42 research outputs found

    First Measurement of Chiral Dynamics in pi(-)gamma -> pi(-)pi(-)pi(+)

    No full text
    Adolph C, Alekseev MG, Alexakhin VY, et al. First Measurement of Chiral Dynamics in pi(-)gamma -&gt; pi(-)pi(-)pi(+). Physical Review Letters. 2012;108(19): 192001.The COMPASS Collaboration at CERN has investigated the pi(-)gamma -> pi(-)pi(-)pi(+) reaction at center-of-momentum energy below five pion masses, root s < 5m(pi), embedded in the Primakoff reaction of 190 GeV pions impinging on a lead target. Exchange of quasireal photons is selected by isolating the sharp Coulomb peak observed at smallest momentum transfers, t' < 0.001 GeV2/c(2). Using partial-wave analysis techniques, the scattering intensity of Coulomb production described in terms of chiral dynamics and its dependence on the 3 pi-invariant mass m(3 pi) = root s were extracted. The absolute cross section was determined in seven bins of root s with an overall precision of 20%. At leading order, the result is found to be in good agreement with the prediction of chiral perturbation theory over the whole energy range investigated

    Exclusive rho(0) muoproduction on transversely polarised protons and deuterons

    No full text
    Adolph C, Alekseev MG, Alexakhin VY, et al. Exclusive rho(0) muoproduction on transversely polarised protons and deuterons. Nuclear Physics B. 2012;865(1):1-20.The transverse target spin azimuthal asymmetry A(UT)(sin(phi-phi s)) in hard exclusive production of rho(0) mesons was measured at COMPASS by scattering 160 GeV/c muons off transversely polarised protons and deuterons. The measured asymmetry is sensitive to the nucleon helicity-flip generalised parton distributions E-q, which are related to the orbital angular momentum of quarks in the nucleon. The Q(2), x-B-j and p(T)(2) dependence of A(UT)(sin(phi-phi s)) is presented in a wide kinematic range: 1 (GeV/c)(2) < Q(2) < 10 (GeV/c)(2), 0.003 < xB(j) < 0.3 and 0.05 (GeV/c)(2) < p(T)(2) < 0.5 (GeV/c)(2) for protons or 0.10 (GeV/c)(2) < p(T)(2) < 0.5 (GeV/c)(2) for deuterons. Results for deuterons are obtained for the first time. The measured asymmetry is small in the whole kinematic range for both protons and deuterons, which is consistent with the theoretical interpretation that contributions from GPDs E-u and E-d approximately cancel. (C) 2012 Elsevier B.V. All rights reserved

    Flavour separation of helicity distributions from deep inelastic muon-deuteron scattering

    No full text
    Alekseev M, Alexakhin VY, Alexandrov Y, et al. Flavour separation of helicity distributions from deep inelastic muon-deuteron scattering. Phys.Lett. B. 2009;680(3):217-224.We present a LO evaluation of helicity densities of valence, Delta u(v) + Delta d(v), non-strange sea, Delta(u) over bar + Delta(d) over bar, and strange quarks, Delta s (assumed to be equal to Delta(s) over bar). They have been obtained from the inclusive asymmetry A(1,d) and the semi-inclusive asymmetries A(1,d)(pi+), A(1,d)(pi-), A(1,d)(K+), A(1,d)(K-) 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 < x < 0.3. Both non-strange densities are found to be in a good agreement with previous measurements. The distribution of Delta s(x) is compatible with zero in the whole measured range, in contrast to the shape of the strange quark helicity distribution obtained in most LO and NLO QCD fits. The sensitivity of the values of Delta s(x) upon the choice of fragmentation functions used in the derivation is discussed. (C) 2009 Elsevier B.V. All rights reserved

