92 research outputs found

    Hadron Spectroscopy with CLAS and CLAS12

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    The CLAS detector completed a highly successful data-taking lifetime in 2012. The mass range accessible with the CEBAF 6 GeV electron beam and the large solid angle of the detector made it the premier facility for studying baryon resonances. The analysis of a great number of final states with differing polarization states is underway and will lead to a greater understanding of the existing nucleon resonances. In the meantime, preparation for an upgraded detector, CLAS12, is progressing. With commissioning due to start in 2016 with a 11 GeV electron beam CLAS12 will kinematically favour the study of mesons. In particular, it will look to resolve outstanding questions as to the make-up of mesonic states in terms of hybrid states with gluonic constituents; states of pure glue; or tetraquark/molecular configurations

    Model discrimination in pseudoscalar-meson photoproduction

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    To learn about a physical system of interest, experimental results must be able to discriminate among models. We introduce a geometrical measure to quantify the distance between models for pseudoscalar-meson photoproduction in amplitude space. Experimental observables, with finite precision, map to probability distributions in amplitude space, and the characteristic width scale of such distributions needs to be smaller than the distance between models if the observable data are going to be useful. We therefore also introduce a method for evaluating probability distributions in amplitude space that arise as a result of one or more measurements, and show how one can use this to determine what further measurements are going to be necessary to be able to discriminate among models

    The CLAS12 software framework and event reconstruction

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    We describe offline event reconstruction for the CEBAF Large Acceptance Spectrometer at 12 GeV (CLAS12), including an overview of the offline reconstruction framework and software tools, a description of the algorithms developed for the individual detector subsystems, and the overall approach for charged and neutral particle identification. We also present the scheme for data processing and the code management procedures

    Dependence of the 12^{12}C(γ⃗\vec{\gamma},pd) reaction on photon linear polarisation

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    The sensitivity of the 12^{12}C(γ⃗,pd)(\vec{\gamma},pd) reaction to photon linear polarisation has been determined at MAMI, giving the first measurement of the reaction for a nucleus heavier than 3^{3}He. Photon asymmetries and cross sections were measured for EγE_{\gamma}=170 to 350 MeV. For EγE_{\gamma} below the Δ\Delta resonance, reactions leaving the residual 9^{9}Be near its ground state show a positive asymmetry of up to 0.3, similar to that observed for 3^{3}He suggesting a similar reaction mechanism for the two nuclei.Comment: 4 pages, 2 figure

    Measurements of 12C(→γ,pp) photon asymmetries for Eγ= 200–450 MeV

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    The 12C (→γ ,pp) reaction has been studied in the photon energy range 200-450 MeV at the Mainz microtron MAMI-C, where linearly polarised photons were energy-tagged using the Glasgow-Mainz Tagged Photon Spectrometer and protons were detected in the Crystal Ball detector. The photon asymmetry Σ has been measured over a wider Eγ range than previous measurements. The strongest asymmetries were found at low missing energies where direct emission of nucleon pairs is expected. Cuts on the difference in azimuthal angles of the two ejected protons increased the magnitude of the observed asymmetries. At low missing energies the Σ data exhibit a strong angular dependence, similar to deuteron photodisintegration

    Nucleon electromagnetic form factors in a quark-gluon core model

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    We study the nucleon electromagnetic form factors in a quark-gluon core model framework, which can be viewed as an extension of the Isgur-Karl model of baryons. Using this picture we derive nucleon electromagnetic dipole form factors at low Q^2 and the deviation from the dipole form at high Q^2, that are consistent with the existing experimental data.Comment: 5 pages, 3 figure

    First measurement of the circular beam asymmetry in the gamma p --> pi0 eta p reaction

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    The circular photon asymmetry for pi0 eta photoproduction on the proton was measured for the first time at the tagged photon facility of the MAMI C accelerator using the Crystal Ball/TAPS photon spectrometer. The experimental results are interpreted within a phenomenological isobar model that confirms the dominant role of the Delta(1700)D33 resonance. The measured asymmetry allows us to identify small contributions from positive-parity resonances via interference terms with the dominant D33 amplitude.Comment: 11 pages, 3 figures, submitted to Phys.Lett.

    Measurement of the beam-helicity asymmetry in photoproduction of π0η pairs on carbon, aluminum, and lead

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    The beam-helicity asymmetry was measured, for the first time, in photoproduction of π0η pairs on carbon, aluminum, and lead, with the A2 experimental setup at MAMI. The results are compared to an earlier measurement on a free proton and to the corresponding theoretical calculations. The Mainz model is used to predict the beam-helicity asymmetry for the nuclear targets. The present results indicate that the photoproduction mechanism for π0η pairs on nuclei is similar to photoproduction on a free nucleon. This process is dominated by the D33 partial wave with the ηΔ(1232) intermediate state
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