One of the main goals of the COMPASS experiment at CERN is the determination of the gluon contribution to the nucleon spin. To achieve this goal COMPASS uses a naturally polarised muon beam with an energy of 160 GeV and a fixed polarised target. Two types of materials are used for the latter: $^{6}$LiD (polarised deuterons) during the years of 2002-2006 and NH$_{3}$ (polarised protons) in 2007. The gluons in the nucleon can be accessed directly via the Photon Gluon Fusion (PGF) process. Among the channels studied by COMPASS, the production of charmed mesons is the one that tags a PGF interaction in the most clean and efficient way. This talk presents a result for the gluon polarisation, $Δg/g$, which is based on a measurement of the spin asymmetry resulting from the production of D$^{0}$ mesons. These mesons are reconstructed through the invariant mass of their decay products. The statistical significance of the $D^{0}$ mass spectra has been improved significantly using a new method based on Neural Networks. The $Δg/g$ result is also presented using a next-to-leading order (NLO-QCD) analysis of the $μN \rightarrow q\bar{q}$ reaction. Such correction is relevant and was for the first time applied to an experimental measurement of the gluon polarisation.
One of the main goals of the COMPASS experiment at CERN is the determination of the gluon polarisation in the nucleon, DeltaG/G. It is extracted from the spin asymmetry in the scattering of a polarized 160 GeV muon beam on a polarized LiD target, by selecting photon-gluon fusion events. These events are tagged by the production of open charm or high pT hadron pairs. We present the results obtained for DeltaG/G(x) in both channels. For the first time, preliminary results of 2004 data in the high pT channel are also shown.
The gluon polarization $Δ{G}/G$ is the key to a further clarification of the spin structure of the nucleon. The COMPASS collaboration at CERN has set out to undertake the direct determination of this quantity. It accesses the gluon distribution via the photon-gluon fusion process (PGF) in scattering polarized muons off a polarized deuteron target. And it explores three different channels to tag the PGF: open charm production and high transverse momentum (high p_T), in either electroproduction (Q^2 > 1 GeV^2) or quasi-real photoproduction (Q^2 < 1 GeV^2). The high p_T quasi-real photoproduction…
COMPASS experiment measurements of the gluon polarisation in nucleon, DeltaG/G are reviewed. Two different approaches based on tagging the Photon Gluon Fusion process are described. They rely on the open charm meson or high-p_T hadron pairs detection.
One of the main aims of the COMPASS experiment at CERN is the study of the spin structure of the nucleon and in particular the determination of the gluon polarization in the nucleon. We present some new results of 2002-2003 data analysis. They concern a precise measurement of the deuteron structure function g1 at small x, some preliminary result on Collins and Sivers asymmetries which are linked to transversity and a new measurement of DeltaG/G.
We make a first crude direct estimate of the net gluon polarization in the proton, DeltaG, combining data on the asymmetries in high-p_T hadron production from the HERMES, SMC and COMPASS collaborations. Although these data sample a restricted range of x, they provide no hint that DeltaG is large. Fixing the normalizations of different theoretical parametrizations using the hadron asymmetry data, we find typical central values of DeltaG \sim 0.5, with uncertainties of similar magnitude. Values of DeltaG \ge 2 are disfavoured by Delta chi^2 \sim 9 to 20, depending on the parametrization used.
One of the main goals of the COMPASS experiment at CERN is the determination of the gluon polarisation in the nucleon, Delta G/G. It is determined from spin asymmetries in the scattering of 160 GeV/c polarised muons on a polarised LiD target. The gluon polarisation is accessed by the selection of photon-gluon fusion (PGF) events. A very clean selection of PGF events can be obtained with charmed mesons in the final state. Their detection is based on the reconstruction of D* and D0 mesons in the COMPASS spectrometer. The analysis method for the first measurement of Delta G/G from the open charm …
One of the missing keys in the present understanding of the spin structure of the nucleon is the contribution from the gluons: the so-called gluon polarisation. This quantity can be determined in DIS through the photon-gluon fusion process, in which two analysis methods may be used: (i) identifying open charm events or (ii) selecting events with high p_T hadrons. The data used in the present work were collected in the COMPASS experiment, where a 160 GeV/c naturally polarised muon beam, impinging on a polarised nucleon fixed target is used. Preliminary results for the gluon polarisation from hi…
The COMPASS experiment will determine the gluon polarization in the nucleon $ΔG/G$ from the double helicity asymmetry measured in the scattering of a 160 GeV muon beam on a longitudinally polarized deuteron target, by selecting the photon-gluon fusion reaction. This reaction can be tagged either by the production of open charm, or by the production of high $p_T$ hadron pairs. The first asymmetry obtained with the latter method is presented.
