0000000000178091
AUTHOR
E. Burtin
Virtual Compton scattering and the generalized polarizabilities of the proton atQ2=0.92and 1.76 GeV2
Virtual Compton Scattering (VCS) on the proton has been studied at Jefferson Lab using the exclusive photon electroproduction reaction (e p --> e p gamma). This paper gives a detailed account of the analysis which has led to the determination of the structure functions P{sub LL}-P{sub TT}/epsilon and P{sub LT}, and the electric and magnetic generalized polarizabilities (GPs) alpha{sub E}(Q{sup 2}) and beta{sub M}(Q{sup 2}) at values of the four-momentum transfer squared Q{sup 2} = 0.92 and 1.76 GeV{sup 2}. These data, together with the results of VCS experiments at lower momenta, help building a coherent picture of the electric and magnetic GPs of the proton over the full measured Q{sup 2}-…
Experimental investigation of transverse spin asymmetries in muon-p SIDIS processes: Sivers asymmetries
The COMPASS Collaboration at CERN has measured the transverse spin azimuthal asymmetry of charged hadrons produced in semi-inclusive deep inelastic scattering using a 160 GeV positive muon beam and a transversely polarised NH_3 target. The Sivers asymmetry of the proton has been extracted in the Bjorken x range 0.003 0.03. The asymmetry is different from zero and positive also in the low x region, where sea-quarks dominate. The kinematic dependence of the asymmetry has also been investigated and results are given for various intervals of hadron and virtual photon fractional energy. In contrast to the case of the Collins asymmetry, the results on the Sivers asymmetry suggest a strong depende…
Search for exclusive photoproduction ofZc±(3900) at COMPASS
A search for the exclusive production of the Z(c)(+/-)(3900) hadron by virtual photons has been performed in the channel Z(c)(+/-)(3900). J/Psi pi(+/-). The data cover the range from 7GeV to 19GeV 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(Z(c)(+/-)(3900)-> J/Psi pi(+/-)) x sigma(gamma N) -> Z(c)(+/-)(3900) N/sigma gamma N -> J/Psi N 3.7 x10(-3) has been established at the confidence level of90%. (C) 2015 The Authors. Published by Elsevier B.V.
Polarised quark distributions in the nucleon from semi-inclusive spin asymmetries
We present a measurement of semi-inclusive spin asymmetries for positively and negatively charged hadrons from deep inelastic scattering of polarised muons on polarised protons and deuterons in the range $0.003$1 GeV$^2$. Compared to our previous publication on this subject, with the new data the statistical errors have been reduced by nearly a factor of two. From these asymmetries and our inclusive spin asymmetries we determine the polarised quark distributions of valence quarks and non-strange sea quarks at $Q^2$=10 GeV$^2$. The polarised $u$ valence quark distribution, $\Delta u_v(x)$, is positive and the polarisation increases with $x$. The polarised $d$ valence quark distribution, $\De…
Interplay among transversity induced asymmetries in hadron leptoproduction
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 dif…
A line-shape analysis for spin-1 NMR signals
An analytic model of the deuteron absorption function has been developed and is compared to experimental NMR signals of deuterated butanol obtained at the SMC experiment in order to determine the deuteron polarization. The absorption function model includes dipolar broadening and a frequency-dependent treatment of the intensity factors. The high-precision TE signal data available are used to adjust the model for Q-meter distortions and dispersion effects. Once the Q-meter adjustment is made, the enhanced polarizations determined by the asymmetry and TE-calibration methods compare well within the accuracy of each method. In analyzing the NMR signals, the quadrupolar coupling constants could …
Spin alignment and violation of the OZI rule in exclusive ω and ϕ production in pp collisions
Exclusive production of the isoscalar vector mesons $\omega$ and $\phi$ 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 $x_{F}$ of the fast proton. A significant violation of the OZI rule is found, confirming earlier findings. Its kinematic dependence on $x_{F}$ and on the invariant mass $M_{p\mathrm{V}}$ of the system formed by fast proton $p_\mathrm{fast}$ and vector meson $V$ is discussed in terms of diffractive production of $p_\mathrm{fast}V$ resonances in competition with central production. The measurement of the spin density matrix element $\rho_{00}$ of the vector mesons…
Transverse spin effects in hadron-pair production from semi-inclusive deep inelastic scattering
First measurements of azimuthal asymmetries in hadron-pair production in deep-inelastic scattering of muons on transversely polarised ^6LiD (deuteron) and NH_3 (proton) targets are presented. The data were taken in the years 2002-2004 and 2007 with the COMPASS spectrometer using a muon beam of 160 GeV/c at the CERN SPS. The asymmetries provide access to the transversity distribution functions, without involving the Collins effect as in single hadron production. The sizeable asymmetries measured on the NH_ target indicate non-vanishing u-quark transversity and two-hadron interference fragmentation functions. The small asymmetries measured on the ^6LiD target can be interpreted as indication …
