T. Hotta

Evidence for the Θ+ in the γd→K+K-pn reaction by detecting K+K- pairs

The gamma d to K^+K^-pn reaction has been studied to search for the evidence of the Theta^+ by detecting K^+K^- pairs at forward angles. The Fermi-motion corrected nK^+ invariant mass distribution shows a narrow peak at 1.524 +- 0.002 + 0.003 GeV/c^2. The statistical significance of the peak calculated from a shape analysis is 5.1 sigma, and the differential cross-section for the gamma n to K^- Theta^+ reaction is estimated to be 12 +- 2 nb/sr in the photon energy range from 2.0 GeV to 2.4 GeV in the LEPS angular range by assuming the isotropic production of the Theta^+ in the gamma n center-of-mass system. The obtained results support the existence of the Theta^+.

Measurement of spin-density matrix elements for φ-meson photoproduction from protons and deuterons near threshold

The LEPS/SPring-8 experiment made a comprehensive measurement of the spin-density matrix elements for gammap->phip, gammad->phipn, and gammad->phid at forward production angles. A linearly polarized photon beam at E{sub g}amma=1.6-2.4 GeV was used for the production of phi mesons. The natural-parity Pomeron exchange processes remain dominant near threshold. The unnatural-parity processes of pseudoscalar exchange is visible in the production from the nucleons but is greatly reduced in the coherent production from deuterons. There is no strong E{sub g}amma dependence, but there is some dependence on momentum transfer. A small but finite value of the spin-density matrix elements reflecting helicity-nonconserving amplitudes in the t-channel is observed.

New projects at SPring-8 with multi-GeV polarized photons

Abstract The GeV photon beam at SPring-8 is produced by backward-Compton scattering of laser photons from 8 GeV electrons. Polarization of the photon beam reaches to 100 % at the maximum energy if laser photons are 100 % circular polarized. The photon energy will be above 3 GeV thanks to the high energy of the circulating electron beam. The photon energy is measured by tagging the recoiled electron. The photon beam well defined in both the polarization and energy becomes a clean and sharp probe to study the baryon and meson structures in terms of quarks and their dynamics. We present the outline of the project. A few selected topics studied in the SPring-8 Laser-Electron Photon Collaboration are also discussed.

Backward-angle photoproduction of π0 mesons on the proton at Eγ=1.5–2.4 GeV

Abstract Differential cross sections and photon beam asymmetries for π 0 photoproduction have been measured at E γ = 1.5 – 2.4 xa0GeV and at the π 0 scattering angles, − 1 cos Θ c . m . − 0.6 . The energy-dependence of differential cross sections for u -channel π 0 production has been determined. An enhancement at backward angles is found above E γ = 1.9 xa0GeV . This is likely to be due to u -channel diagrams. Photon beam asymmetries have been obtained for the first time at backward angles, showing a strong angular dependence at E γ > 2.0 xa0GeV .

Backward-angle η photoproduction from protons at Eγ=1.6-2.4

Differential cross sections for {eta} photoproduction from protons have been measured at E{sub {gamma}}=1.6-2.4 GeV in the backward direction. A bump structure has been observed above 2.0 GeV in the total energy. No such bump is observed in {eta}{sup },{omega}, and {pi}{sup 0} photoproductions. It is inferred that this unique structure in {eta} photoproduction is due to a baryon resonance with a large ss component that is strongly coupled to the {eta}N channel.

Backward-angle eta photoproduction from protons at E(gamma) = 1.6 - 2.4 GeV

Differential cross sections for {eta} photoproduction from protons have been measured at E{sub {gamma}}=1.6-2.4 GeV in the backward direction. A bump structure has been observed above 2.0 GeV in the total energy. No such bump is observed in {eta}{sup },{omega}, and {pi}{sup 0} photoproductions. It is inferred that this unique structure in {eta} photoproduction is due to a baryon resonance with a large ss component that is strongly coupled to the {eta}N channel.

First experiment at RCNP on pp-bremsstrahlung at 400 MeV

Abstract Proton-proton bremsstrahlung events were measured at 400 MeV proton energy. Two arm spectrograph and a liquid-hydrogen target system with well tuned beam provided good event identification.

Backward production of ω mesons by linearly polarized photons at SPring-8/LEPS

Abstract The γp → ωp reaction has been measured at the photon energies from 1.5 GeV to 2.4 GeV by using a linearly polarized photon beam. A forward proton from backward gw photoproduction has been detected with the LEPS spectrometer. Preliminary results from the first SPring-8/LEPS data are presented.

2π0 photoproduction experiment at SPring-8

Abstract The γ A → π 0 π 0 X reactions is one of the tools to effectively investigate partial restoration of chiral symmetry. An experiment was performed at the LEPS/SPring-8 facility using various nuclear targets. In this experiment, a 2π calorimeter consisting of 252 Lead/SCIFI modules was used to detect the photons coming from the π 0 π 0 decay. A preliminary analysis shows that π 0 π 0 X events are observed.