37 research outputs found
Nucleon structure studies with the COMPASS experiment at CERN
No full textThe COMPASS experiment at CERN uses hadron and lepton beams for nucleon structure studies. Most of the data collected so far with a muon beam and either proton or deuteron polarised target were analysed in terms of longitudinally or transversely polarised parton distribution functions and transverse momentum-dependent distributions in the nucleon. A negative hadron beam is used to perform Drell-Yan measurements. The COMPASS large polarised target gives access to several momentum-dependent singlespin asymmetries and provides a stringent test of the fundamental QCD factorisation assumptions. With positive and negative muon beams COMPASS also studies Generalised Parton Distributions using exclusive deeply virtual Compton scattering and meson production experiments. An overview of the most recent COMPASS results is given. The expected physics outcome of the forthcoming measurements is discussed
Nucleon Spin And Structure Studies With COMPASS
No full textThe COMPASS experiment at CERN investigates nucleon structure and spectroscopy with high-intensity muon and hadron beams. Between 2002 and 2004 COMPASS has mainly concentrated on the spin structure of the nucleon via deep-inelastic scattering of polarized muons on polarized nucleons. First results include measurements of the gluon contribution to the nucleon spin via charm production and high- pairs, A polarization, vector-meson production, Collins and Sivers asymmetries, inclusive and semi-inclusive deep-inelastic scattering asymmetries and a search for pentaquark states. A short overview of these results is presented
Determination of the gluon contribution to the nucleon spin with COMPASS
No full textNew double-spin asymmetry data from the COMPASS experiment at CERN are presented. The data are used to determine the polarized gluon distribution , via three different techniques. Next-to-leading order QCD fits to the nucleon polarized structure functions indicate that the first moment of is small (either positive or negative) and compatible with zero. This observation is corroborated by three measurements of $\Delta G/G(x), based on the photon-gluon fusion process
Longitudinal spin transfer to and anti- hyperons
No full textThe polarisations of and hyperons produced in deep inelastic scattering of 160 polarised muons off unpolarised deuterons were studied at the COMPASS experiment at CERN. The longitudinal spin transfer was measured for both hyperons, as a function of the Bjorken and the Feynman variables. The spin transfer to the is small and compatible with zero for both and . In contrast, the spin transfer to is positive for both variables, and shows a steep increase for large . Comparison with theoretical predictions indicates that the polarisation of the hyperon is sensitive to the strange anti-quark distribution in the target nucleon
Initial-state energy loss in cold QCD matter and the Drell-Yan process
No full textInternational audienceThe effects of parton energy loss in nuclear matter on the Drell-Yan process in pA and πA collisions at fixed-target energies are investigated. Calculations are based on the Baier-Dokshitzer-Mueller-Peigné-Schiff (BDMPS) framework embedded in a next-to-leading order calculation, using the transport coefficient extracted from J/ψ measurements. Model calculations prove in good agreement with preliminary measurements by the E906 experiment, despite a slightly different magnitude, supporting a consistent picture between Drell-Yan and J/ψ data. Predictions for the COMPASS future measurements in πA collisions at GeV are also performed. At higher collision energy ( GeV), Drell-Yan measurements are only slightly affected by energy loss effects. On the contrary, the E906 results turn out in clear disagreement with nuclear PDF effects alone. The comparison of E772, E866, and E906 measurements indicates for the first time a clear violation of QCD factorization in Drell-Yan production in pA collisions
