Color Transparency at COMPASS via Exclusive Coherent Vector Meson Production

We examine the potential of the COMPASS experiment at CERN to study color transparency via exclusive coherent vector meson production in hard muon-nucleus scattering. It is demonstrated that COMPASS has high sensitivity to test this important prediction of perturbative QCD.Comment: Contribution to Proceedings of 2002 Praha Advanced Study Institute "Symmetries and Spin" Workshop, Praha-SPIN-2002, July 2002, Prague, Czech Republic. File has 13 pages, four figures. Conference site is http://mfinger.home.cern.ch/mfinger/praha200

Color Transparency at COMPASS - Feasibility Study

We examine the potential of the COMPASS experiment at CERN to study color transparency via exclusive vector meson production in hard muon-nucleus scattering. It is demonstrated that COMPASS has high sensitivity to test this important prediction of perturbative QCD.Comment: Expanded version of the talk presented at the Workshop on "Nucleon Structure and Meson Spectroscopy", Dubna, Russia, 10-11 October 200

Exclusive meson production at COMPASS

We explore the feasibility to study exclusive meson production (EMP) in hard muon-proton scattering at the COMPASS experiment. These measurements constrain the off-forward parton distributions (OFPD's) of the proton, which are related to the quark orbital contribution to the proton spin.Comment: Workshop on Exclusive and Semi-Exclusive Processes at High Momentum Transfer, Jefferson Lab, 20-22 Mai 199

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. © 2015

Recent results from COMPASS on exclusive muoproduction

The 160 GeV polarised muon beam available at CERN, with positive or negative charge, makes COMPASS a unique place for GPD studies. The first GPD related COMPASS results come from exclusive vector meson production on transversely polarised protons and deuterons. The data were taken in 2003-2010 with large solid-state polarised targets, although without detection of recoil particles. Results on various transverse target spin dependent azimuthal asymmetries are presented and their relations to GPDs are discussed. The dedicated COMPASS GPD program started in 2012 with commissioning of a new long liquid hydrogen target and new detectors such as the large recoil proton detector and the large-angle electromagnetic calorimeter. It was followed by a short pilot 'DVCS run'. The performance of the setup and first results on DVCS and exclusive π(0) channels have been demonstrated. The full data taking for the GPD program approved within COMPASS-II proposal is planned for 2016 and 2017

Recent results from COMPASS on the GPD program

The dedicated COMPASS GPD program started in 2012 with a short pilot ’DVCS run’ and the first results from this run on DVCS, exclusive π0 and exclusive ω channels have been obtained. From the sum of cross sections measured with positive and negative beam polarities the DVCS cross-section and its dependence on the squared four-momentum transfer t have been extracted. It allowed a first estimate of the transverse size of the partonic distribution of the nucleon √〈r2⊥〉 i in a range of intermediate xBj with a mean xBj value of 0.056. We report also on the absolute value of the cross section for exclusive π0 production and its dependence on t and azimuthal angle ϕ between the scattering plane and π0 production plane. These measurements are aiming to constrain the GPDs, in particular chiral-odd (transversity) GPDs. Further, we present results on the spin density matrix elements for exclusive ω meson production and discuss the contributions from unnaturaly parity exchanges and from transitions that do not conserve helicity

Diffractive vector meson production at COMPASS and plans for GPDs measurements

Preliminary results on the spin dependence of exclusive $\rho^{0}$ muoproduction at COMPASS are discussed. The $Q^{2}$ dependence of several spin density matrix elements and of $R = \sigma_{L}\sigma_{T}$ is presented. The data are consistent with a substantial increase of $R$ with $Q^{2}$ and a weak violation of SCHC. Next, the longitudinal double-spin asymmetry $A^{\rho}_{1}$ for $\rho^{0}$ production on polarized deuterons is shown as a function of $Q^{2}$ and $x_{Bj}$. The asymmetry is compatible with zero in the whole kinematical range. Finally, we discuss the prospects for measuring DVCS and exclusive meson production at COMPASS in order to investigate GPDs

Transverse extension of partons in the proton probed by DVCS at COMPASS

We present the first result on exclusive single-photon muoproduction on the proton in the range of intermediate values of the Bjorken variable xBj , which was recently published by COMPASS collaboration. The measurements were performed using 160GeV/c polarised μ + and μ − beams of the CERN SPS scattering off a liquid hydrogen target. We determine the dependence of the average of the measured μ + and μ − cross sections for deeply virtual Compton scattering on the squared four-momentum transfer t from the initial to the final proton. The slope B of the t-dependence is fitted with a single exponential function, which yields . This result can be converted into an average transverse extension of partons in the proton, . For this measurement, the average virtuality of the photon mediating the interaction is ⟨Q 2⟩ = 1.8 (GeV/c)2 and the average value of the Bjorken variable is ⟨xBj ⟩ = 0.056. We also compare COMPASS result to earlier estimates at very small xBj from HERA