K. Maeda

Polarization and weak decays of Λ hypernuclei produced by the (π+, K+) reaction on 12C

Abstract Non-mesonic weak-decay protons (NM-p) and mesonic weak-decay pions (M-π) from Λ produced by the (π+, K+) reaction on 12C have been measured. NM-p from the 11 Λ B ( 5 2 + ) produced through the proton-unbound states in 12ΛC and NM-p and M-π associated with the quasi-bound region (Ex ≈ 20 MeV) in 12ΛC show finite asymmetries of A1 = − 0.20 to − 0.35 with respect to the reaction plane. These results indicate finite polarizations of the relevant Λ-hypernuclei and asymmetric NM weak decays.

Mesonic and nonmesonic weak decay widths of medium-heavy {lambda} hypernuclei

We have measured the energy spectra of pions and protons emitted in the weak decay of {sup {lambda}}{sub 12}C, {sup {lambda}}{sub 28}Si, and hypernuclei produced via the ({pi}{sup +},K{sup +}) reaction. The decay widths of the {pi}{sup -} mesonic decay ({lambda}{yields}p{pi}{sup -}) and the nonmesonic decay ({lambda}N{yields}NN) were extracted. The present results demonstrate an increase of the mesonic decay width due to a distortion of the pion wave function in nuclear medium for the first time. The ratios of the neutron- to proton-induced nonmesonic decay widths, {gamma}{sub n}({lambda}n{yields}nn)/{gamma}{sub p}({lambda}p{yields}np), were evaluated by a direct comparison of the measured proton energy spectra with the calculated ones. No theoretical calculation which has been proposed so far can simultaneously account for both the nonmesonic decay widths and the {gamma}{sub n}/{gamma}{sub p} ratios in the present data.

The (π+,K+) reaction and light Λ hypernuclear spectroscopy

Abstract High-quality excitation-energy spectra of Λ 7 Li, Λ 9 Be, Λ 13 C and Λ 16 O as well as Λ 12 C have been obtained with 2 MeV (FWHM) resolution and good statistics through the (π + ,K+) reaction. The spectra of Λ 12 C, Λ 16 0 and Λ 13 C are discussed in relation to the ΛN spindependent interaction, and those of Λ 9 Be and Λ 7 Li within the framework of the cluster structure of Λ hypernuclei. It is emphasized that Λ hypernuclear spectroscopy provides us with valuable information on Λ hypernuclear structure and the ΛN interaction.

The (pi+,K+) reaction and light Lambda hypernuclear spectroscopy

Abstract High-quality excitation-energy spectra of Λ 7 Li, Λ 9 Be, Λ 13 C and Λ 16 O as well as Λ 12 C have been obtained with 2 MeV (FWHM) resolution and good statistics through the (π + ,K+) reaction. The spectra of Λ 12 C, Λ 16 0 and Λ 13 C are discussed in relation to the ΛN spindependent interaction, and those of Λ 9 Be and Λ 7 Li within the framework of the cluster structure of Λ hypernuclei. It is emphasized that Λ hypernuclear spectroscopy provides us with valuable information on Λ hypernuclear structure and the ΛN interaction.

In-medium rho0 spectral function study via the H-2, He-3, C-12(gamma,pi+ pi-) reaction

We report a helicity analysis of sub-threshold rho^0 production on 2H, 3He and 12C at low photo-production energies. The results are indicative of a large longitudinal rho^0 polarization (l=1, m=0) and are consistent with a strong helicity-flip production mechanism. This signature is used to extract in-medium rho^0_L invariant mass distributions for all three nuclei in a manner which is less model-dependent than previous measurements. The results are compared to kinematic and phenomenological models of the rho^0 spectral function. The 2H and 3He data distributions support the role of N*(1520) excitation in shaping the in-medium rho^0_L invariant mass distribution, while the 12C distributions are consistent with quasi-free rho^0_L production. The data support an in-medium modification of the rho^0_L invariant mass distribution.