The spin structure functiong1pof the proton and a test of the Bjorken sum rule
New results for the double spin asymmetry A(1)(p) and the proton longitudinal spin structure function g(1)(p) are presented. They were obtained by the COMPASS Collaboration using polarised 200 GeV muons scattered off a longitudinally polarised NH3 target. The data were collected in 2011 and complement those recorded in 2007 at 160 GeV, in particular at lower values of x. They improve the statistical precision of g(1)(p)(x) by about a factor of two in the region x less than or similar to 0.02. A next-to-leading order QCD fit to the g(1) world data is performed. It leads to a new determination of the quark spin contribution to the nucleon spin, Delta Sigma, ranging from 0.26 to 0.36, and to a…
Leading order determination of the gluon polarisation from DIS events with high-pThadron pairs
We present a determination of the gluon polarisation Delta g/g in the nucleon, based on the longitudinal double-spin asymmetry of DIS events with Q(2) > 1 (GeV/c)(2) including a pair of large transverse-momentum hadrons in the final state. The data were obtained by the COMPASS experiment at CERN using a 160 GeV/c polarised muon beam scattering off a polarised (LiD)-Li-6 target. The gluon polarisation is evaluated by a Neural Network approach for three intervals of the gluon momentum fraction x(g) covering the range 0.04 < x(g) < 0.27. The values obtained at leading order in QCD do not show any significant dependence on x(g). Their average is Delta g/g = 0.125 +/- 0.060 (stat.) +/- 0.063 (sy…
Exclusive muoproduction on transversely polarised protons and deuterons
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 deuteron…
Transverse extension of partons in the proton probed in the sea-quark range by measuring the DVCS cross section
Physics letters / B B793, 188-194 (2019). doi:10.1016/j.physletb.2019.04.038
Corrigendum to “Transverse extension of partons in the proton probed in the sea-quark range by measuring the DVCS cross section” [Phys. Lett. B 793 (2019) 188]
Physics letters / B B 800, 135129 (2020). doi:10.1016/j.physletb.2019.135129
Observation of a New Narrow Axial-Vector Mesona1(1420)
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 J(PC)=1(++). We present a resonance-model study of a subset of the spin-density matrix selecting 3π states with J(PC)=2(++) and 4(++) decaying into ρ(770)π and with J(PC)=1(++) decaying into f0(980)π. We identify a new a1 meson with mass (1414(-13)(+15)) MeV/c2 and wid…
New Measurements of the Transverse Beam Asymmetry for Elastic Electron Scattering from Selected Nuclei
We have measured the beam-normal single-spin asymmetry $A_n$ in the elastic scattering of 1-3 GeV transversely polarized electrons from $^1$H and for the first time from $^4$He, $^{12}$C, and $^{208}$Pb. For $^1$H, $^4$He and $^{12}$C, the measurements are in agreement with calculations that relate $A_n$ to the imaginary part of the two-photon exchange amplitude including inelastic intermediate states. Surprisingly, the $^{208}$Pb result is significantly smaller than the corresponding prediction using the same formalism. These results suggest that a systematic set of new $A_n$ measurements might emerge as a new and sensitive probe of the structure of heavy nuclei.
The COMPASS Setup for Physics with Hadron Beams
The main characteristics of the COMPASS experimental setup for physics with hadron beams are described. This setup was designed to perform exclusive measurements of processes with several charged and/or neutral particles in the final state. Making use of a large part of the apparatus that was previously built for spin structure studies with a muon beam, it also features a new target system as well as new or upgraded detectors. The hadron setup is able to operate at the high incident hadron flux available at CERN. It is characterised by large angular and momentum coverages, large and nearly flat acceptances, and good two and three-particle mass resolutions. In 2008 and 2009 it was successful…
Spin asymmetry A1d and the spin-dependent structure function g1d of the deuteron at low values of x and Q2
Abstract We present a precise measurement of the deuteron longitudinal spin asymmetry A 1 d and of the deuteron spin-dependent structure function g 1 d at Q 2 1 ( GeV / c ) 2 and 4 × 10 −5 x 2.5 × 10 −2 based on the data collected by the COMPASS experiment at CERN during the years 2002 and 2003. The statistical precision is tenfold better than that of the previous measurement in this region. The measured A 1 d and g 1 d are found to be consistent with zero in the whole range of x.
