Gluon contribution to the Sivers effect. COMPASS results on deuteron target

National Centre for Nuclear Research, Warsaw

^a e-mail: adam.szabelski@cern.ch

Published online: 23 January 2015

Abstract

Sivers effect for gluons is connected to gluon orbital angular momentum which may be the missing part of the nucleon spin puzzle. We present a method of extraction of Sivers effect for gluons from COMPASS SIDIS data on transversely polarised target. In order to access the Sivers effect for gluons photon-gluon fusion (PGF) process is used. To enhance the fraction of PGF in the sample high-p_T hadron pair events are selected. The method is based on a assumption that there are 3 processes contributing to the muon-nucleon scattering: PGF, leading process and QCD Compton process. Then one performs a weighting procedure which enables to extract the asymmetries for the 3 contributing processes simultaneously. In order to do that a neural network trained by a Monte Carlo to assign to each event 3 probabilities corresponding to the 3 processes is needed. Finaly we show results of Sivers effect for gluons extraction on COMPASS data with transversely polarised deuteron target. A_PGF^{sinΦ_2h–Φ_S} = −0.14 ± 0.15 (stat.) at ‹X_G› = 0.126.