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 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

Generalized Parton Distributions of the Proton in Position Space : Zero Skewness

We present a study of the parton distributions in transverse position or impact parameter space using a recent parametrization of u and d quark generalized parton distributions (GPDs) $H(x,t)$ and $E(x,t)$ at zero skewness. We make a comparative study between different parametrizations and discuss the region of validity of the positivity condition.Comment: 14 pages, 5 figure

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

Longitudinal double spin asymmetries in single hadron quasi-real photoproduction at high pTp_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 $0.05 < x_g < 0.2$

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 difference of the azimuthal angles of the two hadrons. The similarity of transversity induced single and di-hadron asymmetries is discussed. A new analysis of the data allows to establish quantitative relationships among them, providing for the first time strong experimental indication that the underlying fragmentation mechanisms are all driven by a common physical process.Comment: 6 figure

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 difference of the azimuthal angles of the two hadrons. The similarity of transversity induced single and di-hadron asymmetries is discussed. A new analysis of the data allows to establish quantitative relationships among them, providing for the first time strong experimental indication that the underlying fragmentation mechanisms are all driven by a common physical process.Comment: 6 figure