S. Ajimura

Photoproduction of Lambda(1405) and Sigma0(1385) on the proton at E(gamma) = 1.5-2.4-GeV

Differential cross sections for {gamma}p{yields}K{sup +}{lambda}(1405) and {gamma}p{yields}K{sup +}{sigma}{sup 0}(1385) reactions have been measured in the photon energy range from 1.5 to 2.4 GeV and the angular range of 0.8<cos({theta})<1.0 for the K{sup +} scattering angle in the center-of-mass system. This data is the first measurement of the {lambda}(1405) photoproduction cross section. The line shapes of {lambda}(1405) measured in {sigma}{sup +}{pi}{sup -} and {sigma}{sup -}{pi}{sup +} decay modes were different with each other, indicating a strong interference of the isospin 0 and 1 terms of the {sigma}{pi} scattering amplitudes. The ratios of {lambda}(1405) production to {sigma}{sup 0}(1385) production were measured in two photon energy ranges: near the production threshold (1.5<E{sub {gamma}}<2.0 GeV) and far from it (2.0<E{sub {gamma}}<2.4 GeV). The observed ratio decreased in the higher photon energy region, which may suggest different production mechanisms and internal structures for these hyperon resonances.

Photoproduction ofΛ(1405)andΣ0(1385)on the proton atEγ=1.5−2.4GeV

Differential cross sections for {gamma}p{yields}K{sup +}{lambda}(1405) and {gamma}p{yields}K{sup +}{sigma}{sup 0}(1385) reactions have been measured in the photon energy range from 1.5 to 2.4 GeV and the angular range of 0.8<cos({theta})<1.0 for the K{sup +} scattering angle in the center-of-mass system. This data is the first measurement of the {lambda}(1405) photoproduction cross section. The line shapes of {lambda}(1405) measured in {sigma}{sup +}{pi}{sup -} and {sigma}{sup -}{pi}{sup +} decay modes were different with each other, indicating a strong interference of the isospin 0 and 1 terms of the {sigma}{pi} scattering amplitudes. The ratios of {lambda}(1405) production to {sigma}{sup 0}(1385) production were measured in two photon energy ranges: near the production threshold (1.5<E{sub {gamma}}<2.0 GeV) and far from it (2.0<E{sub {gamma}}<2.4 GeV). The observed ratio decreased in the higher photon energy region, which may suggest different production mechanisms and internal structures for these hyperon resonances.

The superconducting kaon spectrometer — SKS

A superconducting kaon spectrometer has been installed in the north experimental hall of the KEK 12-GeV proton synchrotron. The spectrometer was designed to serve for nuclear physics experiments with meson beams in the 1 GeV/c region, particular emphasis being laid on study of Λ-hypernuclei via (π+, K+) reactions. In order to obtain Λ-hypernuclear data with better statistics and energy resolution, it was designed to have a good momentum resolution of 0.1% FWHM and a large acceptance of 100 msr. It consists of a large superconducting dipole magnet, tracking chambers, and trigger counters that can efficiently select kaons from large background of pions and protons. The overall energy resolution for scattering is realized together with a beam-line spectrometer in the K6 beam line, the momentum resolution of which was also designed to be better than 0.1% FWHM. A good energy resolution of better than 2 MeV FWHM has been confirmed in π−-12C elastic scattering and in the (π+,K+) reaction on 12C.

Hypernuclear spectroscopy of products from Li-6 projectiles on a carbon target at 2 A GeV

Abstract A novel experiment, aiming at demonstrating the feasibility of hypernuclear spectroscopy with heavy ion beams, was conducted. Using the invariant mass method, the spectroscopy of hypernuclear products of 6Li projectiles on a carbon target at 2 A GeV was performed. Signals of the Λ-hyperon and H Λ 3 and H Λ 4 hypernuclei were observed for final states of p + π − , He 3 + π − and He 4 + π − , respectively, with significance values of 6.7, 4.7 and 4.9σ. By analyzing the proper decay time from secondary vertex distribution with the unbinned maximum likelihood fitting method, their lifetime values were deduced to be 262 − 43 + 56 ± 45 ps for Λ, 183 − 32 + 42 ± 37 ps for H Λ 3 , and 140 − 33 + 48 ± 35 ps for H Λ 4 .