The spin-dependent structure function of the proton g1p and a test of the Bjorken sum rule
Abstract The inclusive double-spin asymmetry, A 1 p , has been measured at COMPASS in deep-inelastic polarised muon scattering off a large polarised NH3 target. The data, collected in the year 2007, cover the range Q 2 > 1 ( GeV / c ) 2 , 0.004 x 0.7 and improve the statistical precision of g 1 p ( x ) by a factor of two in the region x 0.02 . The new proton asymmetries are combined with those previously published for the deuteron to extract the non-singlet spin-dependent structure function g 1 NS ( x , Q 2 ) . The isovector quark density, Δ q 3 ( x , Q 2 ) , is evaluated from a NLO QCD fit of g 1 NS . The first moment of Δ q 3 is in good agreement with the value predicted by the Bjorken su…
Longitudinal double spin asymmetries in single hadron quasi-real photoproduction at high $p_T$
We measured the longitudinal double spin asymmetries $A_{LL}$ for single hadron muo-production off protons and deuterons at photon virtuality $Q^2$ < 1(GeV/$\it c$)$^2$ for transverse hadron momenta $p_T$ in the range 0.7 GeV/$\it c$ to 4 GeV/$\it c$ . They were determined using COMPASS data taken with a polarised muon beam of 160 GeV/$\it c$ or 200 GeV/$\it c$ impinging on polarised $\mathrm{{}^6LiD}$ or $\mathrm{NH_3}$ targets. The experimental asymmetries are compared to next-to-leading order pQCD calculations, and are sensitive to the gluon polarisation $\Delta G$ inside the nucleon in the range of the nucleon momentum fraction carried by gluons $0.05 < x_g < 0.2$. We measured the longi…
Flavour Separation of Helicity Distributions from Deep Inelastic Muon-Deuteron Scattering
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<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 wit…
The Polarised Valence Quark Distribution from semi-inclusive DIS
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 …
Backward electroproduction ofπ0mesons on protons in the region of nucleon resonances at four momentum transfer squaredQ2=1.0GeV2
Exclusive electroproduction of pi{sup 0} mesons on protons in the backward hemisphere has been studied at Q2 = 1.0 GeV2 by detecting protons in the forward direction in coincidence with scattered electrons from the 4 GeV electron beam in Jefferson Lab's Hall A. The data span the range of the total (gamma*p) center-of-mass energy W from the pion production threshold to W = 2.0 GeV. The differential cross sections sigma{sub T} + epsilon sigma{sub L}, sigma{sub TL}, and sigma{sub TT} were separated from the azimuthal distribution and are presented together with the MAID and SAID parameterizations.
Collins and Sivers asymmetries for pions and kaons in muon-deuteron DIS
The measurements of the Collins and Sivers asymmetries of identified hadrons produced in deep-inelastic scattering of 160 GeV/c muons on a transversely polarised 6LiD target at COMPASS are presented. The results for charged pions and charged and neutral kaons correspond to all data available, which were collected from 2002 to 2004. For all final state particles both the Collins and Sivers asymmetries turn out to be small, compatible with zero within the statistical errors, in line with the previously published results for not identified charged hadrons, and with the expected cancellation between the u- and d-quark contributions.
A large Streamer Chamber muon tracking detector in a high-flux fixed-target application.
Arrays of limited streamer tubes of the Iarocci type were deployed in our experiment at CERN as part of a forward muon detector system with provisions for the beam to pass through the center of each panel in the array. A total of 16 4 m x 4 m panels were assembled with inductive readout strips on both sides of each panel. An active feedback system. was deployed to regulate the high voltage to the streamer tubes to insure a constant efficiency for minimum ionizing particles. The arrays were operated in this environment for over five years of data taking. Streamer tube track-reconstruction efficiencies and tube replacement rates are reported. (C) 1999 Elsevier Science B.V. All rights reserved.
