1- Results from the SMC experiment at CERN. 2- The COMPASS experiment at CERN

Abstract

SMC : The SMC collaboration has measured the proton and deuteron spin structure function g 1 in the kinematical range 0.00006 < x < 0.8 and 0.02 < Q 2 < 100 GeV 2. A brief summary of the NLO pQCD global analysis of all g 1 world data available in 1998 is given. The first moments of the proton, deuteron and neutron structure function were calculated in two factorization schemes ( MS and AB). They lead to a value of a 0 close to 0.2, which corresponds to ΔΣ= 0.19 in the MS scheme, and 0.38 in the AB one. The Bjorken sum rule is tested and found to agree with the theoretical predictions at the level of 10 %. COMPASS : The first goal of the COMPASS experiment at CERN is the measurement of the contribution of the gluon polarization Δ g to the nucleon spin. The measurement will be done by scattering high energy polarized muons on a polarized proton or deuteron target. The photon gluon fusion process, which is sensitive to the gluon density distribution, will be signed by the presence of charmed particles ( D 0, D 0) or of high p t hadrons pairs of opposite charge. In addition to Δ g, the longitudinal and transversely polarized parton distributions will be measured for the different flavors. A new spectrometer with large acceptance, high rate capability and identification of particles is being commissioned.