Transverse single-spin asymmetry of weak bosons and Drell-Yan production in p+p collisions at STAR: present and future

TL;DR

This paper discusses measurements of transverse single spin asymmetries in proton-proton collisions at STAR, testing the Sivers function's non-universality and exploring the spin structure of the proton, with future plans for more precise data collection.

Contribution

It presents the first STAR measurements of TSSAs in weak boson and Drell-Yan production, and outlines future plans for high-precision tests of the Sivers function.

Findings

Current data favor models with a sign change of the Sivers function.

Results are consistent with small TMD evolution effects.

Future measurements will significantly improve precision.

Abstract

Accessing the Sivers TMD function in proton+proton collisions through the measurement of transverse single spin asymmetries (TSSAs) in Drell-Yan and weak boson production is an effective path to test the fundamental QCD prediction of the non-universality of the Sivers function. Furthermore, it provides data to study the spin-flavor structure of valence and sea quarks inside the proton and to test the evolution of parton distributions. The TSSA amplitude, $A_{N}$, has been measured at STAR in proton+proton collisions at $\sqrt{s} = 500$ GeV, with a recorded integrated luminosity of 25 pb$^{-1}$. Within relatively large statistical uncertainties, the current data favor theoretical models that include a change of sign for the Sivers function relative to observations in SIDIS measurements, if TMD evolution effects are small. RHIC plans to run proton+proton collisions of transversely polarized beams at $\sqrt{s} = 510$ GeV in 2017, delivering an integrated luminosity of 400 pb$^{-1}$. This will allow STAR to perform a precise measurement of TSSAs in both Drell-Yan and weak boson production. The present status and future plans for the Sivers function program at STAR will be discussed as well as other observables sensitive to the non-universality of the Sivers function in the Twist-3 framework, e.g. the TSSA of direct photons.