Erratum to: Hadron transverse momentum distributions in muon deep inelastic scattering at 160 GeV/ $$c$$ c
Author(s): Adolph, C; Alekseev, MG; Alexakhin, VY; Alexandrov, Y; Alexeev, GD; Amoroso, A; Andrieux, V; Austregesilo, A; Badelek, B; Balestra, F; Barth, J; Baum, G; Bedfer, Y; Berlin, A; Bernhard, J; Bertini, R; Bicker, K; Bieling, J; Birsa, R; Bisplinghoff, J; Boer, M; Bordalo, P; Bradamante, F; Braun, C; Bravar, A; Bressan, A; Buchele, M; Burtin, E; Capozza, L; Chiosso, M; Chung, SU; Cicuttin, A; Crespo, ML; Dalla Torre, S; Dasgupta, SS; Dasgupta, S; Denisov, OY; Donskov, SV; Doshita, N; Duic, V; Dunnweber, W; Dziewiecki, M; Efremov, A; Elia, C; Eversheim, PD; Eyrich, W; Faessler, M; Ferrero, A; Filin, A; Finger, M; Finger Jr, M; Fischer, H; Franco, C; du Fresne von Hohenesche, N; Friedri…
Study of Σ(1385) and Ξ(1321) hyperon and antihyperon production in deep inelastic muon scattering
Large samples of \Lambda, \Sigma(1385) and \Xi(1321) hyperons produced in deep-inelastic muon scattering off a ^6LiD target were collected with the COMPASS experimental setup at CERN. The relative yields of \Sigma(1385)^+, \Sigma(1385)^-, \bar{\Sigma}(1385)^-, \bar{\Sigma}(1385)^+, \Xi(1321)^-, and \bar{\Xi}(1321)^+ hyperons decaying into \Lambda(\bar{\Lambda})\pi were measured. The heavy hyperon to \Lambda and heavy antihyperon to \bar{\Lambda} yield ratios 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.
Virtual Compton Scattering and Neutral Pion Electroproduction in the Resonance Region up to the Deep Inelastic Region at Backward Angles
We have made the first measurements of the virtual Compton scattering (VCS) process via the H$(e,e'p)\gamma$ exclusive reaction in the nucleon resonance region, at backward angles. Results are presented for the $W$-dependence at fixed $Q^2=1$ GeV$^2$, and for the $Q^2$-dependence at fixed $W$ near 1.5 GeV. The VCS data show resonant structures in the first and second resonance regions. The observed $Q^2$-dependence is smooth. The measured ratio of H$(e,e'p)\gamma$ to H$(e,e'p)\pi^0$ cross sections emphasizes the different sensitivity of these two reactions to the various nucleon resonances. Finally, when compared to Real Compton Scattering (RCS) at high energy and large angles, our VCS data…
The COMPASS experiment at CERN
The COMPASS experiment makes use of the CERN SPS high-intensitymuon and hadron beams for the investigation of the nucleon spin structure and the spectroscopy of hadrons. One or more outgoing particles are detected in coincidence with the incoming muon or hadron. A large polarized target inside a superconducting solenoid is used for the measurements with the muon beam. Outgoing particles are detected by a two-stage, large angle and large momentum range spectrometer. The setup is built using several types of tracking detectors, according to the expected incident rate, required space resolution and the solid angle to be covered. Particle identification is achieved using a RICH counter and both…
The Deuteron Spin-dependent Structure Function g1(d) and its First Moment
We present a measurement of the deuteron spin-dependent structure function g1d based on the data collected by the COMPASS experiment at CERN during the years 2002-2004. The data provide an accurate evaluation for Gamma_1^d, the first moment of g1d(x), and for the matrix element of the singlet axial current, a0. The results of QCD fits in the next to leading order (NLO) on all g1 deep inelastic scattering data are also presented. They provide two solutions with the gluon spin distribution function Delta G positive or negative, which describe the data equally well. In both cases, at Q^2 = 3 (GeV/c)^2 the first moment of Delta G is found to be of the order of 0.2 - 0.3 in absolute value.
