Global fitting of single spin asymmetry: an attempt

TL;DR

This paper performs a combined analysis of SIDIS and proton-proton collision data to extract Sivers and ETQS functions, addressing the sign mismatch issue and exploring different functional forms.

Contribution

It introduces a combined TMD and twist-3 fitting approach to analyze single spin asymmetries and investigates the sign mismatch problem in spin-dependent functions.

Findings

SIDIS and STAR data are well described by the fit

BRAHMS data are not satisfactorily explained

The sign mismatch issue remains unresolved

Abstract

We present an attempt of global analysis of Semi-Inclusive Deep Inelastic Scattering (SIDIS) $\ell p^\uparrow \to \ell' \pi X$ data on single spin asymmetries and data on left-right asymmetry $A_N$ in $p^\uparrow p \to \pi X$ in order to simultaneously extract information on Sivers function and twist-three quark-gluon Efremov-Teryaev-Qiu-Sterman (ETQS) function. We explore different possibilities such as node of Sivers function in $x$ or $k_\perp$ in order to explain "sign mismatch" between these functions. We show that $\pi^\pm$ SIDIS data and $\pi^0$ STAR data can be well described in a combined TMD and twist-3 fit, however $\pi^\pm$ BRAHMS data are not described in a satisfactory way. This leaves open a question to the solution of the "sign mismatch". Possible explanations are then discussed.