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Register a new userMuoproduction of exotic charmonia at COMPASS
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A. Guskov
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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 $mu^+~N rightarrow mu^+(J/psipi^+pi^- )pi^{pm} N$ has been observed with a statistical significance of around 5 $sigma$. The shape of the $pi^+pi^-$ mass distribution from the decay $X(3872)rightarrow J/psipi^+pi^-$ shows disagreement with previous observations. The product of the cross section and the branching fraction of the $X(3872)$ decay into $J/psipipi$ is estimated as 71$pm$28(stat)$pm$39(syst) pb. The results obtained for the production of the $Z_c^{pm}(3900)$ are also reported as well as future perspectives.
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Exotic charmonium-like states have been observed by various experiments over the last 15 years, but their nature is still under discussion. 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. A signal in the mass spectrum of $J/psipi^+pi^-$ with the statistical significance of 4.1 $sigma$ was observed in the reaction $mu^+~N rightarrow mu^+(J/psipi^+pi^- )pi^{pm} N$. Its mass and width are consistent with those of the $X(3872)$. The shape of the $pi^+pi^-$ mass distribution from the observed decay into $J/psipi^+pi^-$ is different from previous observations for $X(3872)$. The observed signal may be interpreted as possible evidence of a new charmonium state $widetilde{X}(3872)$. It could be associated with a neutral partner of $X(3872)$ with $C = -1$ predicted by a tetraquark model.
We have searched for exclusive production of exotic charmonia in the reaction $mu^+~N rightarrow mu^+ (J!/!psipi^+pi^-)pi^{pm}~N$ using COMPASS data collected with incoming muons of 160 GeV/$c$ and 200 GeV/$c$ momentum. In the $J!/!psipi^+pi^-$ mass distribution we observe a signal with a statistical significance of 4.1 $sigma$. Its mass and width are consistent with those of the $X(3872)$. The shape of the $pi^+pi^-$ mass distribution from the observed decay into $J!/!psipi^+pi^-$ shows disagreement with previous observations for $X(3872)$. The observed signal may be interpreted as a possible evidence of a new charmonium state. It could be associated with a neutral partner of $X(3872)$ with $C = -1$ predicted by a tetraquark model. The product of cross section and branching fraction of the decay of the observed state into $J!/!psipi^+pi^-$ is determined to be 71$pm$28(stat)$pm$39(syst) pb.
A major part of the future COMPASS program is dedicated to the investigation of the nucleon structure through Deeply Virtual Compton Scattering (DVCS) and Deeply Virtual Meson Production (DVMP). COMPASS will measure DVCS and DVMP reactions with a high intensity muon beam of 160 GeV and a 2.5 m-long liquid hydrogen target surrounded by a new TOF system. The availability of muon beams with high energy and opposite charge and polarization will allow to access the Compton form factor related to the dominant GPD $H$ and to study the $x_{B}$-dependence of the $t$-slope of the pure DVCS cross section and to study nucleon tomography. Projections on the achievable accuracies and preliminary results of pilot measurements will be presented.
Proton transverse-spin azimuthal asymmetries are extracted from the COMPASS 2010 semi-inclusive hadron measurements in deep inelastic muon-nucleon scattering in those four regions of the photon virtuality $Q^2$, which correspond to the four regions of the di-muon mass $sqrt{Q^2}$ used in the ongoing analysis of the COMPASS Drell-Yan measurements. This allows for a future direct comparison of the nucleon transverse-momentum-dependent parton distribution functions extracted from these two alternative measurements. Various two-dimensional kinematic dependences are presented for the azimuthal asymmetries induced by the Sivers transverse-momentum-dependent parton distribution function. 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.
The aim of the COMPASS hadron programme is to study the light-quark hadron spectrum, and in particular, to search for evidence of hybrids and glueballs. COMPASS is a fixed-target experiment at the CERN SPS and features a two-stage spectrometer with high momentum resolution, large acceptance, particle identification and calorimetry. A short pilot run in 2004 resulted in the observation of a spin-exotic state with $J^{PC} = 1^{-+}$ consistent with the debated $pi1(1600)$. In addition, Coulomb production at low momentum transfer data provide a test of Chiral Perturbation Theory. During 2008 and 2009, a world leading data set was collected with hadron beam which is currently being analysed. The large statistics allows for a thorough decomposition of the data into partial waves. The COMPASS hadron data span over a broad range of channels and shed light on several different aspects of QCD.
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