First measurements of Collins and Sivers asymmetries at COMPASS

COMPASS is a fixed-target experiment on the SPS M2 beamline at CERN. Its 6 LiD target can be polarised both longitudinally and transversally with respect to the longitudinally polarised 160GeV/c µ + beam. Approximately 20% of the beam-time in 2002, 2003 and 2004 was spent in the transverse configuration, allowing the first measurement of both the Collins and Sivers asymmetries on a deuterium target. First results from the the transverse data of the COMPASS run in 2002 are reported here. 1. The Theoretical Background The cross-sections for polarised deep inelastic scattering [1] of leptons on spin-1/2 hadrons can be expressed at leading twist as a function of three independent quark distribution functions: the unpolarised function q(x), the longitudinally polarised " helicity " function ∆q(x) and the transversely polarised or " transversity " function ∆ T q(x). This latter function is chiral-odd and therefore decouples from inclusive DIS. It can however be measured in semi-inclusive DIS where it appears in combination with a chiral-odd fragmentation function, the Collins function ∆D h a (z, p h T), in an azimuthal single-spin asymmetry (SSA) in the hadronic end-product [2]. A similar asymmetry could however also arise from a modulation of the transverse momentum k T of unpolarised quarks in a transversely polarised nucleon represented by the Sivers function, ∆ T 0 q [3]. The Collins and Sivers effects can be disentangled in leptoproduction on a transversely polarised nucleon, since they exhibit a dependence on linearly independent kinematic variables. The Collins hypothesis holds that the fragmentation function of a quark of flavour a in a hadron h can be written as [4]: D h a (z, p h T) = D h a (z, p h T) + ∆D h a (z, p h T) · sinΦ C (1) where p h T is the final hadron transverse momentum with respect to the quark direction – i.e. the virtual photon direction – and z = E h /(E l − E l ′) is the fraction of available energy carried by the hadron (E h is the hadron energy, and E l and E l ′ are the incoming and scattered lepton energies respectively). The angle Φ C appearing in the fragmentation function is known as " Collins angle " and is conveniently defined in the system where the z-axis is the virtual photon direction and the x-z plane is the muon scattering plane …