The HERMES Experiment

The study of the spin structure of the nucleon is a fundamental problem in strong interaction physics. The HERMES experiment at DESY, Hamburg, Germany is carrying out measurements to probe the spin structure of the nucleon using a new technique. Polarized internal gas targets of hydrogen, deuterium, and 3He are used with the 27.5 GeV longitudinally polarized positron (or electron) beam of the HERA collider to measure both inclusive and semi-inclusive spin-dependent deep-inelastic scattering from the nucleon. In addition, HERMES has observed a negative spin asymmetry in the photoproduction of hadron pairs with high transverse momenta. This is interpreted as the first direct experimental evidence for a positive gluon polarization in the nucleon. The azimuthal single-spin asymmetry measured in semi-inclusive pion production at HERMES is presented and interpreted as an effect of a new T-odd fragmentation function. HERMES also has carried out precision measurements of the ratios of unpolarized nuclear cross-sections. The data indicate a sizable nuclear dependence in the ratio of longitudinal to transverse cross-sections at low Q2.