Measurements of the gluon polarization $\frac{ΔG}{G}$ via the open charm channel and based on the helicity asymmetry of large transverse-momentum hadrons in the final state are presented. The data have been collected in the years 2002-2004 by the COMPASS experiment at CERN using a 160 GeV/c polarized muon beam scattered off a polarized $^6$LiD target. The new result for $\frac{ΔG}{G}$ from the charm channel is $-0.57 \pm 0.41 (stat.)$ at $x_G \simeq 0.15$ and scale $μ^2 \simeq 13 $ (GeV/c)$^2$. The gluon polarization from high-$p_T$ hadron pairs is $\frac{ΔG}{G} = 0.016\pm 0.058 (stat.)\pm 0.0…
Exotic charmonium-like states have been targeted by various experiments in the last 15 years, but their nature still is unknown. Photo-(muo)production is a new promising instrument to study them. COMPASS, a fixed target experiment at CERN, analyzed the full set of the data collected with a muon beam between 2002 and 2011, covering the range from 7 GeV to 19 GeV in the centre-of-mass energy of the (virtual)photon-nucleon system. Production of the X(3872) state in the reaction $μ^+~N \rightarrow μ^+(J/ψπ^+π^- )π^{\pm} N'$ has been observed with a statistical significance of around 5 $σ$. The sha…
One of the important goals of the COMPASS experiment is the determination of the gluonic contribution to the spin of the nucleon. By using the photon gluon fusion (PGF) process in polarised deep inelastic scattering direct access to the gluon polarisation can be obtained. The PGF is selected via the detection of open charm or high $p_T$ hadron pairs. In 2001 a large part of the experiment had been setup and during a two weeks period first data was taken. In this paper some achievements of the run in 2001 are presented together with first preliminary analysis results.
The recently proposed COMPASS experiment at CERN attempts a measurement of the gluon polarisation with a precision of delta(Delta g/g) = 0.1. The experiment uses open charm muoproduction to tag the photon-gluon fusion process.
One of the key objectives of the COMPASS experiment at CERN is the determination of the gluon contribution to the nucleon spin. The gluon polarization is measured via photon-gluon fusion in deep-inelastic scattering of 160 GeV/c polarized muons on a polarized 6LiD solid-state target. Photon-gluon fusion is tagged by the observation of charmed mesons or the production of hadron pairs with large transverse momenta p_t. The status of the analysis of the D0 and D0* events and of the high-p_t hadron pairs is shown. The gluon polarization ΔG/G has been determined from the asymmetry of high-p_t hadro…
We present the first measurement of the gluon polarisation in the nucleon based on the photon-gluon fusion process tagged by charmed meson production and decay to charged K and pi. The data were collected in polarised muon scattering off a polarised deuteron target by the COMPASS collaboration at CERN during 2002-2004. The result of this LO analysis is <Delta g/g>_x = -0.47 +- 0.44 (stat) +- 0.15 (syst) at <x> ~= 0.11 and a scale mu^2 ~ 13 (GeV/c)^2.
One of the goals of the COMPASS experiment is the determination of the gluon polarisation ΔG/G, for a deep understanding of the spin structure of the nucleon. In DIS the gluon polarisation can be measured via the Photon-Gluon-Fusion (PGF) process, identified by open charm production or by selecting high p_T hadron pairs in the final state. The data used for this work were collected by the COMPASS experiment during the years 2002-2004, using a 160 GeV naturally polarised positive muon beam scattering on a polarised nucleon target. A new preliminary result of the gluon polarisation ΔG/G from hig…
The COMPASS experiment is a fixed target experiment at the CERN SPS using muon and hadron beams for the investigation of the spin structure of the nucleon and hadron spectroscopy. The main objective of the muon physics program is the study of the spin of the nucleon in terms of its constituents, quarks and gluons. COMPASS has accumulated data during 6 years scattering polarized muons off a longitudinally or a transversely polarized deuteron (6LiD) or proton (NH3) target. Results for the gluon polarization are obtained from longitudinal double spin cross section asymmetries using two different …
The gluon polarisation in the nucleon has been determined by detecting charm production via D0 meson decay to charged K and pi in polarised muon scattering off a longitudinally polarised deuteron target. The data were taken by the COMPASS Collaboration at CERN between 2002 and 2006 and corresponds to an integrated luminosity of 2.8 fb^-1. The dominant underlying process of charm production is the photon-gluon fusion to a cc-bar pair. A leading order QCD approach gives an average gluon polarisation of (Delta g/g)_x= -0.49 +- 0.27(stat) +- 0.11(syst) at a scale mu^2 ~ 13 (GeV/c)^2 and at an aver…
The measurement of time-dependent CP asymmetries in charm decays can provide a unique insight into the flavor changing structure of the Standard Model. We examine a number of different CP eigenstate decays of D mesons and describe a method that can be used to measure time-dependent CP asymmetries at existing and future experiments, with a preliminary assessment, based on statistical considerations, of their various capabilities. Any asymmetry observed in time-dependent analysis of neutral D mesons could signify new physics. We discuss the measurements required to perform direct and indirect te…
