High-Statistics Measurement of Collins and Sivers Asymmetries for Transversely Polarized Deuterons

During a two-month research stay at CERN as part of the COMPASS experiment, I contributed to the study of the nucleon’s transverse spin structure using semi-inclusive deep inelastic scattering (SIDIS) of a 160 GeV/c polarized muon beam off a transversely polarized $^6{L}iD$ target. This work is part of the high-statistics 2022 COMPASS run, which aims to provide the most precise deuteron-target measurements of the Collins and Sivers asymmetries to date, thereby enabling an improved extraction of the transversity distribution and Sivers function for u− and d− quarks.
My contribution focused on the experimental analysis chain for asymmetry determination. After applying standard DIS kinematic cuts to select high-quality events, the data were binned in the kinematic variables x, z, $p_{T}$ and W , and separated for positive and negative hadrons. The asymmetries were extracted using the quadruple-ratio method, which cancels acceptance effects by exploiting the three-cell target design with regular polarization reversal. Raw Collins and Sivers amplitudes were obtained from sinusoidal fits to the azimuthal distributions of hadrons, then corrected for target polarization, dilution, and depolarization factors to yield the physical asymmetries. These results, obtained period-by-period and averaged over all stable running periods, show significantly reduced statistical uncertainties—up to a factor of three improvement over previous deuteron measurements. The high precision of this dataset is expected to strongly impact global analyses of transverse-momentum-dependent parton distribution functions (T M DP DF s), improve the determination of the nucleon tensor charge, and deepen our understanding of the three-dimensional spin structure of the nucleon.