Measurement of Spin-Density Matrix Elements for $phi$-Meson Photoproduction from Protons and Deuterons Near Threshold

The LEPS/SPring-8 experiment made a comprehensive measurement of the spin-density matrix elements for $gamma p to phi p$, $gamma d to phi p n$ and $gamma d to phi d$ at forward production angles. A linearly polarized photon beam at $E_{gamma}$=1.6-2.4 GeV was used for the production of $phi$ mesons. The natural-parity Pomeron exchange processes remains dominant nearthreshold. The unnatural-parity processes of pseudoscalar exchange is visible in the production from nucleons but is greatly reduced in the coherent production from deuterons. There is no strong $E_{gamma}$-dependence, but 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.

Time and Amplitude of Afterpulse Measured with a Large Size Photomultiplier Tube

We have studied the afterpulse of a hemispherical photomultiplier tube for an upcoming reactor neutrino experiment. The timing, the amplitude, and the rate of the afterpulse for a 10 inch photomultiplier tube were measured with a 400 MHz FADC up to 16 ms time window after the initial signal generated by an LED light pulse. The time and amplitude correlation of the afterpulse shows several distinctive groups. We describe the dependencies of the afterpulse on the applied high voltage and the amplitude of the main light pulse. The present data could shed light upon the general mechanism of the afterpulse.

Backward-angle photoproduction of eta meson from protons at E_gamma = 1.6 - 2.4 GeV

Differential cross sections for eta photoproduction off protons have been measured at E_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 cross sections for eta, omega and pi0 photoproductions. It is inferred that this unique structure in eta photoproduction is due to a baryon resonance with a large portion of ss-bar strongly coupled to the etaN channel.

Near-threshold Lambda(1520) production by the gamma p -> K+Lambda(1520) reaction at forward K+ angles

Differential cross sections and photon-beam asymmetries for the gamma p -> K+ Lambda(1520) reaction have been measured with linearly polarized photon beams at energies from the threshold to 2.4 GeV at 0.6<cos(theta)<1. A new bump structure was found at W=2.11 GeV in the cross sections. The bump is not well reproduced by theoretical calculations introducing a nucleon resonance with J<=3/2. This result suggests that the bump might be produced by a nucleon resonance possibly with J>=5/2 or by a new reaction process, for example an interference effect with the phi photoproduction having a similar bump structure in the cross sections.

Backward-angle photoproduction of $pi^0$ mesons on the proton at $E_gamma$ = 1.5--2.4 GeV

Differential cross sections and photon beam asymmetries for $pi^0$ photoproduction have been measured at $E_gamma$ = 1.5--2.4 GeV and at the $pi^0$ scattering angles, --1 $<$ cos$Theta_{c.m.} <$ --0.6. The energy-dependent slope of differential cross sections for $u$-channel $pi^0$ production has been determined. An enhancement at backward angles is found above $E_gamma$ = 2.0 GeV. This is inferred to be due to the $u$-channel contribution and/or resonances. Photon beam asymmetries have been obtained for the first time at backward angles. A strong angular dependence has been found at $E_gamma >$ 2.0 GeV, which may be due to the unknown high-mass resonances.