[en] COMPASS (COmmon Muon and Proton Apparatus for Structure and Spectroscopy) is a fixed target experiment at CERN dedicated to studies of the spin structure of the nucleon and of the spectroscopy of hadrons. During the years 2002-2004, 2006-2007 and 2010-2011 the COMPASS collaboration has collected a large amount of data by scattering polarized 160(200) GeV/c muons on polarized ⁶LiD and NH₃ targets. The COMPASS results on quark and gluon helicities are discussed, as well as results on transverse spin and transverse momentum effects in semi-inclusive deeply inelastic scattering

[en] COmmon Muon and Proton Apparatus for Structure and Spectroscopy (COMPASS) is a fixed target experiment at CERN dedicated to studies of the spin structure of the nucleon and of the spectroscopy of hadrons. During the years 2002–2004 and 2006–2007, the COMPASS collaboration has collected a large amount of data by scattering polarized 160 GeV/c muons on polarized ⁶LiD and NH₃ targets. These data were used to evaluate the gluon contribution to the nucleon spin. The gluon polarization (Δg)/g was directly measured from the cross-section helicity asymmetry of D⁰ mesons production in the photon-gluon fusion reaction. During 2008 and 2009, the world leading data sets were collected with hadron beams which are currently being analyzed using Partial Wave Analysis (PWA) technique. COMPASS is performing a search for J^PC-exotic mesons, glueballs and hybrids, through light hadron spectroscopy in high energy 190 GeV/c hadron-proton reactions using both centrally produced and diffractive events. Preliminary results from these searches are discussed

[en] The study of exclusive reactions like Deeply Virtual Compton Scattering (DVCS) and Deeply Virtual Meson Production (DVMP) is one major part of the future COMPASS program to investigate nucleon structure through Generalized Parton Distributions (GPD). The high energy of the muon beam allows to measure the x_Bj-dependence of the t-slope of the DVCS cross section and to study nucleon tomography. The use of positive and negative polarized muon beams allows to determine the Beam Charge and Spin Difference of the DVCS cross sections to access the real part of the Compton form factor related to the dominant GPD H. As a second phase a transversely polarized proton target will be used to collect data to constrain the GPD E. In preparation of the future measurements two DVCS test runs were performed in 2008 and 2009.

[en] The strange quark helicity distribution Δs(x) was derived at LO from the inclusive asymmetry A_a,d and the semi-inclusive asymmetries A_1,d^π+, A_1,d^π-, A_1,d^K+, A_1,d^K-, measured by COMPASS in polarised deep inelastic muon-deuteron scattering. The distribution of Δs(x) is compatible with zero in the whole measured range. The value of the first moment of Δs and its error are very sensitive to the assumed value of the ratio of the s-bar-quark to u-quark fragmentation functions into positive kaons ∫D(K+/s)(z)dz/∫D_u^K+(z)dz.

[en] COMPASS (COmmon Muon and Proton Apparatus for Structure and Spectroscopy) is a fixed target experiment at CERN dedicated to studies of the spin structure of the nucleon and of the spectroscopy of hadrons. During the years 2002-2004 and 2006-2007, the COMPASS collaboration has collected a large amount of data by scattering polarized 160 GeV/c muons on polarized ⁶LiD and NH₃ targets. These data were used to evaluate the gluon contribution to the nucleon spin. The gluon polarization was directly measured from the cross-section helicity asymmetry of D⁰ mesons production in the photon-gluon fusion reaction. In 2008 COMPASS will perform a search for J^PC-exotic mesons, glueballs or hybrids, through light hadron spectroscopy in high energy (190 GeV/c π^-) pion-proton reactions using both centrally produced and diffractive events. Preliminary results from diffractive pion dissociation into a π^-π^-π⁺ final state obtained in 2004 are also discussed.

