820 research outputs found

    Interplay among transversity induced asymmetries in hadron leptoproduction

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    In the fragmentation of a transversely polarized quark several left-right asymmetries are possible for the hadrons in the jet. When only one unpolarized hadron is selected, it exhibits an azimuthal modulation known as Collins effect. When a pair of oppositely charged hadrons is observed, three asymmetries can be considered, a di-hadron asymmetry and two single hadron asymmetries. In lepton deep inelastic scattering on transversely polarized nucleons all these asymmetries are coupled with the transversity distribution. From the high statistics COMPASS data on oppositely charged hadron-pair production we have investigated for the first time the dependence of these three asymmetries on the difference of the azimuthal angles of the two hadrons. The similarity of transversity induced single and di-hadron asymmetries is discussed. A new analysis of the data allows to establish quantitative relationships among them, providing for the first time strong experimental indication that the underlying fragmentation mechanisms are all driven by a common physical process.Comment: 6 figure

    Longitudinal double spin asymmetries in single hadron quasi-real photoproduction at high pTp_T

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    We measured the longitudinal double spin asymmetries ALLA_{LL} for single hadron muo-production off protons and deuterons at photon virtuality Q2Q^2 < 1(GeV/c\it c)2^2 for transverse hadron momenta pTp_T in the range 0.7 GeV/c\it c to 4 GeV/c\it c . They were determined using COMPASS data taken with a polarised muon beam of 160 GeV/c\it c or 200 GeV/c\it c impinging on polarised 6LiD\mathrm{{}^6LiD} or NH3\mathrm{NH_3} targets. The experimental asymmetries are compared to next-to-leading order pQCD calculations, and are sensitive to the gluon polarisation őĒG\Delta G inside the nucleon in the range of the nucleon momentum fraction carried by gluons 0.05<xg<0.20.05 < x_g < 0.2

    Interplay among transversity induced asymmetries in hadron leptoproduction

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    In the fragmentation of a transversely polarized quark several left-right asymmetries are possible for the hadrons in the jet. When only one unpolarized hadron is selected, it exhibits an azimuthal modulation known as Collins effect. When a pair of oppositely charged hadrons is observed, three asymmetries can be considered, a di-hadron asymmetry and two single hadron asymmetries. In lepton deep inelastic scattering on transversely polarized nucleons all these asymmetries are coupled with the transversity distribution. From the high statistics COMPASS data on oppositely charged hadron-pair production we have investigated for the first time the dependence of these three asymmetries on the difference of the azimuthal angles of the two hadrons. The similarity of transversity induced single and di-hadron asymmetries is discussed. A new analysis of the data allows to establish quantitative relationships among them, providing for the first time strong experimental indication that the underlying fragmentation mechanisms are all driven by a common physical process.Comment: 6 figure

    Collins and Sivers asymmetries in muonproduction of pions and kaons off transversely polarised proton

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    Measurements of the Collins and Sivers asymmetries for charged pions and charged and neutral kaons produced in semi-inclusive deep-inelastic scattering of high energy muons off transversely polarised protons are presented. The results were obtained using all the available COMPASS proton data, which were taken in the years 2007 and 2010. The Collins asymmetries exhibit in the valence region a non-zero signal for pions and there are hints of non-zero signal also for kaons. The Sivers asymmetries are found to be positive for positive pions and kaons and compatible with zero otherwise.Comment: 15 pages, 13 figures and 1 tabl

    The Spin Structure Function g1pg_1^{\rm p} of the Proton and a Test of the Bjorken Sum Rule

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    New results for the double spin asymmetry A1pA_1^{\rm p} and the proton longitudinal spin structure function g1pg_1^{\rm p} are presented. They were obtained by the COMPASS collaboration using polarised 200 GeV muons scattered off a longitudinally polarised NH3_3 target. The data were collected in 2011 and complement those recorded in 2007 at 160\,GeV, in particular at lower values of xx. They improve the statistical precision of g1p(x)g_1^{\rm p}(x) by about a factor of two in the region x‚Č≤0.02x\lesssim 0.02. A next-to-leading order QCD fit to the g1g_1 world data is performed. It leads to a new determination of the quark spin contribution to the nucleon spin, őĒő£\Delta \Sigma ranging from 0.26 to 0.36, and to a re-evaluation of the first moment of g1pg_1^{\rm p}. The uncertainty of őĒő£\Delta \Sigma is mostly due to the large uncertainty in the present determinations of the gluon helicity distribution. A new evaluation of the Bjorken sum rule based on the COMPASS results for the non-singlet structure function g1NS(x,Q2)g_1^{\rm NS}(x,Q^2) yields as ratio of the axial and vector coupling constants ‚ą£gA/gV‚ą£=1.22¬Ī0.05¬†(stat.)¬Ī0.10¬†(syst.)|g_{\rm A}/g_{\rm V}| = 1.22 \pm 0.05~({\rm stat.}) \pm 0.10~({\rm syst.}), which validates the sum rule to an accuracy of about 9\%.Comment: 19 pages, 8 figures and table

