Spin Physics with COMPASS

COMPASS is a fixed target experiment at CERN studying the spin structure of the nucleon via deep inelastic muon-nucleon scattering. Hadron spectroscopy and structure are investigated using hadronic beams. The main goal of the muon program is the determination of the gluon helicity contribution, ∆G/G, to the nucleon spin. Experimentally this quantity is accessible via the measurement of double spin asymmetries in two different processes: The first one is the production of hadron pairs with large transverse momentum. The second one is open charm production which provides the cleanest and most direct measurement. The first method has a higher statistical accuracy but is affected by larger systematic uncertainties due to background processes. First results from COMPASS indicate that the helicity contribution of the gluons to the nucleon spin is small. In parallel to the ∆G measurement longitudinal inclusive and semi-inclusive asymmetries are studied as well. These allow a flavor decomposition of the helicity contribution, ∆Σ, of the quarks to the nucleon spin. The spin physics program of COMPASS also includes measurements of single spin asymmetries on a transversely polarized target, giving access to transverse quark distributions in the nucleon. Recent results of the muon program from the data taking periods 2002-2003 will be presented.