Single Spin Asymmetries on a Transversely Polarised Proton Target at Compass

COMPASS is a running fixed-target experiment at the CERN SPS with a rich physics program focused on nucleon spin structure and on hadron spectroscopy. One of the main goals of the spin program is the measurement of the transverse spin effects in semi-inclusive DIS off transversely polarised nucleons. In the years 2002, 2003 and 2004 data have been taken using a 160 GeV /c naturally polarised µ + beam and a deuterium target (6 LiD) transversely polarised respect to the beam direction. In 2007 the run year has been devoted to collect data with a proton (N H 3) target. The preliminary results for the Collins and Sivers asymmetries, extracted from the 2007 data with transverse target po-larisation, are presented here. Results are also compared with existing model predictions. The cross-section for polarised deep inelastic scattering 1 of leptons off spin 1/2 hadrons can be expressed, at leading twist, as a function of three independent quark distribution functions: q(x), ∆q(x) and ∆ T q(x). The latter, which describes the probability of finding a quark with spin parallel to the nucleon spin in a transversely polarised nucleon, is chiral-odd and can be measured in transversely polarised nucleon (anti)-nucleon hard scattering or in semi-inclusive deep inelastic scattering (SIDIS) on a transversely po-larised target. In SIDIS ∆ T q(x) can be measured in combination with the chiral odd Collins fragmentation function ∆ T 0 D h q , via azimuthal single spin asymmetries (SSA) in single hadron production. According to Collins, 2 the fragmentation of a transversely polarised quark in unpolarised hadrons presents