Spin asymmetries in deep-inelastic electron-nucleon scatering - selected HERMES results

Abstract

The HERMES collaboration has collected many millions of deep-inelastic lepton-nucleon scattering events during the years 1995–2000, i. e. the first phase of the experiment. Longitudinally polarised electron or positron beams in the HERA electron storage ring were incident on longitudinally polarised internal atomic gas targets as well as several nuclear gas targets. The primary goal of the HERMES experiment is the study of the spin structure of the nucleon. High precision measurements of double-spin asymmetries in inclusive and semi-inclusive scattering from undiluted polarised atomic hydrogen, deuterium and 3He gas targets are presented. These data represent the world's most precise experimental determination to date of the separate contributions of the spin of up, down and strange quarks to the spin of the nucleon, and the first direct indication of a positive gluon polarisation. The observation of single-spin asymmetries in the azimuthal distributions of hadrons in semi-inclusive deep-inelastic scattering from longitudinally polarised targets is also presented. These single-spin asymmetries can be related to transversity, the only unmeasured of the three leading twist parton distribution functions. Moreover, first measurements of single-spin asymmetries in hard exclusive production of real photons and of pions are reported. Such data can be interpreted in the framework of generalised parton distributions, which is also used when discussing new data on hard exclusive electro-production of the vector mesons ϱ, ω and φ. Finally some results from measurement of longitudinal spin transfer in electro-production of Λ hyperons, and of transverse Λ polarisation in quasi-real photo-production are discussed.