Measurement of target-polarization dependent azimuthal asymmetries in SIDIS and Drell-Yan processes at COMPASS experiment

Abstract

According to QCD the quark structure of the nucleon spin within the twist-2 approximation is parameterized by eight transverse-momentum-dependent (TMD) parton distribution functions (PDFs). The PDFs describe the distributions of longitudinal and transverse momenta of partons and their correlations with nucleon and quark polarizations and can be accessed through measurement of spin (in)dependent azimuthal asymmetries in semi-inclusive DIS (SIDIS) and Drell-Yan processes. The COMPASS experiment at CERN, as part of its programme, addresses the exploration of the spin structure of the nucleon by measuring spin (in)dependent azimuthal asymmetries in semi-inclusive DIS and, recently, also in Drell-Yan processes. Between 2002 and 2011 COMPASS performed a series of SIDIS measurements, using a longitudinally polarized muon beam impinging on longitudinally and transversely polarized 6 LiD or NH 3 targets. Drell-Yan measurements with a π − beam interacting with a transversely polarized NH 3 target started with the 2015 run and will be continued in 2018. The measurement of the Sivers and other azimuthal asymmetries at practically the same hard scale in polarized SIDIS and Drell-Yan provides a unique possibility to test predicted in QCD (pseudo-)universal features of transversemomentum-dependent parton distribution functions. The main focus of this review is set on the first ever polarized Drell-Yan measurements performed by COMPASS and on the recent results obtained for target longitudinal and transverse spin dependent SIDIS asymmetries.

Semi-inclusive deep inelastic scattering in Wandzura-Wilczek-type approximation

“…for details. This asymmetry was measured at JLab [172], COMPASS [173][174][175] and HERMES [176,177] (for the latter two experiments only preliminary results are available so far). Figure 11 shows the preliminary results from the 2010 COMPASS data [166], in addition to our calculation, where we approximate the charged hadrons (70-80 % of which are π ± at COMPASS) by charged pions, see App.…”

“…10. The structure function F /F U U was studied at HERMES [178,181], COM-PASS [175,179], and JLab [127,180]. In Fig.…”

“…In Fig. 12 proton data are shown for π ± in the vs. x from HERMES [178] and COMPASS [175,179] (a, b), A sin(2φ h ) U L, y from JLab [180] (c), and from our calculation for COMPASS kinematics in the WW-type approximation (d) shown separately for reasons explained in the caption of Fig. 9.…”

“…as a function of x: preliminary COMPASS data [175,179] (a,b), and our calculation for COMPASS kinematics in WW-type approximation (c) shown separately for reasons explained in the caption of Fig. 9.…”

Working Group 6 Summary

“…The COMPASS experiment also presented the full series of longitudinal target-spin and doublespin asymmetries [30]. The asymmetries are extracted from data collected with longitudinally polarized proton and deuteron targets and a longitudinally polarized muon beam.…”