Polarised Quark Distributions from Deep Inelastic Scattering

Polarised quark distributions are extracted from semi-inclusive deep inelastic scattering data, which include the additional detection of a leading hadron to providestatisticaìàvour-tagging'. Results are combined from both proton and`neutron' targets to isolate the contributions to the nucleon spin of the individual quark avours, including the sea quarks. Spin asymmetries in the semi-inclusive cross sections for the production of positively and negatively charged hadrons were measured in deep-inelastic scattering of polarised positrons on polarised hydrogen and 3 He targets, in the kinematic range 0:023 < x < 0:6 and 1 GeV 2 < Q 2 < 10 GeV 2. The polarised quark distributions were extracted as a function of x for up (u + u) and down (d + d) avors. In this analysis the sea polarisation is assumed to be independent of avor. The up-quark polarisation is positive and the down-quark polarisation is negative in the measured range. The understanding of the spin structure of the nu-cleon in terms of quarks and gluons has been the goal of intense investigation since it was demonstrated by EMC 1 using inclusive deep-inelastic scattering (DIS) that the net contribution of the quark spins to that of the nucleon is small. It is not possible to extract separate avor distributions from only inclusive data, but it is possible to derive rst moments of up, down and strange quark spin distributions by assuming SU(3) f avor symmetry. This kind of analysis lead to the constraint on the net quark contribution mentioned above, and also indicated that the strange quark sea seems to be negatively polarised 14. Semi-inclusive polarised deep-inelastic scattering experiments are able to measure the separate spin contributions q f of quark and antiquark avors f to the total spin of the nucleon, as a function of the Bjorken scaling variable x. They are also able to measure the sea polarisation directly and to test SU(3) f symmetry. Hadron production in DIS is described by the absorption of a virtual photon by a point-like quark and the fragmentation of this quark into a hadronic nal state. The two processes are characterized by two independent functions: the quark distribution function q f (x; Q 2), and the fragmentation function D h f (z; Q 2). The semi-inclusive DIS cross section h (x; Q 2 ; z) to produce a hadron of type h with energy fraction z = E h == is given The sum …