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Search Results: 1 - 10 of 12262 matches for " for the COMPASS Collaboration "
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Transversity signals at COMPASS
By COMPASS Collaboration
Physics , 2007, DOI: 10.1016/j.nuclphysbps.2007.08.088
Abstract: COMPASS is a fixed target experiment at the CERN SPS, with a rich physics program focused on nucleon spin structure and on hadron spectroscopy. One of the main goals of the spin program is the measurement of the transverse spin distribution function $\Delta_T q(x)$ in semi-inclusive DIS off transversely polarized nucleons. For this purpose approximately 20% of the running time in the years 2002 to 2004 with the longitudinally polarized muon beam of 160 GeV and with $^6$LiD polarized target was used to collect data with the target polarized transversely with respect to the beam direction. The 2002 data have been already analysed and published. We present here the preliminary results from the full statistics for the Collins and Sivers single hadron asymmetries and for the transverse spin asymmetry in hadron pair production.
Azimuthal asymmetries of charged hadrons produced by high-energy muons scattered off longitudinally polarised deuterons
COMPASS Collaboration
Physics , 2010, DOI: 10.1140/epjc/s10052-010-1461-9
Abstract: 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 phi 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 phi-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 phi-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 phi-modulation term is less pronounced than that in the corresponding HERMES data. All other dependencies of the phi-modulation amplitudes are consistent with zero within the statistical errors.
Quark helicity distributions from longitudinal spin asymmetries in muon-proton and muon-deuteron scattering
COMPASS Collaboration
Physics , 2010, DOI: 10.1016/j.physletb.2010.08.034
Abstract: Double-spin asymmetries for production of charged pions and kaons in semi-inclusive deep-inelastic muon scattering have been measured by the COMPASS experiment at CERN. The data, obtained by scattering a 160 GeV muon beam off a longitudinally polarised NH_3 target, cover a range of the Bjorken variable x between 0.004 and 0.7. A leading order evaluation of the helicity distributions for the three lightest quarks and antiquark flavours derived from these asymmetries and from our previous deuteron data is presented. The resulting values of the sea quark distributions are small and do not show any sizable dependence on x in the range of the measurements. No significant difference is observed between the strange and antistrange helicity distributions, both compatible with zero. The integrated value of the flavour asymmetry of the helicity distribution of the light-quark sea, \Delta u-bar - \Delta d-bar, is found to be slightly positive, about 1.5 standard deviations away from zero.
First Measurement of the Transverse Spin Asymmetries of the Deuteron in Semi-Inclusive Deep Inelastic Scattering
The COMPASS Collaboration
Physics , 2005, DOI: 10.1103/PhysRevLett.94.202002
Abstract: First measurements of the Collins and Sivers asymmetries of charged hadrons produced in deep-inelastic scattering of muons on a transversely polarized 6-LiD target are presented. The data were taken in 2002 with the COMPASS spectrometer using the muon beam of the CERN SPS at 160 GeV/c. The Collins asymmetry turns out to be compatible with zero, as does the measured Sivers asymmetry within the present statistical errors.
