Hermes Measurement of the Spin Structure of the Nucleon

The HERMES experiment at the DESY Laboratory in Hamburg, Germany is designed to probe the spin structure of the nucleon by using deep inelastic scattering (DIS) of longitudinally polarized positrons on longitudinally and transversely polarized targets. The HERMES spectrometer has the ability to not only identify the scattered DIS positrons but as well the hadrons in coincidence with the scattered positrons, enabling spin decomposition through avor tagging. This paper presents an overview of the HERMES experiment and gives physics results of analyses performed with the 1995 data. The HERMES experiment was designed to answer the question of how the constituents of the nucleon combine to produce its spin. The Quark-Parton Model (QPM) provides a picture of the structure of the nucleon. The nucleon consists of three valence quarks (u and d quarks). Gluons are exchanged between the valence quarks and sometimes split to produce quark-antiquark pairs (uu; dd, or ss) known as sea quarks. All of these components could contribute to the spin of the nucleon. The spin of the nucleon, S N z , can be expressed as a sum of its components where u v ; d v , and q s are the spin contributions from valence and sea quark helicities, g is the contribution from the gluon spin, and L q +L g is the component from the orbital angular momentum of the partons in the nucleon. Although g, L q , and L g are not separately gauge invariant, their sum is 1. Polarized deep inelastic experiments both at CERN and at SLAC have provided some information on the spin structure of the nucleon. Several ex