymmetries for kaons and protons in jets, highlighting flavor-dependent spin effects within the TMD framework.

Findings

Azimuthal asymmetries for charged kaons and protons in jets were measured.

Results shed light on flavor-dependent spin dynamics and the Collins effect.

Data contribute to understanding the nucleon spin structure.

Abstract

Understanding the origin of transverse single-spin asymmetries is a long-standing challenge in strong interaction physics. Recent precise measurements of the azimuthal distribution of charged pions in jets from STAR have shed new light on the spin momentum correlations within the Transverse Momentum Dependent (TMD) formalism. This measurement, particularly sensitive to the Collins effect, correlates the quark transversity with the Collins fragmentation functions, elucidating the spin-dependent fragmentation process. Moreover, similar measurements involving kaons and protons, which have unique quark compositions, provide vital insights into the flavor-dependent spin dynamics of the parton distribution functions and fragmentation functions. These studies are crucial for developing a more comprehensive understanding of nucleon spin structure. In these proceedings, selection of the recent Cold QCD STAR results are presented, focusing on the asymmetries for charged kaons and protons in jets from transversely polarized $pp$ data at $\sqrt{s}$ = 200 GeV.