Flavour Separation of Helicity Distributions from Deep Inelastic Muon-Deuteron Scattering
COMPASS Collaboration,M. Alekseev
Physics , 2009, DOI: 10.1016/j.physletb.2009.08.065
Abstract: We present a LO evaluation of helicity densities of valence, \Delta u_v+\Delta d_v, non-strange sea, \Delta\bar{u}+\Delta\bar{d}, and strange quarks, \Delta s (assumed to be equal to \Delta\bar{s}). They have been obtained from the inclusive asymmetry A_{3,d} and the semi-inclusive asymmetries A^{\pi+}_{1,d}, A^{\pi-}_{1,d}, A^{K+}_{1,d}, A^{K-}_{1,d} measured in polarised deep inelastic muon-deuteron scattering. The full deuteron statistics of COMPASS (years 2002-2004 and 2006) has been used. The data cover the range Q^2 > 1 (GeV/c)^2 and 0.004
Transversity Signal in two Hadron Pair Production in COMPASS
H. Wollny,for the COMPASS collaboration
Physics , 2009,
Abstract: Measuring single spin asymmetries in semi-inclusive deep-inelastic scattering (SIDIS) on a transversely polarized target gives a handle to investigate the transversity distribution and transverse momentum dependent distribution functions. In the years 2002, 2003 and 2004 COMPASS took data with a transversely polarized deuteron target and in the year 2007 with a proton target. Three channels for accessing transversity have been analysed. Azimuthal asymmetries in the production of hadron pairs, involving the polarized two hadron interference fragmentation function, azimuthal asymmetries in the production of single hadrons, involving the Collins fragmentation function and polarization measurements of spin-${1/2} \hbar$ particles like $\Lambda$-Hyperons via their self analyzing weak decay. In the following we will focus on new preliminary results from the analysis of two hadron pair asymmetries measured with the proton target.
Measurement of the Longitudinal Spin Transfer to Lambda and Anti-Lambda Hyperons in Polarised Muon DIS
The COMPASS Collaboration,M. Alekseev
Physics , 2009, DOI: 10.1140/epjc/s10052-009-1143-7
Abstract: The longitudinal polarisation transfer from muons to lambda and anti-lambda hyperons, D_LL, has been studied in deep inelastic scattering off an unpolarised isoscalar target at the COMPASS experiment at CERN. The spin transfers to lambda and anti-lambda produced in the current fragmentation region exhibit different behaviours as a function of x and xF . The measured x and xF dependences of D^lambda_LL are compatible with zero, while D^anti-lambda_LL tends to increase with xF, reaching values of 0.4 - 0.5. The resulting average values are D^lambda_LL = -0.012 +- 0.047 +- 0.024 and D^anti-lambda_LL = 0.249 +- 0.056 +- 0.049. These results are discussed in the frame of recent model calculations.
COMPASS Results on Collins and Sivers Asymmetries
Andrea Bressan,for the COMPASS Collaboration
Physics , 2009,
Abstract: In the list of the main items studied by the CERN COMPASS experiment there are the transverse spin and momentum effects visible in the azimuthal distributions of hadrons produced in the deep inelastic scattering. In the years 2002-2004 COMPASS has collected data with a 6LiD target with the polarization oriented transversely with respect to the muon beam direction for about 20% of the running time; in 2007, COMPASS has used for the first time a proton NH_3 target with the data taking time equally shared between longitudinal and transverse polarization of the target. After reviewing the results obtained with the deuteron, the new results for the Collins and Sivers asymmetries of the proton will be presented.
Azimuthal asymmetries in SIDIS off unpolarized targets at COMPASS
Andrea Bressan,for the COMPASS Collaboration
Physics , 2009,
Abstract: Azimuthal asymmetries measured in unpolarized semi-inclusive deep inelastic scattering bring important information on the inner structure of the nucleons, and can be used both to estimate the average quark transverse momentum k_T and to access the so-far unmeasured Boer-Mulders functions. COMPASS results using part of the 2004 data collected with a 6LiD target and a 160 GeV positive muon beam are presented separately for positive and negative hadrons.
Measurements of unpolarised azimuthal asymmetries at COMPASS
Giulio Sbrizzai,for the COMPASS Collaboration
Physics , 2009,
Abstract: Azimuthal asymmetries in unpolarized SIDIS can be used to probe the transverse momentum of the quarks inside the nucleon. Furthermore they give access to the so-far unmeasured Boer-Mulders function. We report on the extraction of these asymmetries from the COMPASS data taken with a 160 GeV/c $\mu ^+$ beam impinging on a deuteron target. This asymmetries have been extracted separately for positive and negative hadrons, and the results have been compared with theoretical predictions.
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