The COMPASS experiment at CERN

A new fixed target experiment is being constructed at CERN to investigate hadron structure and hadron spectroscopy, by using either muon or hadron beams. The goal of the muon programme of COMPASS is the study of semi-inclusive polarised muon scattering from a polarised target. The identification of the hadrons will allow, for the first time, a direct measurement of the gluon polarisation ΔG/G from the cross-section asymmetry of open charm production, and, more generally, a flavour analysis of the spin structure function g1(x). With a transversely polarised target the spin dependent hadron azimuthal asymmetries will be measured. These asymmetries depend on the hitherto unmeasured twist-2 distribution functions ΔTq of transverse spin and the Collins asymmetries in fragmentation. The present measurements of g1 and g2 will also be improved. (author)