A new measurement of the Collins and Sivers asymmetries on a transversely polarised deuteron target
New high precision measurements of the Collins and Sivers asymmetries of charged hadrons produced in deep-inelastic scattering of muons on a transversely polarised 6LiD target are presented. The data were taken in 2003 and 2004 with the COMPASS spectrometer using the muon beam of the CERN SPS at 160 GeV/c. Both the Collins and Sivers asymmetries turn out to be compatible with zero, within the present statistical errors, which are more than a factor of 2 smaller than those of the published COMPASS results from the 2002 data. The final results from the 2002, 2003 and 2004 runs are compared with naive expectations and with existing model calculations.
A new measurement of the spin-dependent structure function $g_{1}(x)$ of the deuteron
Abstract We present a new measurement of the spin-dependent structure function g 1 d of the deuteron in deep inelastic scattering of 190 GeV polarised muons on polarised deuterons, in the kinematic range 0.003 x 2 Q 2 2 . This structure function is found to be negative at small x . The first moment Γ 1 d =∫ 0 1 g 1 d d x evaluated at Q 0 2 = 10 GeV 2 is 0.034 ± 0.009 (stat.) ± 0.006 (syst.). This value is below the Ellis-Jaffe sum rule prediction by three standard deviations. Using our earlier determination of Γ 1 p , we obtain Γ 1 p − Γ 1 n = 0.199 ± 0.038 which agrees with the Bjorken sum rule.
Collins and Sivers asymmetries in muonproduction of pions and kaons off transversely polarised protons
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.
The polarized double cell target of the SMC
The polarized target of the Spin Muon Collaboration at CERN was used for deep inelastic muon scattering experiments during 1993-1996 with a polarized muon beam to investigate the spin structure of the nucleon. Most of the experiments were carried out with longitudinal target polarization and 190 GeV muons, and some were done with transverse polarization and 100 GeV muons. Protons as well as deuterons were polarized by dynamic nuclear polarization (DNP) in three kinds of solid materials - butanol, ammonia, and deuterated butanol - with maximum degrees of polarization of 94%, 91% and 60%, respectively. Considerable attention was paid to the accuracies of the NMR polarization measurements and …
Measurement of the Spin Structure of the Deuteron in the DIS Region
We present a new measurement of the longitudinal spin asymmetry A_1^d and the spin-dependent structure function g_1^d of the deuteron in the range 1 GeV^2 < Q^2 < 100 GeV^2 and 0.004< x <0.7. The data were obtained by the COMPASS experiment at CERN using a 160 GeV polarised muon beam and a large polarised 6-LiD target. The results are in agreement with those from previous experiments and improve considerably the statistical accuracy in the region 0.004 < x < 0.03.
Measurement of the SMC muon beam polarisation using the asymmetry in the elastic scattering off polarised electrons
A muon beam polarimeter was built for the SMC experiment at the CERN SPS, for beam energies of 100 and 190 GeV. The beam polarisation is determined from the asymmetry in the elastic scattering off the polarised electrons of a ferromagnetic target whose magnetisation is periodically reversed. At muon energies of 100 and 190 GeV the measured polarisation is P-mu = -0.80 +/- 0.03 (stat.) +/- 0.02 (syst.) and P-mu = - 0.797 +/- 0.011 (stat.) +/- 0.012 (syst.), respectively. These results agree with measurements of the beam polarisation using a shape analysis of the decay positron energy spectrum. (C) 2000 Elsevier Science B.V. All rights reserved.
Measurement of azimuthal hadron asymmetries in semi-inclusive deep inelastic scattering off unpolarised nucleons
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 $160$ GeV/c and a $^6$LiD target. The amplitudes of the three azimuthal modulations $\cos\phi_h$, $\cos2\phi_h$ and $\sin\phi_h$ were obtained binning the data separately in each of the relevant kinematic variables $x$, $z$ or $p_T^{\,h}$ and binning in a three-dimensional grid of these three variables. The amplitudes of the $\cos \phi_h$ and $\cos 2\phi_h$ modulations show strong kinematic dependencies both for positive and negative hadrons.