    Measurement of the charged-pion polarisability

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    The COMPASS collaboration at CERN has investigated pion Compton scattering, ŌÄ‚ąíő≥‚ÜíŌÄ‚ąíő≥\pi^-\gamma\rightarrow \pi^-\gamma, at centre-of-mass energy below 3.5 pion masses. The process is embedded in the reaction ŌÄ‚ąíNi‚ÜíŌÄ‚ąíő≥‚ÄÖ‚ÄäNi\pi^-\mathrm{Ni}\rightarrow\pi^-\gamma\;\mathrm{Ni}, which is initiated by 190\,GeV pions impinging on a nickel target. The exchange of quasi-real photons is selected by isolating the sharp Coulomb peak observed at smallest momentum transfers, Q2<0.0015Q^2<0.0015\,(GeV/cc)2^2. From a sample of 63\,000 events the pion electric polarisability is determined to be $\alpha_\pi\ =\ (\,2.0\ \pm\ 0.6_{\mbox{\scriptsize stat}}\ \pm\ 0.7_{\mbox{\scriptsize syst}}\,) \times 10^{-4}\,\mbox{fm}^3undertheassumption under the assumption \alpha_\pi=-\beta_\pi$, which relates the electric and magnetic dipole polarisabilities. It is the most precise measurement of this fundamental low-energy parameter of strong interaction, that has been addressed since long by various methods with conflicting outcomes. While this result is in tension with previous dedicated measurements, it is found in agreement with the expectation from chiral perturbation theory. An additional measurement replacing pions by muons, for which the cross-section behavior is unambigiously known, was performed for an independent estimate of the systematic uncertainty.Comment: Published version: 9 pages, 3 figures, 1 tabl

    Spin alignment and violation of the OZI rule in exclusive ŌČ\omega and Ōē\phi production in pp collisions

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    Exclusive production of the isoscalar vector mesons ŌČ\omega and Ōē\phi is measured with a 190 GeV/c/c proton beam impinging on a liquid hydrogen target. Cross section ratios are determined in three intervals of the Feynman variable xFx_{F} of the fast proton. A significant violation of the OZI rule is found, confirming earlier findings. Its kinematic dependence on xFx_{F} and on the invariant mass MpVM_{p\mathrm{V}} of the system formed by fast proton pfastp_\mathrm{fast} and vector meson VV is discussed in terms of diffractive production of pfastVp_\mathrm{fast}V resonances in competition with central production. The measurement of the spin density matrix element ŌĀ00\rho_{00} of the vector mesons in different selected reference frames provides another handle to distinguish the contributions of these two major reaction types. Again, dependences of the alignment on xFx_{F} and on MpVM_{p\mathrm{V}} are found. Most of the observations can be traced back to the existence of several excited baryon states contributing to ŌČ\omega production which are absent in the case of the Ōē\phi meson. Removing the low-mass MpVM_{p\mathrm{V}} resonant region, the OZI rule is found to be violated by a factor of eight, independently of xFx_\mathrm{F}.Comment: 23 pages, 13 figures and 5 table

    Resonance Production and ŌÄŌÄ\pi\pi S-wave in ŌÄ‚ąí+p‚ÜíŌÄ‚ąíŌÄ‚ąíŌÄ++precoil\pi^- + p \rightarrow \pi^- \pi^- \pi^+ + p_{recoil} at 190 GeV/c

