Measurement of transverse single-spin asymmetries of $\pi^0$ and electromagnetic jets at forward rapidity in 200 and 500 GeV transversely polarized proton-proton collisions

TL;DR

This paper measures transverse single-spin asymmetries of $^0$ and electromagnetic jets at forward rapidity in polarized proton-proton collisions at 200 and 500 GeV, revealing energy-independent behaviors and potential new physics mechanisms.

Contribution

It provides new measurements of TSSA for $^0$ and jets at two energies, exploring initial and final state effects and comparing results to QCD models, highlighting discrepancies and new physics possibilities.

Findings

TSSA increases with Feynman-x and is energy-independent from 19.4 to 500 GeV.

$^0$ with no nearby particles have higher TSSA than inclusive $^0$.

Small Collins asymmetry inside jets suggests limited final-state spin effects.

Abstract

The STAR Collaboration reports measurements of the transverse single-spin asymmetry (TSSA) of inclusive $\pi^0$ at center-of-mass energies ($\sqrt s$) of 200 GeV and 500 GeV in transversely polarized proton-proton collisions in the pseudo-rapidity region 2.7 to 4.0. The results at the two different energies show a continuous increase of the TSSA with Feynman-$x$, and, when compared to previous measurements, no dependence on $\sqrt s$ from 19.4 GeV to 500 GeV is found. To investigate the underlying physics leading to this large TSSA, different topologies have been studied. $\pi^0$ with no nearby particles tend to have a higher TSSA than inclusive $\pi^0$. The TSSA for inclusive electromagnetic jets, sensitive to the Sivers effect in the initial state, is substantially smaller, but shows the same behavior as the inclusive $\pi^0$ asymmetry as a function of Feynman-$x$. To investigate final-state effects, the Collins asymmetry of $\pi^0$ inside electromagnetic jets has been measured. The Collins asymmetry is analyzed for its dependence on the $\pi^0$ momentum transverse to the jet thrust axis and its dependence on the fraction of jet energy carried by the $\pi^0$. The asymmetry was found to be small in each case for both center-of-mass energies. All the measurements are compared to QCD-based theoretical calculations for transverse-momentum-dependent parton distribution functions and fragmentation functions. Some discrepancies are found, which indicates new mechanisms might be involved.