This paper will review the data collected by the H1 and ZEUS Collaborations concerning the production of heavy quarks (charm and beauty) at the electron-proton collider HERA. Heavy quark production is an important testing ground for quantum chromodynamics (QCD), because QCD calculations are expected to be reliable if a hard scale is present in the process. In heavy quarks production a hard scale is provided by the quark mass. Moreover heavy quarks production can give direct access to the gluon density in the proton due to the fact that it proceeds, in QCD, almost exclusively via photon-gluon f…
We investigate the semi-inclusive production of hidden-charm exotic states, including $X(3872)$, $Z_c$, $Z_{cs}$ and the pentaquark $P_c$ states, in lepton-proton scattering processes. These hadrons are close to the thresholds of a pair of charm and anticharm hadrons, and are assumed to have hadronic molecular structure as their main components. In order to give order-of-magnitude estimates of the cross sections, we use Pythia to simulate the short-distance productions of the constituent hadrons, which then rescatter to form the exotic hadrons. The estimates for the $X(3872)$ and $Z_c(3900)^\p…
We present estimates of Single Spin Asymmetry (SSA) in the electroproduction of $J/ψ$ taking into account the transverse momentum dependent (TMD) evolution of the gluon Sivers function and using Color Evaporation Model of charmonium production. We estimate SSA for JLab, HERMES, COMPASS and eRHIC energies using recent parameters for the quark Sivers functions which are fitted using an evolution kernel in which the perturbative part is resummed up to next-to-leading logarithms (NLL) accuracy. We find that these SSAs are much smaller as compared to our first estimates obtained using DGLAP evoluti…
We propose measurement of transverse single spin asymmetry (SSA) in charmonium production as a probe of gluon Sivers function. We estimate SSA in low virtuality electroproduction of $J/ψ$ using color evaporation model of charmonium production and existing models of the gluon Sivers function and find sizable asymmetry at JLab, HERMES, COMPASS and eRHIC energies.
We present the study of the production of double $J/ψ$ mesons using COMPASS data collected with a 190 GeV/$c$ $π^-$ beam scattering off NH$_{3}$, Al and W targets. Kinematic distributions of the collected double $J/ψ$ events are analysed, and the double $J/ψ$ production cross section is estimated for each of the COMPASS targets. The results are compared to predictions from single- and double-parton scattering models as well as the pion intrinsic charm and the tetraquark exotic resonance hypotheses. It is demonstrated that the single parton scattering production mechanism gives the dominant con…
We present a first global determination of spin-dependent parton distribution functions (PDFs) and their uncertainties using the NNPDF methodology: NNPDFpol1.1. Longitudinally polarized deep-inelastic scattering data, already used for the previous NNPDFpol1.0 PDF set, are supplemented with the most recent polarized hadron collider data for inclusive jet and $W$ boson production from the STAR and PHENIX experiments at RHIC, and with open-charm production data from the COMPASS experiment, thereby allowing for a separate determination of the polarized quark and anti-quark PDFs, and an improved de…
The gluon polarisation in the nucleon was measured using open charm production by scattering 160 GeV/c polarised muons off longitudinally polarised protons or deuterons. The data were taken by the COMPASS collaboration between 2002 and 2007. A detailed account is given of the analysis method that includes the application of neural networks. Several decay channels of D^0 mesons are investigated. Longitudinal spin asymmetries of the D meson production cross-sections are extracted in bins of D^0 transverse momentum and energy. At leading order QCD accuracy the average gluon polarisation is determ…
The $a_1(1420)$ with $I^G(J^{PC})= 1^-(1^{++})$ observed in the $π^+ f_0(980)$ final state in the $π^-p\to π^+π^-π^- p$ process by the COMPASS collaboration seems unlikely to be an ordinary $\bar qq$ mesonic state. Available theoretical explanations include tetraquark or rescattering effects due to $a_1(1260)$ decays. If the $a_1(1420)$ were induced by the rescattering, its production rates are completely determined by those of the $a_1(1260)$. In this work, we propose to explore the ratios of branching fractions of heavy meson weak decays into the $a_1(1420)$ and $a_1(1260)$, and testing the …
The production of D* and D mesons in inelastic scattering of 160 GeV/c muons off a ^6LiD target has been investigated with the COMPASS spectrometer at CERN for 0.003 (GeV/c)^2 < Q^2 < 10 (GeV/c)^2 and 3x10^-5< x_Bj < 0.1. The study is based on 8100 events where a D^0 or anti D^0 is detected subsequently to a D*+ or D*- decay, and on 34000 events, where only a D^0 or anti D^0 is detected. Kinematic distributions of D*, D and K*_2 are given as a function of their energy E, transverse momentum p_T, energy fraction z, and of the virtual photon variables nu, Q^2 and x_Bj. Semi-inclusive differentia…