    The Polarised Valence Quark Distribution from semi-inclusive DIS

    No full text
    Alekseev M, Alexakhin VY, Alexandrov Y, et al. The Polarised Valence Quark Distribution from semi-inclusive DIS. Phys.Lett. B. 2008;660(5):458-465.The semi-inclusive difference asymmetry A^{h^{+}-h^{-}} for hadrons of opposite charge has been measured by the COMPASS experiment at CERN. The data were collected in the years 2002-2004 using a 160 GeV polarised muon beam scattered off a large polarised ^6LiD target and cover the range 0.006 < x < 0.7 and 1 < Q^2 < 100 (GeV/c)^2. In leading order QCD (LO) the asymmetry A_d^{h^{+}-h^{-}} measures the valence quark polarisation and provides an evaluation of the first moment of Delta u_v + Delta d_v which is found to be equal to 0.40 +- 0.07 (stat.) +- 0.05 (syst.) over the measured range of x at Q^2 = 10 (GeV/c)^2. When combined with the first moment of g_1^d previously measured on the same data, this result favours a non-symmetric polarisation of light quarks Delta u-bar = - Delta d-bar at a confidence level of two standard deviations, in contrast to the often assumed symmetric scenario Delta u-bar = Delta d-bar = Delta s-bar = Delta s

    Resonance production and pi pi S-wave in pi(-) + p -&gt; pi(-) pi(-) pi(+) + p(recoil) at 190 GeV/c

    No full text
    The COMPASS collaboration has collected the currently largest data set on diffractively produced pi(-) pi(-) pi(+) final states using a negative pion beam of 190 GeV/c momentum impinging on a stationary proton target. This data set allows for a systematic partial-wave analysis in 100 bins of three-pion mass, 0.5 &lt; m(3 pi) &lt; 2.5 GeV/c(2), and in 11 bins of the reduced four-momentum transfer squared, 0.1 &lt; t' &lt; 1.0 (GeV/c)(2). This two-dimensional analysis offers sensitivity to genuine one-step resonance production, i.e. the production of a state followed by its decay, as well as to more complex dynamical effects in nonresonant 3 pi production. In this paper, we present detailed studies on selected 3p partial waves with J(PC) = 0(-+) ,1(++) ,2(-+) ,2(++) ,and 4(++). In these waves, we observe the well-known groundstate mesons as well as a new narrow axial-vector meson a(1)(1420) decaying into f(0) (980)pi. In addition, we present the results of a novel method to extract the amplitude of the pi(-)pi(+) subsystem with I(G)J(PC) = 0(+)0(++) in various partial waves from the pi(-)pi(-)pi(+) data. Evidence is found for correlation of the f (0)(980) and f(0)(1500) appearing as intermediate pi(-)pi(+) isobars in the decay of the known pi(1800) and pi(2)(1880)

    Sivers asymmetry extracted in SIDIS at the hard scales of the Drell–Yan process at COMPASS

    No full text
    Eight proton transverse-spin-dependent azimuthal asymmetries are extracted in four regions of the photon virtuality Q2 from the COMPASS 2010 semi-inclusive hadron measurements in deep inelastic muon–nucleon scattering. These Q2 regions correspond to the four regions of the di-muon mass View the MathML source used in the ongoing analyses of the COMPASS Drell–Yan measurements, which allows for a future direct comparison of the nucleon transverse-momentum-dependent parton distribution functions extracted from these two alternative measurements. In addition, for the azimuthal asymmetries induced by the Sivers transverse-momentum-dependent parton distribution function various two-dimensional kinematic dependences are presented. The integrated Sivers asymmetries are found to be positive with an accuracy that appears to be sufficient to test the sign change of the Sivers function predicted by Quantum Chromodynamics

    Multiplicities of charged kaons from deep-inelastic muon scattering off an isoscalar target

    No full text
    Precise measurements of charged-kaon multiplicities in deep inelastic scattering were performed. The results are presented in three-dimensional bins of the Bjorken scaling variable x, the relative virtual-photon energy y, and the fraction z of the virtual-photon energy carried by the produced hadron. The data were obtained by the COMPASS Collaboration by scattering 160 GeV muons off an isoscalar (LiD)-Li-6 target. They cover the kinematic domain 1 (GeV/c)(2) &lt; Q(2) &lt; 60 (GeV/c)(2) in the photon virtuality, 0.004 &lt; x &lt; 0.4, 0.1 &lt; y &lt; 0.7, 0.20 &lt; z &lt; 0.85, and W &gt; 5 GeV/c(2) in the invariant mass of the hadronic system. The results from the sum of the z-integrated K+ and K- multiplicities at high x point to a value of the non-strange quark fragmentation function larger than obtained by the earlier DSS fit

