88 research outputs found
Observation of a new narrow axial-vector meson
No full textThe COMPASS collaboration at CERN has measured diffractive dissociation of 190 GeV pions into the final state using a stationary hydrogen target. A partial-wave analysis (PWA) was performed in bins of mass and four-momentum transfer using the isobar model and the so far largest PWA model consisting of 88~waves. A narrow signal is observed in the channel. We present a resonance-model study of a subset of the spin-density matrix selecting states with and decaying into and with decaying into . We identify a new meson with mass MeV and width MeV. Within the final states investigated in our analysis, we observe the new decaying only into , suggesting its exotic nature. To our knowledge, such a state has never been predicted.The COMPASS Collaboration at CERN has measured diffractive dissociation of 190 GeV/c pions into the π-π-π+ final state using a stationary hydrogen target. A partial-wave analysis (PWA) was performed in bins of 3π mass and four-momentum transfer using the isobar model and the so far largest PWA model consisting of 88 waves. A narrow peak is observed in the f0(980)π channel with spin, parity and C-parity quantum numbers JPC=1++. We present a resonance-model study of a subset of the spin-density matrix selecting 3π states with JPC=2++ and 4++ decaying into ρ(770)π and with JPC=1++ decaying into f0(980)π. We identify a new a1 meson with mass (1414-13+15) MeV/c2 and width (153-23+8) MeV/c2. Within the final states investigated in our analysis, we observe the new a1(1420) decaying only into f0(980)π, suggesting its exotic nature.The COMPASS collaboration at CERN has measured diffractive dissociation of 190 GeV/ pions into the final state using a stationary hydrogen target. A partial-wave analysis (PWA) was performed in bins of mass and four-momentum transfer using the isobar model and the so far largest PWA model consisting of 88 waves. A narrow signal is observed in the channel. We present a resonance-model study of a subset of the spin-density matrix selecting states with and decaying into and with decaying into . We identify a new meson with mass MeV/ and width MeV/. Within the final states investigated in our analysis, we observe the new decaying only into , suggesting its exotic nature
Transverse-spin-dependent azimuthal asymmetries of pion and kaon pairs produced in muon-proton and muon-deuteron semi-inclusive deep inelastic scattering
No full textA set of measurements of azimuthal asymmetries in the production of pairs of identified hadrons in deep-inelastic scattering of muons on transversely polarised 6LiD (deuteron) and NH3 (proton) targets is presented. All available data collected in the years 2002–2004 and 2007/2010 with the COMPASS spectrometer using a muon beam of 160 GeV=c at the CERN SPS were analysed. The asymmetries provide access to the transversity distribution functions via a fragmentation function that in principle may be independently obtained from e+e- annihilation data. Results are presented, discussed and compared to existing measurements as well as to model predictions. Asymmetries of pi+pi- pairs measured with the proton target as a function of the Bjorken scaling variable are sizeable in the range x > 0:032, indicating non-vanishing transversity distribution and di-hadron interference fragmentation functions. As already pointed out by several authors, the small asymmetries of pi+pi- measured on the 6LiD target can be interpreted as indication for a cancellation of u and d-quark transversity distributions.A set of measurements of azimuthal asymmetries in the production of pairs of identified hadrons in deep-inelastic scattering of muons on transversely polarised 6LiD (deuteron) and NH3 (proton) targets is presented. All available data collected in the years 2003–2004 and 2007/2010 with the COMPASS spectrometer using a muon beam of 160GeV/c at the CERN SPS were analysed. The asymmetries provide access to the transversity distribution functions via a fragmentation function that in principle may be independently obtained from e+e− annihilation data. Results are presented, discussed and compared to existing measurements as well as to model predictions. Asymmetries of π+π− pairs measured with the proton target as a function of the Bjorken scaling variable are sizeable in the range x>0.032, indicating non-vanishing transversity distribution and di-hadron interference fragmentation functions. As already pointed out by several authors, the small asymmetries of π+π− measured on the 6LiD target can be interpreted as indication for a cancellation of u and d-quark transversity distributions.A set of measurements of azimuthal asymmetries in the production of pairs of identified hadrons in deep-inelastic scattering of muons on transversely polarised LiD (deuteron) and NH (proton) targets is presented. All available data collected in the years 2002-2004 and 2007/2010 with the COMPASS spectrometer using a muon beam of 160 GeV/ at the CERN SPS were analysed. The asymmetries provide access to the transversity distribution functions via a fragmentation function that in principle may be independently obtained from annihilation data. Results are presented, discussed and compared to existing measurements as well as to model predictions. Asymmetries of pairs measured with the proton target as a function of the Bjorken scaling variable are sizeable in the range , indicating non-vanishing transversity distribution and di-hadron interference fragmentation functions. As already pointed out by several authors, the small asymmetries of measured on the LiD target can be interpreted as indication for a cancellation of and -quark transversity distributions
Collins and Sivers transverse-spin asymmetries in inclusive muoproduction of mesons