Measurement of the Charged-Pion Polarizability
The COMPASS collaboration at CERN has investigated pion Compton scattering, $\pi^-\gamma\rightarrow \pi^-\gamma$, at centre-of-mass energy below 3.5 pion masses. The process is embedded in the reaction $\pi^-\mathrm{Ni}\rightarrow\pi^-\gamma\;\mathrm{Ni}$, 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, $Q^2<0.0015$\,(GeV/$c$)$^2$. From a sample of 63\,000 events the pion electric polarisability is determined to be $\alpha_\pi\ =\ (\,2.0\ \pm\ 0.6_{\mbox{\scriptsize stat}}\ \pm\ 0.7_{\mbox{\scriptsize syst}}\,) \times 10^{-4}\,\mbox{fm}^3$ under the …
Measurement of the Collins and Sivers asymmetries on transversely polarised protons
The Collins and Sivers asymmetries for charged hadrons produced in deeply inelastic scattering on transversely polarised protons have been extracted from the data collected in 2007 with the CERN SPS muon beam tuned at 160 GeV/c. At large values of the Bjorken x variable non-zero Collins asymmetries are observed both for positive and negative hadrons while the Sivers asymmetry for positive hadrons is slightly positive over almost all the measured x range. These results nicely support the present theoretical interpretation of these asymmetries, in terms of leading-twist quark distribution and fragmentation functions.
Corrigendum to “Odd and even partial waves of ηπ− and η′π− in π−p → η(′)π−p at 191 GeV/c” [Phys. Lett. B 740 (2015) 303–311]
Abstract In Fig. 5 on p. 311 of our Phys. Lett. B 740 (2015) 303 an adjustment by 180 ∘ is required for the phases with respect to the L = 2 , M = 1 wave, of the following waves: L = 1 , 3 , 5 with M = 1 , and L = 2 with M = 2 . After this correction (Fig. 5 (corrected) below), the extracted partial waves describe the angular distribution of the η ( ′ ) in the Gottfried-Jackson (GJ) frame, using Eq. (4) with implicit Condon-Shortley phase convention. The other results of our paper are not affected. The right-handed GJ coordinate system was defined by the z-axis pointing in the direction of the beam in the η ( ′ ) π − center-of-mass system and the y-axis pointing in the direction of p recoil…
The spin-dependent structure function g1(x) of the deuteron from polarized deep-inelastic muon scattering
We present a new measurement of the spin-dependent structure function $g_{1}^{\rm d}$ of the deuteron from deep inelastic scattering of 190 GeV polarized muons on polarized deuterons. The results are combined with our previous measurements of $g_{1}^{\rm d}$. A perturbative QCD evolution in next-to-leading order is used to compute $g_{1}^{\rm d}(x)$ at a constant $Q^{2}$. At $Q^{2} = 10$ GeV$^{2}$, we obtain a first moment $\Gamma_{1}^{\rm d} = \int_{0}^{1} g_{1}^{\rm d}{\rm d}x = 0.041 \pm 0.008$, a flavour-singlet axial charge of the nucleon $a_{0} = 0.30 \pm 0.08$, and an axial charge of the strange quark $a_{s} = -0.09 \pm 0.03$. Using our earlier determination of $\Gamma_{1}^{\rm p}$, …
Gluon polarization in the nucleon from quasi-real photoproduction of high-pT hadron pairs
Abstract We present a determination of the gluon polarization Δ G / G in the nucleon, based on the helicity asymmetry of quasi-real photoproduction events, Q 2 1 ( GeV / c ) 2 , with a pair of large transverse-momentum hadrons in the final state. The data were obtained by the COMPASS experiment at CERN using a 160 GeV polarized muon beam scattered on a polarized 6 LiD target. The helicity asymmetry for the selected events is 〈 A ∥ / D 〉 = 0.002 ± 0.019 ( stat ) ± 0.003 ( syst ) . From this value, we obtain in a leading-order QCD analysis Δ G / G = 0.024 ± 0.089 ( stat ) ± 0.057 ( syst ) at x g = 0.095 and μ 2 ≃ 3 ( GeV / c ) 2 .
Measurement of the spin-dependent structure function g1(x) of the deuteron
We report on the first measurement of the spin-dependent structure function g1d of the deuteron in the deep inelastic scattering of polarised muons off polarised deuterons, in the kinematical range 0.006<x<0.6, 1 GeV2<Q2<30 GeV2. The first moment, Γ1d=sh{phonetic}01 g1d dx=0.023±0.020 (stat.) ± 0.015 (syst.), is smaller than the prediction of the Ellis-Jaffe sum rules. Using earlier measurements of g1p, we infer the first moment of the spin-dependent neutron structure function g1n. The difference Γ1p-Γ1n=0.20 ±0.05 (stat.) ± 0.04 (syst.) agrees with the prediction of the Bjorken sum rule, Γ1p-Γ1n=0.191 ±0.002.