    Leading and Next-to-Leading Order Gluon Polarisation in the Nucleon and Longitudinal Double Spin Asymmetries from Open Charm Muoproduction

    No full text
    Adolph C, Alekseev MG, Alexakhin VY, et al. Leading and Next-to-Leading Order Gluon Polarisation in the Nucleon and Longitudinal Double Spin Asymmetries from Open Charm Muoproduction. Physical Review D. 2013;87(5): 52018.The gluon polarisation in the nucleon was measured using open charmproduction by scattering 160 GeV/c polarised muons off longitudinally polarisedprotons or deuterons. The data were taken by the COMPASS collaboration between2002 and 2007. A detailed account is given of the analysis method that includesthe application of neural networks. Several decay channels of D^0 mesons areinvestigated. Longitudinal spin asymmetries of the D meson productioncross-sections are extracted in bins of D^0 transverse momentum and energy. Atleading order QCD accuracy the average gluon polarisation is determined as(Delta g/G)^LO=-0.06 +/- 0.21 (stat.) +/- 0.08 (syst.) at the scale ~13(GeV/c)^2 and an average gluon momentum fraction ~ 0.11. For the first time,the average gluon polarisation is also obtained at next-to-leading order QCDaccuracy as (Delta g/G)^NLO = -0.13 +/- 0.15 (stat.) +/- 0.15 (syst.) at thescale ~ 13 (\GeV/c)^2 and ~ 0.20

    Transverse target spin asymmetries in exclusive ρ0ρ^0 muoproduction

    No full text
    Adolph C, Alekseev MG, Alexakhin VY, et al. Transverse target spin asymmetries in exclusive ρ0ρ^0 muoproduction. Physics Letters B. 2014;731:19-26.Exclusive production of ρ0\rho^0 mesons was studied at the COMPASS experimentby scattering 160 GeV/cc muons off transversely polarised protons. Fivesingle-spin and three double-spin azimuthal asymmetries were measured as afunction of Q2Q^2, xBjx_{Bj}, or pT2p_{T}^{2}. The sinϕS\sin \phi_S asymmetry isfound to be 0.019±0.008(stat.)±0.003(syst.)-0.019 \pm 0.008(stat.) \pm 0.003(syst.). All other asymmetriesare also found to be of small magnitude and consistent with zero withinexperimental uncertainties. Very recent calculations using a GPD-based modelagree well with the present results. The data is interpreted as evidence forthe existence of chiral-odd, transverse generalized parton distributions

    Hadron Transverse Momentum Distributions in Muon Deep Inelastic Scattering at 160 GeV/cc

    No full text
    Adolph C, Alekseev MG, Alexakhin VY, et al. Hadron Transverse Momentum Distributions in Muon Deep Inelastic Scattering at 160 GeV/cc. European Physical Journal C. 2013;73(8): 2531.Multiplicities of charged hadrons produced in deep inelastic muon scatteringoff a 6^6LiD target have been measured as a function of the DIS variablesxBjx_{Bj}, Q2Q^2, W2W^2 and the final state hadron variables pTp_T and zz. ThepT2p_T^2 distributions are fitted with a single exponential function at lowvalues of pT2p_T^2 to determine the dependence of pT2\langle p_T^2 \rangle onxBjx_{Bj}, Q2Q^2, W2W^2 and zz. The zz-dependence of pT2\langle p_T^2 \rangleis shown to be a potential tool to extract the average intrinsic transversemomentum squared of partons, k2\langle k_{\perp}^2 \rangle, as a function ofxBjx_{Bj} and Q2Q^2 in a leading order QCD parton model
    corecore