• LEPS2/BGOegg Experiments

  • Fig.1 A picture of BGOegg inside the thermostatic booth (Left) and the drawings of BGOegg (Right).
    A large acceptance electromagnetic (EM) calorimeter BGOegg (Fig.1) was constructed at ELPH, Tohoku University. This calorimeter system has been transferred to the new laser Compton scattering beamline LEPS2 at SPring-8, where a 1.3-2.9 GeV photon beam with high linear polarization is available. New hadron experiments have started from 2014 April with the EM calorimeter BGOegg and the additional detectors for charged particles.

    Physics

    The origin of hadron mass is being studied by searching for mesic nuclei. We are particularly interested in the η' meson, whose mass is distictively larger than the other pseudoscalar meson masses due to the UA(1) anomaly. It is expected that the partial restoration of the chiral symmetry breaking, which may strongly affect the UA(1) anomaly, reduces the η' mass by 80-150 MeV at the nuclear density in the theoretical calculations based on the Nambu-Jona-Lasinio (NJL) model and the linear sigma model. The LEPS2/BGOegg experiments measure the momentum of an extremely forward proton by the time-of-flight and remove the large amout of backgrounds by the EM calorimeter.

    We also aim to investigate a variety of hadron pictures through resonance studies. In the combination with the 0.6-1.3 GeV bremsstrahlung photon beam available at the booster synchrotron ring of ELPH, we can cover the wide range of photon beam energies continuously. For example, excited nucleon resonances are searched for by the total cross section measurements of the η, η', ω, π0π0, π0η photoproduction. The spin observables with the linearly polarized photon beam must help to clarify the resonance natures.

    Detectors

    Fig.2 The detector setup of the LEPS2/BGOegg experiments.
    Figure 2 shows the detector setup of the LEPS2/BGOegg experiments. The EM calorimeter BGOegg consists of 1320 BGO crystals (20 radiation lengths), surrounding a target in the polar angle region of 24 to 144 degrees with the world's best energy resolution (1.3% at 1 GeV). Charged particles are identified at the inner plastic scintillators (IPS) and the cylindrical drift chamber (CDC) inside the BGOegg. The forward acceptance hole is covered by the planar drift chamber (DC, θ<21 degree) and the resistive plate chambers (RPC, θ<7 degree). The resolution of the time-of-flight measurement at the RPC is about 80 psec, resulting in the proton momentum resolution of about 1% at 2 GeV/c with the flight length of 12.5 m. As a target material, we use the liquid hydrogen / deuterium target system with a long nose or a nuclear target (Carbon, polyethylene, etc) depending on the physics programs.

    Status

    The LEPS2/BGOegg experiments are carried out under the collaboration of ELPH (Tohoku University), RCNP (Osaka University), Kyoto University, KEK, RIKEN, JASRI (SPring-8), and several other institutes in the world. ELPH and RCNP cooperate the LEPS2 facility.

    2012Dec The EM calorimeter BGOegg was transferred to the LEPS2 experimental building at SPring-8
    after the completion of its construction at ELPH.
    2013Jan The construction of the LEPS2 beamline was completed.
    The first photon beam was observed at the new beamline.
    2013Dec Test data was taken after the preparations of individual detectors and DAQ.
    2014Apr Started the collection of physics data with a nuclear target.
    2014Nov Started the collection of physics data with a liquid hydrogen target after several operation tests.
    2015Apr Restarted the collection of physics data with a nuclear target.

    Collaboration List

    Publications

    Proposal / Letter of Intent

    BGOegg exp. LOI [Nov. 2012] English Version, Japanese Version

    Links

    Quark-Nuclear Physics Division at ELPH

    ELPH, Tohoku University

    Laser Electron Photon Experiments at SPring-8 (LEPS/LEPS2)

    RCNP, Osaka University

    SPring-8

    Advanced Meson Science Lab., RIKEN Nishina Center