The HERMES experiment

A selection of data obtained by the HERMES experiment at DESY are presented, which provide new insight into the QCD structure of the nucleon. Using longitudinally polarized lepton beams and longitudinally polarized targets, the spin-dependent structure function g1(x) has been determined for 1H, 2H and 3He. By also observing one of the hadrons produced it has been possible to extract the helicity distributions of individual quark flavours in the nucleon as well. Further information on the spin structure of the nucleon has been obtained by investigating whether the gluons are also polarized in the nucleon. In addition, first results are presented of an entirely new approach to the study of the spin structure of the nucleon by measuring the asymmetry for deep inelastic scattering of a transversely polarized target. Such data give access to the transversity distribution, which probes the spin of the nucleon while switching off the gluon contribution. Moreover, the possible role of quark orbital momentum is addressed by studying (almost) exclusive reactions, which can be interpreted in terms of the recently re-introduced generalized parton distributions (GPDs). As an example of such data measurements of both the beam-spin and beam-charge asymmetries in deeply virtual Compton scattering (DVCS) are presented. By embedding the deep inelastic scattering process in the nuclear environment, additional information can be obtained on parton propagation effects in the nuclear medium. Such information is of importance in order to better understand the hadronization process in both cold and hot nuclei, the latter being needed for the interpretation of relativistic heavy ion collisions.