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    The COMPASS collaboration has collected the currently largest data set on diffractively produced ŌÄ‚ąíŌÄ‚ąíŌÄ+\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<m3ŌÄ<2.50.5 < m_{3\pi} < 2.5 GeV/c2^2 , and in 11 bins of the reduced four-momentum transfer squared, 0.1<t<1.00.1 < t < 1.0 (GeV/c)2^2 . 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 3ŌÄ3\pi production. In this paper, we present detailed studies on selected 3ŌÄ3\pi partial waves with JPC=0‚ąí+J^{PC} = 0^{-+}, 1++1^{++}, 2‚ąí+2^{-+}, 2++2^{++}, and 4++4^{++}. In these waves, we observe the well-known ground-state mesons as well as a new narrow axial-vector meson a1(1420)a_1(1420) decaying into f0(980)ŌÄf_0(980) \pi. In addition, we present the results of a novel method to extract the amplitude of the ŌÄ‚ąíŌÄ+\pi^-\pi^+ subsystem with IGJPC=0+0++I^{G}J^{PC} = 0^+ 0^{++} in various partial waves from the ŌÄ‚ąíŌÄ‚ąíŌÄ+\pi^-\pi^-\pi^+ data. Evidence is found for correlation of the f0(980)f_0(980) and f0(1500)f_0(1500) appearing as intermediate ŌÄ‚ąíŌÄ+\pi^- \pi^+ isobars in the decay of the known ŌÄ(1800)\pi(1800) and ŌÄ2(1880)\pi_2(1880).Comment: 96 page

    Search for exclusive photoproduction of Zc¬Ī_c^{\pm}(3900) at COMPASS

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    A search for the exclusive production of the Zc¬Ī(3900)Z_c^{\pm}(3900) hadron by virtual photons has been performed in the channel Zc¬Ī(3900)‚ÜíJ/ŌąŌĬĪZ_c^{\pm}(3900)\rightarrow J/\psi \pi^{\pm}. The data cover the range from 7 GeV to 19 GeV in the centre-of-mass energy of the photon-nucleon system. The full set of the COMPASS data set collected with a muon beam between 2002 and 2011 has been used. An upper limit for the ratio BR(Zc¬Ī(3900)‚ÜíJ/ŌąŌĬĪ)√óŌÉő≥¬†N‚ÜíZc¬Ī(3900)¬†N/ŌÉő≥¬†N‚ÜíJ/Ōą¬†NBR(Z_c^{\pm}(3900)\rightarrow J/\psi \pi^{\pm} )\times \sigma_{ \gamma~N \rightarrow Z_c^{\pm}(3900)~ N} /\sigma_{\gamma~N \rightarrow J/\psi~ N} of 3.7√ó10‚ąí33.7\times10^{-3} has been established at the confidence level of 90%.Comment: 8 pages, 3 figures, 1 tabl

    Transverse-momentum-dependent Multiplicities of Charged Hadrons in Muon-Deuteron Deep Inelastic Scattering

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    A semi-inclusive measurement of charged hadron multiplicities in deep inelastic muon scattering off an isoscalar target was performed using data collected by the COMPASS Collaboration at CERN. The following kinematic domain is covered by the data: photon virtuality Q2>1Q^{2}>1 (GeV/cc)2^2, invariant mass of the hadronic system W>5W > 5 GeV/c2c^2, Bjorken scaling variable in the range 0.003<x<0.40.003 < x < 0.4, fraction of the virtual photon energy carried by the hadron in the range 0.2<z<0.80.2 < z < 0.8, square of the hadron transverse momentum with respect to the virtual photon direction in the range 0.02 (GeV/c)2<PhT2<3c)^2 < P_{\rm{hT}}^{2} < 3 (GeV/cc)2^2. The multiplicities are presented as a function of PhT2P_{\rm{hT}}^{2} in three-dimensional bins of xx, Q2Q^2, zz and compared to previous semi-inclusive measurements. We explore the small-PhT2P_{\rm{hT}}^{2} region, i.e. PhT2<1P_{\rm{hT}}^{2} < 1 (GeV/cc)2^2, where hadron transverse momenta are expected to arise from non-perturbative effects, and also the domain of larger PhT2P_{\rm{hT}}^{2}, where contributions from higher-order perturbative QCD are expected to dominate. The multiplicities are fitted using a single-exponential function at small PhT2P_{\rm{hT}}^{2} to study the dependence of the average transverse momentum ⟨PhT2⟩\langle P_{\rm{hT}}^{2}\rangle on xx, Q2Q^2 and zz. The power-law behaviour of the multiplicities at large PhT2P_{\rm{hT}}^{2} is investigated using various functional forms. The fits describe the data reasonably well over the full measured range.Comment: 28 pages, 20 figure
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