Single Spin Asymmetries on a transversely polarised proton target at COMPASS

TL;DR

The paper reports preliminary COMPASS measurements of Collins and Sivers single-spin asymmetries in SIDIS off a transversely polarized proton target. Using a 2007 NH3 target with a three-cell, multi-period setup and standard DIS kinematic cuts, the analysis extracts azimuthal modulations in hadron yields to isolate Collins and Sivers effects, and compares results with theoretical predictions. The Collins asymmetry is observed to be nonzero with opposite signs for positive and negative hadrons, while the Sivers asymmetry is consistent with zero within current statistics, challenging some previous measurements and model expectations. These results provide constraints on transversity and the Sivers function and motivate higher-statistics studies to map the proton's transverse spin structure.

Abstract

COMPASS is a running fixed-target experiment at the CERN SPS with a rich physics program focused on nucleon spin structure and on hadron spectroscopy. One of the main goals of the spin program is the measurement of the transverse spin effects in semi-inclusive DIS off transversely polarised nucleons. In the years 2002, 2003 and 2004 data have been taken using a 160 $GeV/c$ naturally polarised $μ^{+}$ beam and a deuterium target ($^{6}LiD$) transversely polarised respect to the beam direction. In 2007 the run year has been devoted to collect data with a proton ($NH_{3}$) target. The preliminary results for the Collins and Sivers asymmetries, extracted from the 2007 data with transverse target polarisation, are presented here. Results are also compared with existing model predictions.

Figures (6)

• Figure 1: Scatter plot of $Q^2$ as function of x Bjorken for the events after all the kinematic cuts. Most of the statistics is concentrated at low x Bjorken values.
• Figure 2: Collins asymmetries for positive unidentified hadrons (triangles) and for negative unidentified hadrons (circles) as function of $x$, $p^h_T$, and $z$.
• Figure 3: Collins asymmetries for positive unidentified hadrons (triangles) and for negative unidentified hadrons (circles) with the cut $x>0.05$ as function of $x$, $p^h_T$, and $z$.
• Figure 4: Compass preliminary Collins asymmetries on proton with the latest prediction of Anselmino et al, (A. Prokudin at DIS 08).
• Figure 5: Sivers asymmetries for positive unidentified hadrons (black) and for negative unidentified hadrons (circles) as function of $x$, $p^h_T$, and $z$.