No full textThe production of vector mesons in deep inelastic scattering is an interesting yet scarsely explored channel to study the transverse spin structure of the nucleon and the related phenomena. The COMPASS collaboration has performed the first measurement of the Collins and Sivers asymmetries for inclusively produced mesons. The analysis is based on the data set collected in deep inelastic scattering in using a beam impinging on a transversely polarized target. The mesons are selected from oppositely charged hadron pairs, and the asymmetries are extracted as a function of the Bjorken- variable, the transverse momentum of the pair and the fraction of the energy carried by the pair. Indications for positive Collins and Sivers asymmetries are observed.The production of vector mesons in deep inelastic scattering is an interesting yet scarcely explored channel to study the transverse spin structure of the nucleon and the spin-dependence of fragmentation. The COMPASS collaboration has performed the first measurement of the Collins and Sivers asymmetries for inclusively produced ρ0 mesons. The analysis is based on the data set collected in deep inelastic scattering in 2010 using a 160GeV/cμ+ beam impinging on a transversely polarized NH3 target. The ρ0 mesons are selected from oppositely charged hadron pairs, and the asymmetries are extracted as a function of the Bjorken-x variable, the transverse momentum of the pair and the fraction of the energy z carried by the pair. Indications for positive Collins and Sivers asymmetries are observed.The production of vector mesons in deep inelastic scattering is an interesting yet scarsely explored channel to study the transverse spin structure of the nucleon and the related phenomena. The COMPASS collaboration has performed the first measurement of the Collins and Sivers asymmetries for inclusively produced mesons. The analysis is based on the data set collected in deep inelastic scattering in using a beam impinging on a transversely polarized target. The mesons are selected from oppositely charged hadron pairs, and the asymmetries are extracted as a function of the Bjorken- variable, the transverse momentum of the pair and the fraction of the energy carried by the pair. Indications for positive Collins and Sivers asymmetries are observed
Measurement of azimuthal hadron asymmetries in semi-inclusive deep inelastic scattering off unpolarised nucleons
No full textSpin-averaged asymmetries in the azimuthal distributions of positive and negative hadrons produced in deep inelastic scattering were measured using the CERN SPS muon beam at GeV/c and a LiD target. The amplitudes of the three azimuthal modulations , and were obtained binning the data separately in each of the relevant kinematic variables , or and binning in a three-dimensional grid of these three variables. The amplitudes of the and modulations show strong kinematic dependencies both for positive and negative hadrons.Spin-averaged asymmetries in the azimuthal distributions of positive and negative hadrons produced in deep inelastic scattering were measured using the CERN SPS longitudinally polarised muon beam at 160 GeV/c and a 6 LiD target. The amplitudes of the three azimuthal modulations cosϕh , cos2ϕh and sinϕh were obtained binning the data separately in each of the relevant kinematic variables x , z or pTh and binning in a three-dimensional grid of these three variables. The amplitudes of the cosϕh and cos2ϕh modulations show strong kinematic dependencies both for positive and negative hadrons.Spin-averaged asymmetries in the azimuthal distributions of positive and negative hadrons produced in deep inelastic scattering were measured using the CERN SPS muon beam at GeV/c and a LiD target. The amplitudes of the three azimuthal modulations , and were obtained binning the data separately in each of the relevant kinematic variables , or and binning in a three-dimensional grid of these three variables. The amplitudes of the and modulations show strong kinematic dependencies both for positive and negative hadrons
Quark helicity distributions from longitudinal spin asymmetries in muon-proton and muon-deuteron scattering
No full textAlekseev MG, Alexakhin VY, Alexandrov Y, et al. Quark helicity distributions from longitudinal spin asymmetries in muon-proton and muon-deuteron scattering. Phys.Lett. B. 2010;693(3):227-235.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
Multiplicities of positive and negative pions, kaons, and unidentified hadrons from deep-inelastic scattering of muons off a liquid hydrogen target
No full textThe multiplicities of positive and negative pions, kaons, and unidentified hadrons produced in deep-inelastic scattering are measured in bins of the Bjorken scaling variable x, the relative virtual-photon energy y, and the fraction of the virtual-photon energy transferred to the final-state hadron z. Data were obtained by the COMPASS Collaboration using a 160 GeV muon beam of both electric charges and a liquid hydrogen target. These measurements cover the kinematic domain with photon virtuality Q2>1 (GeV/c)2, 0.004<x<0.4, 0.1<y<0.7, and 0.2<z<0.85, in accordance with the kinematic domain used in earlier published COMPASS multiplicity measurements with an isoscalar target. The calculation of radiative corrections was improved by using the Monte Carlo generator djangoh, which results in up to 12% larger corrections in the low-x region.The multiplicities of positive and negative pions, kaons and unidentified hadrons produced in deep-inelastic scattering are measured in bins of the Bjorken scaling variable , the relative virtual-photon energy and the fraction of the virtual-photon energy transferred to the final-state hadron . Data were obtained by the COMPASS Collaboration using a 160 GeV muon beam of both electric charges and a liquid hydrogen target. These measurements cover the kinematic domain with photon virtuality (GeV/, , and , in accordance with the kinematic domain used in earlier published COMPASS multiplicity measurements with an isoscalar target. The calculation of radiative corrections was improved by using the Monte Carlo generator DJANGOH, which results in up to 12% larger corrections in the low- region
