II – Experimental investigation of transverse spin asymmetries in μ-p SIDIS processes: Sivers asymmetries

Abstract

The COMPASS Collaboration at CERN has measured the transverse spin azimuthal asymmetry of\ud charged hadrons produced in semi-inclusive deep inelastic scattering using a 160 GeV μ+ beam and a\ud transversely polarised NH3 target. The Sivers asymmetry of the proton has been extracted in the Bjorken\ud x range 0.003 < x < 0.7. The new measurements have small statistical and systematic uncertainties of\ud a few percent and confirm with considerably better accuracy the previous COMPASS measurement. The\ud Sivers asymmetry is found to be compatible with zero for negative hadrons and positive for positive\ud hadrons, a clear indication of a spin–orbit coupling of quarks in a transversely polarised proton. As\ud compared to measurements at lower energy, a smaller Sivers asymmetry for positive hadrons is found\ud in the region x > 0.03. The asymmetry is different from zero and positive also in the low x region,\ud where sea-quarks dominate. The kinematic dependence of the asymmetry has also been investigated and\ud results are given for various intervals of hadron and virtual photon fractional energy. In contrast to the\ud case of the Collins asymmetry, the results on the Sivers asymmetry suggest a strong dependence on the\ud four-momentum transfer to the nucleon, in agreement with the most recent calculations

Sivers Effect in Dihadron Electroproduction

“…The Sivers effect is introduced as a modulation of the struck quark momentum's azimuthal angle according to the empirical parametrizations of Sivers function of Ref. [7], that were fitted to the experimental data [4,5].…”

“…In mLEPTO simulations we generated 10 11 DIS events in the kinematical region of the the COMPASS experiment [5], where E μ = 160 GeV. The following kinematic cuts were applied: Q 2 > 1 GeV 2 , 0.1 < y < 0.9, 0.03 < x < 0.7, W > 5 GeV.…”

“…3 we compare mLEPTO results for the SSAs in single hadron SIDIS production off transversely polarized proton target with the measurements by the COM-PASS collaboration [5], with the additional kinematic cuts P T > 0.1 GeV and z > 0.2 used in the experiment. This allows us to validate mLEPTO by demonstrating that it provides a good description of the data used in extracting the parametrizations of Sivers PDFs.…”

“…The corresponding SSA in one hadron SIDIS have been measured in HERMES [4], COMPASS [5], and JLab [6] experiments and allow to extract the Sivers PDF, e.g. [7].…”

Structure and spin of the nucleon

“…A wider range in Q 2 provided by the CLAS12 detector at JLab would also allow studies of the Q 2 -evolution, important for understanding and controlling possible higher-twist contributions. By using QCD evolved TMDs one can explain the observed discrepancies between HERMES [57] and COMPASS [58,59] data, and predictions have been made for the non-trivial behavior of the Sivers asymmetry as a function of Q 2 [60]. Measuring the Q 2 -dependence of the Sivers function is one of the main goals of the upgraded CLAS12 experiment using a transversely polarized HD target [1].…”