γ-ray spectroscopy in Λ hypernuclei

Abstract The present status of hypernuclear γ -ray spectroscopy with Hyperball is summarized. We observed two γ transitions of 16 Λ O( 1 − → 1 − , 0 − ) and obtained the strength of the ΛN tensor force. In 10 B( K − , π − γ ) data, we did not observe the spin-flip M1 transition of 10 Λ B( 2 − → 1 − ), but γ rays from hyperfragments such as 7 Λ Li( 7 / 2 + → 5 / 2 + ) and 9 Λ Be( 3 / 2 + → 1 / 2 + ) were observed. In 11 B( π + , K + γ ) data, we observed six γ transitions of 11 Λ B. We also attempted an inclusive γ -ray measurement with a stopped K − beam.

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.

Neutron and proton energy spectra from the non-mesonic weak decays of HeΛ5 and CΛ12

We have simultaneously measured the energy spectra of neutrons and protons emitted in the non-mesonic weak decays of 5_Lambda-He and 12_Lambda-C hypernuclei produced via the (pi^+,k^+) reaction with much higher statistics over those of previous experiments. The neutron-to-proton yield ratios for both hypernuclei at a high energy threshold (60 MeV) were approximately equal to two, which suggests that the ratio of the neutron- and proton-induced decay channels, Gn(Lambda n ->nn)/Gp(Lambda p ->np), is about 0.5. In the neutron energy spectra, we found that the yield of the low-energy component is unexpectedly large, even for 5_Lambda-He.

Exclusive measurement of the nonmesonic weak decay of the lambda(5)He hypernucleus.

We performed a coincidence measurement of two nucleons emitted from the nonmesonic weak decay of lambda(5)He formed via the 6Li(pi+, K+) reaction. The energies of the two nucleons and the pair number distributions in the opening angle between them were measured. In both np and nn pairs, we observed a clean back-to-back correlation coming from the two-body weak reactions of lambda p --> np and lambda n --> nn, respectively. The ratio of the nucleon pair numbers was N(nn)/N(np) = 0.45 +/- 0.11(stat) +/- 0.03(syst) in the kinematic region of cos theta(NN) < -0.8. Since each decay mode was exclusively detected, the measured ratio should be close to the ratio of gamma(lambda p --> np)/gamma(lambda n --> nn). The ratio is consistent with recent theoretical calculations based on the heavy meson and/or direct-quark exchange picture.We performed a coincidence measurement of two nucleons emitted from the nonmesonic weak decay (NMWD) of ^{5}_{\Lambda}He formed via the ^{6}Li(\pi^+,K^+) reaction. The energies of two nucleons and the pair number distributions in the opening angle between them were measured. In both np and nn pairs, we observed a clean back-to-back correlation coming from the two-body decay of \Lambda p -->n p and \Lambda n -->n n, respectively. The ratio of the nucleon pair numbers was N_{nn}/N_{np}=0.45 \pm 0.11(stat)\pm 0.03(syst) in the kinematic region of cos(theta_{NN})<-0.8. Since each decay mode was exclusively detected, the measured ratio should be close to the ratio of \Gamma(\Lambda p -->np)/\Gamma(\Lambda n -->nn). The ratio is consistent with recent theoretical calculations based on the heavy meson/direct quark exchange picture.

Coincidence measurement of the nonmesonic weak decay of 12ΛC

We have measured the angular correlation of the pair nucleons np and nn emitted from the nonmesonic weak decay (NMWD) of ^{12}_{Lambda}C produced via the (pi^+,K^+) reaction in coincidence measurement. The Lambda p -> np and Lambda n -> nn modes were clearly identified by measuring the back-to-back correlation of the emitted nucleon pairs which is the characteristic of two-body kinematics. From the measured nucleon pair numbers N_{nn} and N_{np}, the ratio Gamma_n/Gamma_p of the partial decay widths Gamma_n(Lambda n -> nn) and Gamma_p(Lambda p -> np) of ^{12}_{Lambda}C was extracted to be 0.51+-0.13(stat) +- 0.05(syst); this result is almost free from the ambiguity due to the nuclear final state interaction and 3-body decay process, which were inherent in the previous results. The obtained Gamma_n/Gamma_p ratio of ^{12}_{Lambda}C (p-shell) is close to that of ^5_{Lambda}He (s-shell). The results are consistent with those of recent theoretical calculations.

Search for

A search forKpp bound state (the lightest kaonic nucleus) has been performe d using theγd → KπX reaction at Eγ=1.5-2.4 GeV at LEPS / Pring-8. The di fferential cross section of Kπ photo-production o ff deuterium has been measured for the first time in this energy region, and a bump st ructure was searched for in the inclusive missing mass spectrum. A statistically significant bump structure was no t observed in the region from 2.22 to 2.36 GeV /c2, and the upper limits of the di fferential cross section for the Kpp bound state production were determined to be 0.1 − 7μ b (95% confidence level) for a set of assumed binding energy and width values.