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 $Q2.$