Measurement of Transverse Single Spin Asymmetries in $\pi^0$ Production from $p^{\uparrow}+p$ and $p^{\uparrow}+A$ Collisions at STAR

TL;DR

This paper reports measurements of transverse single spin asymmetries in neutral pion production from polarized proton-proton and proton-nucleus collisions at RHIC, providing insights into nuclear effects and spin phenomena at high energies.

Contribution

It presents the first comparison of spin asymmetries in $p+p$ and $p+A$ collisions at RHIC, exploring nuclear dependence and saturation effects in transverse spin phenomena.

Findings

Large $ au^0$ asymmetries observed in $p+p$ collisions persist in $p+A$ collisions.

Asymmetry dependences on $p_T$ and event topology are compared between $p+p$ and $p+A$.

Results offer new insights into nuclear modifications of spin asymmetries.

Abstract

In 2015 the first collisions between polarized protons and nuclei occurred at the Relativistic Heavy Ion Collider (RHIC), at a center-of-mass energy of $\sqrt{s_{NN}}=200$ GeV. Comparisons between spin asymmetries and cross-sections in $p+p$ production to those in $p+A$ production provide insight into nuclear structure, namely nuclear modification factors, nuclear dependence of spin asymmetries, and comparison to models with saturation effects. The transverse single-spin asymmetry, $A_{N}$, has been measured in $\pi^{0}$ production in the STAR Forward Meson Spectrometer (FMS), an electromagnetic calorimeter covering a forward psuedorapidity range of $2.6<\eta<4$. Within this kinematic range, STAR has previously reported the persistence of large $\pi^0$ asymmetries with unexpected dependences on $p_T$ and event topology in $p+p$ collisions. This talk will compare these dependences to those in $p+A$ production.