Differential Cross Section and Photon-Beam Asymmetry for the γ[over →]p → π^{-}Δ^{++}(1232) Reaction at Forward π^{-} Angles for E_{γ}=1.5-2.95 GeV

A multi-stage scintillation counter for GeV-scale multi-species ion spectroscopy in laser-driven particle acceleration experiments

Particle counting analysis (PCA) with a multi-stage scintillation detector shows a new perspective on angularly resolved spectral characterization of GeV-scale, multi-species ion beams produced by high-power lasers. The diagnosis provides a mass-dependent ion energy spectrum based on time-of-flight and pulse-height analysis of single particle events detected through repetitive experiments. With a novel arrangement of multiple scintillators with different ions stopping powers, PCA offers potential advantages over commonly used diagnostic instruments (CR-39, radiochromic films, Thomson parabola, etc.) in terms of coverage solid angle, detection efficiency for GeV-ions, and real-time analysis during the experiment. The basic detector unit was tested using 230-MeV proton beam from a synchrotron facility, where we demonstrated its potential ability to discriminate major ion species accelerated in laser-plasma experiments (i.e., protons, deuterons, carbon, and oxygen ions) with excellent energy and mass resolution. The proposed diagnostic concept would be essential for a better understanding of laser-driven particle acceleration, which paves the way toward all-optical compact accelerators for a range of applications.

Monitoring the build-up of hydrogen polarization for polarized hydrogen-deuteride (HD) targets with nuclear magnetic resonance (NMR) at 17 T

We report on the frozen-spin polarized hydrogen-deuteride (HD) targets for photoproduction experiments at SPring-8/LEPS. Pure HD gas with a small amount of ortho-H₂ (∼0.1%) and a very small amount of para-D₂ (∼0.001%) was liquefied and solidified by liquid helium. The temperature of the produced solid HD was reduced to about 30 mK with a dilution refrigerator. A magnetic field (17 T) was applied to the HD to grow the polarization with the static method. After the aging of the HD at low temperatures in the presence of a high-magnetic field strength for three months, the polarization froze. Almost all ortho-H₂ molecules were converted to para-H₂ molecules. Most remaining para-D₂ molecules were converted to ortho-D₂ molecules. The para-H₂ and ortho-D₂ molecules exhibited weak spin interactions with the HD. If the concentrations of the ortho-H₂ and para-D₂ were reduced appropriately at the beginning of the aging process, the aging time can be shortened. We have developed a new nuclear magnetic resonance (NMR) system to measure the relaxation times (T₁) of the ¹H and ²H nuclei with two frequency sweeps at the respective frequencies of 726 MHz and 111 MHz and succeeded in the monitoring of the polarization build-up at decreasing temperatures from 600 mK to 30 mK at 17 T. Automatic NMR measurements with the frequency sweeps enabled us to omit the use of a manual tuning circuit and to remove magnetic field sweeps with eddy current heat. This technique enables us to optimize the concentration of the ortho-H₂ and to efficiently polarize the HD target within a shortened aging time.

Recent results of charged pion and kaon photoproduction on the proton at SPring-8/LEPS

We carry out photoproduction experiments by using linearly polarized photon beams with energies of 1.5-3 GeV at SPring-8/LEPS. One of our physics motivations is to understand reaction mechanisms of hadron photoproduction. We study (1) y→p→ π-Δ++, (2) y→p→ π+n, (3) y→p→ K+Λ(K+Σ0), and (4) y→p→ π+Δ0 reactions which produce (1) uu, (2) dd, (3) ss, and (4) dd quark-antiquark pairs in the final state. Differential cross sections and photon beam asymmetries have been measured at forward meson angles of 0.6 < cos θcm <1. Precise comparison between the π-Δ++ and π+Δ0 reactions plays an important role in the study of nucleon resonances. It enables to distinguish between N* and Δ* by taking the cross section ratios of the two reactions. We have measured preliminary cross section ratios dσ(π+Δ0)/dσ(π-Δ++). The ratios are found to be close to 1/3, which is expected from the exchange of isospin=1 in the t channel, for 0.966< cos θcm <1 and all the photon energies. The ratios become larger than 1/3 and close to 1 for larger angles and higher energy regions. The larger ratios might be due to A resonances, giving the ratio 4/3, excited in the intermediate state

Recent results from LEPS and status of LEPS2

Two high-energy photon beamlines, LEPS and LEPS2, have been operated at SPring-8. In both beamlines, linearly polarized photon beams up to 2.9 GeV produced by laser-induced backward Compton scattering from 8 GeV electrons have been used to study quark-nuclear physics via the photo-production of hadrons. In this article, I present some recent results from LEPS including the coherent ø photoproduction from 4He and γp → π− Δ++ reaction, and report on the current status of the developments of the LEPS2 solenoid spectrometer.