GPD program at COMPASS

A major part of the future COMPASS program is dedicated to investigation of the nucleon structure by studying Generalised Parton Distributions (GPDs) through Deeply Virtual Compton Scattering (DVCS) andMeson Production (DVMP). Already new results of the transverse target spin azimuthal asymmetry Asin(φ−φs) UT for hard exclusive ρ0-meson production on a transversely polarized target have been obtained. Now the realisation of the first DVCS test run with a 2.5m long liquid hydrogen target surrounded by a new ToF system and an extended calorimetry has been achieved. 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 measure the xB-dependence of the t-slope of the pure DVCS cross section to study nucleon tomography. In the future we consider to use a transversely polarized proton target to constrain the GPD E

Hard Exclusive Processes at COMPASS and COMPASS-II

A major part of the future COMPASS program is dedicated to the study of Deeply Virtual Compton Scattering (DVCS) and Meson Production (DVMP) to investigate nucleon structure through Generalised Parton Distributions (GPD). The high energy of the muon beam allows us to measure the xB-dependence of the t-slope of the pure DVCS cross section and to study nucleon tomography. In a first phase the use of positive and negative polarised muon beams and of an unpolarised proton target allow measurements of the Beam Charge and Spin Difference of the DVCS cross sections to access the Compton form factor related to the dominant GPD H. In a second phase we consider to use a transversely polarized proton target to constrain the GPD E

Possible measurements of GPDs at COMPASS

This paper presents the reactions which can be performed at COMPASS to study the Generalized Parton Distributions (GPDs). The high energy muon beam at CERN allows to measure Hard Exclusive Meson Production or Deeply Virtual Compton Scattering (DVCS) in the Bjorken regime in a large range of Q^2 and x_Bj (1.5<Q^2<7.5 GeV^2 and 0.03<x_Bj<0.25). Exploratory measurements dedicated to rho_0 or pi_0 production can be investigated with the present setup. DVCS measurement require an upgrade of the COMPASS setup.Comment: 20 pages, 9 figure

New pixelized Micromegas detector for the COMPASS experiment

New Micromegas (Micro-mesh gaseous detectors) are being developed in view of the future physics projects planned by the COMPASS collaboration at CERN. Several major upgrades compared to present detectors are being studied: detectors standing five times higher luminosity with hadron beams, detection of beam particles (flux up to a few hundred of kHz/mm^2, 10 times larger than for the present detectors) with pixelized read-out in the central part, light and integrated electronics, and improved robustness. Studies were done with the present detectors moved in the beam, and two first pixelized prototypes are being tested with muon and hadron beams in real conditions at COMPASS. We present here this new project and report on two series of tests, with old detectors moved into the beam and with pixelized prototypes operated in real data taking condition with both muon and hadron beams.Comment: 11 pages, 5 figures, proceedings to the Micro-Pattern Gaseous Detectors conference (MPGD2009), 12-15 June 2009, Kolympari, Crete, Greece Minor details added and language corrections don

New pixelized Micromegas detector with low discharge rate for the COMPASS experiment

New Micromegas (Micro-mesh gaseous detectors) are being developed in view of the future physics projects planned by the COMPASS collaboration at CERN. Several major upgrades compared to present detectors are being studied: detectors standing five times higher luminosity with hadron beams, detection of beam particles (flux up to a few hundred of kHz/mm^{2}, 10 times larger than for the present Micromegas detectors) with pixelized read-out in the central part, light and integrated electronics, and improved robustness. Two solutions of reduction of discharge impact have been studied, with Micromegas detectors using resistive layers and using an additional GEM foil. Performance of such detectors has also been measured. A large size prototypes with nominal active area and pixelized read-out has been produced and installed at COMPASS in 2010. In 2011 prototypes featuring an additional GEM foil, as well as an resistive prototype, are installed at COMPASS and preliminary results from those detectors presented very good performance. We present here the project and report on its status, in particular the performance of large size prototypes with an additional GEM foil.Comment: 11 pages, 5 figures, proceedings to the Micro-Pattern Gaseous Detectors conference (MPGD2011), 29-31 August 2011, Kobe, Japa

Measurement of the Beam-Recoil Polarization in Low-Energy Virtual Compton Scattering from the Proton

Double-polarization observables in the reaction $\vec{e}p \rightarrow e'\vec{p'}\gamma{}$ have been measured at $Q^2=0.33 (GeV/c)^2$. The experiment was performed at the spectrometer setup of the A1 Collaboration using the 855 MeV polarized electron beam provided by the Mainz Microtron (MAMI) and a recoil proton polarimeter. From the double-polarization observables the structure function $P_{LT}^\perp$ is extracted for the first time, with the value $(-15.4 \pm 3.3 (stat.)^{+1.5}_{-2.4} (syst.)) GeV^{-2}$, using the low-energy theorem for Virtual Compton Sattering. This structure function provides a hitherto unmeasured linear combination of the generalized polarizabilities of the proton

The helicity amplitudes A1/2_{1/2} and A3/2_{3/2} for the D13(1520)_{13}(1520) resonance obtained from the γ⃗p⃗→pπ0\vec{\gamma} \vec{p} \to p \pi^0 reaction}

The helicity dependence of the $\vec{\gamma} \vec{p} \to p \pi^0$ reaction has been measured for the first time in the photon energy range from 550 to 790 MeV. The experiment, performed at the Mainz microtron MAMI, used a 4$\pi$-detector system, a circularly polarized, tagged photon beam, and a longitudinally polarized frozen-spin target. These data are predominantly sensitive to the $D_{13}(1520)$ resonance and are used to determine its parameters.Comment: 5 pages, 4 figure

The first determination of Generalized Polarizabilities of the proton by a Virtual Compton Scattering experiment

Absolute differential cross sections for the reaction (e+p -> e+p+gamma) have been measured at a four-momentum transfer with virtuality Q^2=0.33 GeV^2 and polarization \epsilon = 0.62 in the range 33.6 to 111.5 MeV/c for the momentum of the outgoing photon in the photon-proton center of mass frame. The experiment has been performed with the high resolution spectrometers at the Mainz Microtron MAMI. From the photon angular distributions, two structure functions which are a linear combination of the generalized polarizabilities have been determined for the first time.Comment: 4 pages, 3 figure

First measurement of the Gerasimov-Drell-Hearn integral for Hydrogen from 200 to 800 MeV

A direct measurement of the helicity dependence of the total photoabsorption cross section on the proton was carried out at MAMI (Mainz) in the energy range 200 < E_gamma < 800 MeV. The experiment used a 4$\pi$ detection system, a circularly polarized tagged photon beam and a frozen spin target. The contributions to the Gerasimov-Drell-Hearn sum rule and to the forward spin polarizability $\gamma_0$ determined from the data are 226 \pm 5 (stat)\pm 12(sys) \mu b and -187 \pm 8 (stat)\pm 10(sys)10^{-6} fm^4, respectively, for 200 < E_\gamma < 800 MeV.Comment: 6 pages, 3 figures, 3 table

First Measurement of the Transverse Spin Asymmetries of the Deuteron in Semi-Inclusive Deep Inelastic Scattering

First measurements of the Collins and Sivers asymmetries of charged hadrons produced in deep-inelastic scattering of muons on a transversely polarized 6-LiD target are presented. The data were taken in 2002 with the COMPASS spectrometer using the muon beam of the CERN SPS at 160 GeV/c. The Collins asymmetry turns out to be compatible with zero, as does the measured Sivers asymmetry within the present statistical errors.Comment: 6 pages, 2 figure