A new measurement of the Collins and Sivers asymmetries on a transversely polarised deuteron target

TL;DR

This work presents high-precision measurements of Collins and Sivers asymmetries in semi-inclusive deep-inelastic scattering of muons on a transversely polarized deuteron target with the COMPASS experiment. Using a ratio-product method and extensive systematic checks, the study finds asymmetries compatible with zero across kinematic bins, indicating strong cancellation between u and d quark contributions in the isoscalar deuteron. The results, when compared with proton data and model predictions, support the presence of transversity and Collins fragmentation functions from proton measurements while constraining the Sivers function for the d quark; they also underscore the need for a global analysis combining all available data. Overall, the findings establish important benchmarks for TMD distributions and motivate future proton-target measurements and global fits to disentangle flavor-separated transversity and Sivers functions.

Abstract

New high precision measurements of the Collins and Sivers asymmetries of charged hadrons produced in deep-inelastic scattering of muons on a transversely polarised 6LiD target are presented. The data were taken in 2003 and 2004 with the COMPASS spectrometer using the muon beam of the CERN SPS at 160 GeV/c. Both the Collins and Sivers asymmetries turn out to be compatible with zero, within the present statistical errors, which are more than a factor of 2 smaller than those of the published COMPASS results from the 2002 data. The final results from the 2002, 2003 and 2004 runs are compared with naive expectations and with existing model calculations.

Figures (25)

• 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 spin of the initial and struck quarks respectively.
• Figure 2: Top view of the lay-out of the spectrometer for the COMPASS experiment in 2003. The labels and the arrows refer to the major components of the trigger and PID systems. The thin vertical lines represent the tracking detectors.
• Figure 3: General architecture of the DAQ system.
• Figure 4: Schematic view of the off-line system and reconstruction and analysis flow.
• Figure 5: $\pi \pi$ invariant mass in the $K^0$ region as a function of time from August 13 to August 19, 2004.