Photo-production of neutral kaons on 12C in the threshold region

Abstract The kaon photo-production process on 12C has been studied by measuring neutral kaons in the photon energy range of 0.8–1.1 GeV. Neutral kaons were identified by the invariant mass constructed from two charged pions emitted in the K S 0 → π + π − decay channel. The differential and integrated cross sections in the threshold photon energy region were obtained. The obtained momentum spectra were compared with a Spectator model calculation using elementary amplitudes of kaon photo-production given by recent isobar models. The present results provide the first information on the n ( γ , K 0 ) Λ reaction, which is expected to play an important role in constructing models of strangeness production by electromagnetic interactions. The experimental results show that the cross section of C 12 ( γ , K 0 ) is of the same order as that of C 12 ( γ , K + ) and suggest that a slightly backward K 0 angular distribution is favored in the γ n → K 0 Λ process.

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.

Spectroscopy of 24Al through the 24Mg(p, n)24Al reaction at Ep = 35 MeV

Abstract An experimental study of the 24 Mg(p, n) 24 Al reaction at E p = 35 MeV was carried out. High-resolution neutron time-of-flight spectra were measured at 17 angles in a range of 0°–110°. Neutron peaks corresponding to seventeen states in 24 A1 have been resolved. A number of spin-parity assignments have been made based on distorted-wave Born-approximation (DWBA) analyses for the measured angular distributions, using microscopic transition amplitudes obtained from the 1s0d shell model for positive-parity states and those from the 0s0p-1s0d-1p0f open-shell randomphase approximation (OSRPA) for negative-parity states. Reasonable correlation between the low-energy (p, n) cross section and the β-decay strength has been found for the observed 0 + →1 + Gamow-Teller transitions leading to the 0.439, 1.116 and 3.023 MeV states in 24 Al. The 1.56 MeV 5 + state has been identified as a 0 h ω stretched state, in addition to the 5.54 MeV, 6 state of 1 h ω character. Candidates for the 1 h ω negative-parity transitions leading to the 2, 3, 4 states are also presented.

12C(p, n)12N and 16O(p, n)16F reactions at Ep = 35 and 40 MeV: Reliability of the information obtained from DWBA analysis of low-energy (p, n) data

Abstract A high-resolution study of the 12C(p, n)12N and 16O(p, n)16F reactions was made at Ep = 35 and 40 MeV. The low-lying states in 12N(1+, 2+ and 2−) and in 16F(0−, 1−, 2− and 3−) were clearly resolved, and their angular distributions were measured. Extensive DWBA analysis was made and compared with the data. The calculated angular distribution shapes are found to be in agreement with the data and insensitive to the choice of the parameters involved. On the other hand, the magnitudes of the DWBA cross sections depend strongly on the bound state parameters in the case of a transition from a tightly bound state to a loosley bound state. In the other cases the overall uncertainty of the DWBA cross section magnitudes was estimated to be about ±30%. Within this uncertainty the experimental cross sections for the 12C(p, n) reaction were explained by the calculation, but those for the 16O(p, n) reaction were not: the observed strengths were about a half of the calculated values. Since these results agree with those at intermediate energies, the origin of the discrepancy is considered to be in the structure of the mass 16 nuclei rather than in reaction dynamics. In general, the present results compare well with those at intermediate energies, indicating that the structure information extracted from low-energy, high-resolution (p, n) data is basically sound if careful analysis of the data is made.

A comparative study of the 13C(p, p′)13C and 13C(p, n)13N reactions at Ep = 35 MeV☆

Abstract Differential cross sections were measured at Ep = 35 MeV for the 13C(p, n) and 13C(p, p′) reactions leading to the four low-lying states in the mirror nuclei 13N and 13C. In addition, the analyzing powers were measured for the 13C(p, p′) reaction. The data are generally well accounted for by DWBA calculations except for the 13 C(p, p′) 13 C (3.09 MeV , 1 2 + ) reaction, for which the calculations cannot even reproduce the qualitative features of the data. A comparison of the (p, n) and the (p, p′) results suggests that the isoscalar part of the 13 C ( g.s. , 1 2 − ) → 13 C (3.09 MeV , 1 2 + ) transition is not correctly described by currently available shell-model wave functions.