Measurements of Collins and Sivers asymmetries at COMPASS

TL;DR

The paper investigates azimuthal single-spin asymmetries in semi-inclusive DIS to access transversity and Sivers functions using COMPASS 2002 transversely polarized data on a deuteron target. It outlines the theoretical framework linking quark helicity distributions and transversity to Collins and Sivers mechanisms and describes the experimental methodology and kinematic selections used to extract $A_{Coll}$ and $A_{Siv}$. The key finding is that both Collins and Sivers asymmetries are small and statistically consistent with zero for the deuteron, with only a hint of a Collins effect at large $z$. These results provide constraints on transversity and the Sivers function, contributing to global fits and the understanding of transverse spin structure.

Abstract

COMPASS is a fixed target experiment presently running at CERN. In 2002, 2003, and 2004 the experiment used a 160 GeV polarized muon beam coming from SPS and scattered off a $^6$LiD target. The nucleons in the target can be polarized either longitudinally or transversely with respect to the muon beam and 20% of the running time has been devoted to transverse polarization. From the transverse polarization data collected in 2002, which correspond to a total integrated luminosity of about 200 pb$^{-1}$, the Collins and the Sivers asymmetries have been determined separately and the preliminary results are presented here.

Figures (1)

• Figure 1: Collins and Sivers asymmetry for positive (full points) and negative (open points) hadrons as a function of $x$, $z$ and $p_T^{\, h}$. Leading hadrons analysis on top canvas, all hadrons on bottom canvas.