COMPASS Results on Transverse Single-Spin Asymmetries

TL;DR

COMPASS investigates transverse-spin structure via SIDIS using a transversely polarized deuteron target. The study measures Collins and Sivers single-hadron asymmetries, two-hadron asymmetries, and Λ polarimetry with a 160 GeV muon beam, across 2002-2004 data. All observed asymmetries for charged and identified hadrons, as well as the two-hadron channel, are compatible with zero within ~1% statistical errors, indicating cancellations between u and d quark contributions in the deuteron. The results constrain transversity and Sivers functions and motivate global analyses and future proton-target measurements to disentangle flavor structure.

Abstract

New results on single spin asymmetries of charged hadrons produced in deep-inelastic scattering of muons on a transversely polarised LiD target are presented. The data were taken in the years 2002, 2003 and 2004 with the COMPASS spectrometer using the muon beam of the CERN SPS at 160 GeV/c. Preliminary results are given for the Sivers asymmetry and for all the three ``quark polarimeters'' presently used in COMPASS to measure the transversity distributions. The Collins and the Sivers asymmetries for charged hadrons turn out to be compatible with zero, within the small (~1%) statistical errors, at variance with the results from HERMES on a transversely polarised proton target. Similar results have been obtained for the two hadron asymmetries and for the Lambda polarisation. First attempts to describe the Collins and the Sivers asymmetries measured by COMPASS and HERMES allow to give a consistent picture of these transverse spin effects.

Figures (10)

• 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 spins of the initial and struck quarks respectively.
• Figure 2: Collins asymmetry (top) and Sivers asymmetry (bottom) against the Bjorken variable $x$ (or $x_{Bj}$), $z$ and $p_T^h$ for positive (full circles) and negative leading hadrons (open circles) from the 2002, 2003, and 2004 data.
• Figure 3: Collins asymmetry (top) and Sivers asymmetry (bottom) against $x$, $z$ and $p_T^h$ for all positive (full circles) and all negative hadrons (open circles) from 2002 - 2004 data.
• Figure 4: Collins asymmetry: comparison between the 2002-2004 COMPASS results for positive (left) and negative (right) hadrons with the calculations of Ref. vy (upper plots) and Ref. efre (lower plots). The upper and lower curves represent the theoretical uncertainty.
• Figure 5: Sivers asymmetry: comparison between the 2002-2004 COMPASS results for positive (left) and negative (right) hadrons with the calculations of Ref. vy (upper plots) and Ref. sanse (lower plots).