[en] Single photons detected by the DELPHI experiment at LEP2 in the years 1997-2000 are reanalysed to investigate the existence of a single extra dimension in a modified ADD scenario with slightly warped large extra dimensions. The data collected at centre-of-mass energies between 180 and 209 GeV for an integrated luminosity of ∝650 pb^-1 agree with the predictions of the Standard Model and allow a limit to be set on graviton emission in one large extra dimension. The limit obtained on the fundamental mass scale M_D is 1.69 TeV/c² at 95% CL, with an expected limit of 1.71 TeV/c². (orig.)

[en] The COMPASS Collaboration at CERN has investigated the reaction π^-γ → π^-π^-π⁺ embedded in the Primakoff reaction of 190 GeV pions scattering in the Coulomb field of a lead target, π^-Pb → π^-π^-π⁺ Pb. Exchange of quasi-real photons is selected by isolating the sharp Coulomb peak observed at momentum transfer below 0.001 (GeV/c)². Using a partial-wave analysis the amplitudes and relative phases of the a₂(1320) and π₂(1670) mesons have been extracted, and the Coulomb and the diffractive contributions have been disentangled. Measuring absolute production cross sections we have determined the radiative width of the a₂(1320) to be Γ₀(a₂(1320) → πγ) = (358 ± 6_stat ± 42_syst) keV. As the first measurement, Γ₀(π₂(1670) → πγ) = (181 ± 11_stat ± 27_syst) keV . (BR^PDG_f2π/BR_f2π) is obtained for the radiative width of the π₂(1670), where in this analysis the branching ratio BR^PDG_f2π = 0.56 has been used. We compare these values to previous measurements and theoretical predictions. (orig.)

[en] Large samples of Λ, Σ(1385) and Ξ(1321) hyperons produced in the deep-inelastic muon scattering off a ⁶LiD target were collected with the COMPASS experimental setup at CERN. The relative yields of Σ(1385)⁺, Σ(1385)^-, anti Σ(1385)^-, anti Σ(1385)⁺, Ξ(1321)^-, and anti Ξ(1321)⁺ hyperons decaying into Λ(anti Λ)π were measured. The ratios of heavy-hyperon to Λ and heavy-antihyperon to anti Λ were found to be in the range 3.8 % to 5.6 % with a relative uncertainty of about 10 %. They were used to tune the parameters relevant for strange particle production of the LEPTO Monte Carlo generator. (orig.)

[en] Multiplicities of charged hadrons produced in deep inelastic muon scattering off a ⁶LiD target have been measured as a function of the DIS variables x_Bj, Q², W² and the final state hadron variables p_T and z. The p_T² distributions are fitted with a single exponential function at low values of p_T² to determine the dependence of left angle p_T² right angle on x_Bj, Q², W² and z. The z-dependence of left angle p_T² right angle is shown to be a potential tool to extract the average intrinsic transverse momentum squared of partons, left angle k _{perpendicularto}² right angle, as a function of x_Bj and Q² in a leading order QCD parton model. (orig.)

[en] Azimuthal asymmetries in semi-inclusive production of positive (h⁺) and negative hadrons (h^-) have been measured by scattering 160 GeV muons off longitudinally polarised deuterons at CERN. The asymmetries were decomposed in several terms according to their expected modulation in the azimuthal angle φ of the outgoing hadron. Each term receives contributions from one or several spin and transverse-momentum-dependent parton distribution and fragmentation functions. The amplitudes of all φ-modulation terms of the hadron asymmetries integrated over the kinematic variables are found to be consistent with zero within statistical errors, while the constant terms are nonzero and equal for h ⁺ and h ^- within the statistical errors. The dependencies of the φ-modulated terms versus the Bjorken momentum fraction x, the hadron fractional momentum z, and the hadron transverse momentum p_h^T were studied. The x dependence of the constant terms for both positive and negative hadrons is in agreement with the longitudinal double-spin hadron asymmetries, measured in semi-inclusive deep-inelastic scattering. The x dependence of the sin φ-modulation term is less pronounced than that in the corresponding HERMES data. All other dependencies of the φ-modulation amplitudes are consistent with zero within the statistical errors. (orig.)