Measurement of single-spin azimuthal asymmetries in semi-inclusive electroproduction of pions and kaons on a longitudinally polarised deuterium target HERMES Collaboration

Single-spin asymmetries have been measured for semi-inclusive electroproduction of π+, π−, π0 and K+ mesons in deep-inelastic scattering off a longitudinally polarised deuterium target. The asymmetries appear in the distribution of the hadrons in the azimuthal angle φ around the virtual photon direction, relative to the lepton scattering plane. The corresponding analysing powers in the sinφ moment of the cross section are 0.012 ± 0.002(stat.) ± 0.002(syst.) for π+, 0.006 ± 0.003(stat.)± 0.002(syst.) for π−, 0.021 ± 0.005(stat.)± 0.003(syst.) for π0 and 0.013 ± 0.006(stat.)± 0.003(syst.) for K+. The sin 2φ moments are compatible with zero for all particles.  2003 Elsevier Science B.V. All rights reserved. 184 HERMES Collaboration / Physics Letters B 562 (2003) 182–192 PACS: 13.87.Fh; 13.60.-r; 13.88.+e; 14.20.Dh Deep-inelastic lepton scattering (DIS) on polarised nucleons has provided much of our present understanding of the spin structure of the nucleon. Recently, measurements of single-spin azimuthal asymmetries have been recognised as a powerful source of information about the spin structure of the nucleon [1], complementary to inclusive deep-inelastic scattering. Significant azimuthal target-spin asymmetries in electroproduction of π+ and π0 mesons on a longitudinally polarised hydrogen target have been reported in Refs. [2,3]. Evidence for azimuthal asymmetries of pions has also been reported for deep-inelastic lepton scattering off transversely polarised protons [4]. It has been suggested [5] that these single-spin asymmetries may provide information on the transversity distribution, which describes in a transverse polarisation basis the probability to find a quark with its spin parallel or antiparallel to the spin of the nucleon that is polarised transversely to its (infinite) momentum [6–8]. Transversity is a chiral-odd distribution function, which implies that it is not observable in an inclusive measurement, because chirality is conserved in electromagnetic and strong interactions in the limit of massless on-shell quarks. Therefore, a second chiral-odd object has to be involved in the process [9,10]. In semi-inclusive scattering this can be a chiral-odd fragmentation function—for example, the Collins function [5]. The HERMES results on target single-spin asymmetries [2,3] have elicited a number of phenomenological studies to evaluate these asymmetries in the framework of the Collins mechanism using various models as input for the chiral-odd distribution and fragmentation functions [11–21]. Theoretical predictions have also been made for single-spin asymmetries in DIS off the nucleons in a deuterium target [21,22]. Recently, it has been shown that another mechanism can also cause a single-spin azimuthal asymmetry in semi-inclusive deep-inelastic scattering [23]. In this case, the observed asymmetry is attributed to the interaction of the struck quark with the target remnant * Corresponding author. E-mail address: [email protected] (D. Ryckbosch). through the exchange of a single gluon. This mechanism was shown to be identical [24] to the Sivers effect known already for a long time [25], involving a chiral-even time-odd distribution function. Other theoretical studies [26–29] have revealed that factorisation applies to this process, which leads to gauge-invariant momentum dependent parton distributions. In the case of a longitudinally polarised target the Collins and the Sivers mechanism cannot be distinguished. However, for the two mechanisms a different kinematic dependence on the fractional energy z of the hadron has been predicted [29]. This Letter reports the first observation of targetspin azimuthal asymmetries for semi-inclusive pion and kaon production on a longitudinally polarised deuterium target. The data were recorded during the 1998, 1999 and 2000 running periods of the HERMES experiment. The experiment was performed with a beam of 27.6 GeV polarised electrons/positrons from the HERA storage ring at DESY and polarised nucleons in a deuterium gas target. The average target polarisation was 0.84 with a fractional uncertainty of 5%. The data were taken with an electron beam in 1998 and with a positron beam in 1999 and 2000. The measured single-spin asymmetries show no dependence on the beam charge. Therefore, all datasets were combined. In the following, electrons and positrons will be jointly referred to as positrons. The process considered is the production of a pseudoscalar meson (m = π or K) in deep-inelastic positron scattering off a longitudinally polarised deuterium target: