Energy Independence of the Collins Asymmetry in $p^{\uparrow}p$ Collisions

TL;DR

This paper presents high-precision measurements of the Collins asymmetry in polarized proton collisions at 510 GeV, showing that the asymmetry is nearly energy independent and providing constraints for theoretical models of spin-dependent fragmentation.

Contribution

It provides the first high-precision measurements of Collins asymmetries at 510 GeV, demonstrating their energy independence and testing the universality of transverse-momentum-dependent functions.

Findings

Collins asymmetries are nearly energy independent across 200 GeV to 510 GeV.

Results extend to high momentum transfer scales up to 3400 GeV$^2$.

Data constrains models of Collins fragmentation functions.

Abstract

The STAR experiment reports new, high-precision measurements of the transverse single-spin asymmetries for $\pi^{\pm}$ within jets, namely the Collins asymmetries, from transversely polarized ${p^{\uparrow}p}$ collisions at $\sqrt{s}$ = 510 GeV. The energy-scaled distribution of jet transverse momentum, $x_{\mathrm{T}} = 2p_{\mathrm{T,jet}}/\sqrt s$, shows a remarkable consistency for Collins asymmetries of $\pi^{\pm}$ in jets between $\sqrt{s}$ = 200 GeV and 510 GeV. This indicates that the Collins asymmetries are nearly energy independent with, at most, a very weak scale dependence in $p^{\uparrow}p$ collisions. These results extend to high-momentum scales ($Q^2 \leq 3400$ GeV$^2$) and enable unique tests of evolution and universality in the transverse-momentum-dependent formalism, thus providing important constraints for the Collins fragmentation functions.