Spin alignment and violation of the OZI rule in exclusive and production in pp collisions
No full textExclusive production of the isoscalar vector mesons and is measured with a 190 GeV proton beam impinging on a liquid hydrogen target. Cross section ratios are determined in three intervals of the Feynman variable of the fast proton. A significant violation of the OZI rule is found, confirming earlier findings. Its kinematic dependence on and on the invariant mass of the system formed by fast proton and vector meson is discussed in terms of diffractive production of resonances in competition with central production. The measurement of the spin density matrix element 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 and on are found. Most of the observations can be traced back to the existence of several excited baryon states contributing to production which are absent in the case of the meson. Removing the low-mass resonant region, the OZI rule is found to be violated by a factor of eight, independently of .Exclusive production of the isoscalar vector mesons ω and ϕ is measured with a 190 GeV/c proton beam impinging on a liquid hydrogen target. Cross section ratios are determined in three intervals of the Feynman variable xF of the fast proton. A significant violation of the OZI rule is found, confirming earlier findings. Its kinematic dependence on xF and on the invariant mass MpV of the system formed by fast proton pfast and vector meson V is discussed in terms of diffractive production of pfastV resonances in competition with central production. The measurement of the spin density matrix element ρ00 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 xF and on MpV are found. Most of the observations can be traced back to the existence of several excited baryon states contributing to ω production which are absent in the case of the ϕ meson. Removing the low-mass MpV resonant region, the OZI rule is found to be violated by a factor of eight, independently of xF .Exclusive production of the isoscalar vector mesons and is measured with a 190 GeV proton beam impinging on a liquid hydrogen target. Cross section ratios are determined in three intervals of the Feynman variable of the fast proton. A significant violation of the OZI rule is found, confirming earlier findings. Its kinematic dependence on and on the invariant mass of the system formed by fast proton and vector meson is discussed in terms of diffractive production of resonances in competition with central production. The measurement of the spin density matrix element 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 and on are found. Most of the observations can be traced back to the existence of several excited baryon states contributing to production which are absent in the case of the meson. Removing the low-mass resonant region, the OZI rule is found to be violated by a factor of eight, independently of
Exclusive rho(0) muoproduction on transversely polarised protons and deuterons
No full textAdolph 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
Measurement of the Charged-Pion Polarizability
No full textThe COMPASS collaboration at CERN has investigated pion Compton scattering, , at centre-of-mass energy below 3.5 pion masses. The process is embedded in the reaction , 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, Q^2The COMPASS collaboration at CERN has investigated pion Compton scattering, π-γ→π-γ, at center-of-mass energy below 3.5 pion masses. The process is embedded in the reaction π-Ni→π-γNi, which is initiated by 190 GeV pions impinging on a nickel target. The exchange of quasireal photons is selected by isolating the sharp Coulomb peak observed at smallest momentum transfers, Q2<0.0015 (GeV/c)2. From a sample of 63 000 events, the pion electric polarizability is determined to be απ=(2.0±0.6stat±0.7syst)×10-4 fm3 under the assumption απ=-βπ, which relates the electric and magnetic dipole polarizabilities. 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 unambiguously known, was performed for an independent estimate of the systematic uncertainty.The COMPASS collaboration at CERN has investigated pion Compton scattering, \pi^-\gamma\rightarrow \pi^-\gamma\pi^-\mathrm{Ni}\rightarrow\pi^-\gamma\;\mathrm{Ni}Q^2<0.0015c^2\alpha_\pi\ =\ (\,2.0\ \pm\ 0.6_{\mbox{\scriptsize stat}}\ \pm\ 0.7_{\mbox{\scriptsize syst}}\,) \times 10^{-4}\,\mbox{fm}^3\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
On a problem concerning stratified graphs
No full textAdolph C, Akhunzyanov R, Alekseev MG, et al. A high-statistics measurement of transverse spin effects in dihadron production from muon-proton semi-inclusive deep-inelastic scattering. Physics Letters B. 2014;736:124-131.A measurement of the azimuthal asymmetry in dihadron production indeep-inelastic scattering of muons on transversely polarised proton (NH)targets are presented. They provide independent access to the transversitydistribution functions through the measurement of the Collins asymmetry insingle hadron production. The data were taken in the year with theCOMPASS spectrometer using a 160\,\mbox{GeV}/c muon beam of the CERN SPS,increasing by a factor of about three the available statistics of thepreviously published data taken in the year . The measured sizeableasymmetry is in good agreement with the published data. An approximate equalityof the Collins asymmetry and the dihadron asymmetry is observed, suggesting acommon physical mechanism in the underlying fragmentation
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