New COMPASS results on Collins and Sivers asymmetries

TL;DR

The study addresses the nucleon's transverse-spin structure by measuring Collins and Sivers asymmetries in SIDIS using a 2010 COMPASS proton run on a transversely polarized target. The analysis extracts $A_{Coll}$ and $A_{Siv}$, with DIS cuts $Q^2>1$, $0.1<y<0.9$, $W>5$, and hadron cuts $p_T^h>0.1$ GeV/$c$, $z>0.2$, achieving a substantial reduction in uncertainties (factor >2). Results show a non-zero Collins effect in the valence region up to about $\sim 0.10$ and a positive Sivers effect for positive hadrons, smaller than HERMES, with extended kinematic scans revealing possible $Q^2$ or $W$-dependent behavior at low $y$ and low $z$. These findings provide important constraints for transversity and Sivers functions and motivate global analyses and future tests of TMD universality via Drell–Yan and other measurements, while highlighting COMPASS's ongoing role and upgrades to the facility.

Abstract

The study of transverse spin and transverse momentum effects is an important part of the scientific program of COMPASS, a fixed target experiment at the CERN SPS. For these studies a 160 GeV/c momentum muon beam is scattered on a transversely polarized nucleon target, and the scattered muon and the forward going hadrons produced in DIS processes are reconstructed and identified in a magnetic spectrometer. The measurements have been performed on a deuteron target in 2002, 2003 and 2004, and on a proton target in 2007 and 2010. The results obtained for the Collins and Sivers asymmetries from the data collected in 2010 are here presented for the first time. They nicely confirm the findings of the 2007 run and allow for reduction of the errors by more than a factor of two.

Figures (6)

• Figure 1: Definition of the Collins and Sivers angles. The vectors $\vec{p}_T^{\, h}$ , $\vec{s}$ and $\vec{s'}$ are the hadron transverse momentum and the spin of the initial and struck quarks respectively.
• Figure 2: Left: distribution of the DIS events in the $x-Q^2$ plane. Right: mean $Q^2$ values in the different $x$-bins.
• Figure 3: Collins asymmetry as function of $x$, $z$, and $p_T^h$, for positive and negative hadrons.
• Figure 4: Sivers asymmetry as function of $x$, $z$, and $p_T^h$, for positive and negative hadrons.
• Figure 5: Sivers asymmetry for positive hadrons as function of $x$, $z$, and $p_T^h$, for $x>0.032$. The full points refer to the $0.1<y<0.9$ sample, the open points to the $0.05<y<0.1$ sample.