A. Toyoda

First results on 12ΛC production at DAΦNE

Lambda-hypernuclei are produced and studied, with the FINUDA spectrometer, for the first time at an e+e- collider: DAPHNE, the Frascati phi-factory. The slow negative kaons from phi(1020) decay are stopped in thin (0.2 g/cm^2) nuclear targets, and Lambda-hypernuclei formation is detected by measuring the momentum of the outgoing pi^-. A preliminary analysis on 12Lambda-C shows an energy resolution of 1.29 MeV FWHM on the hypernuclear levels, the best obtained so far with magnetic spectrometers at hadron facilities. Capture rates for the ground state and the excited ones are reported, and compared with previous experiments.

Anthropogenic radionuclides in the atmosphere observed at Tsukuba: characteristics of the radionuclides derived from Fukushima

During a serious accident at the Fukushima Dai-ichi Nuclear Power Plant (FDNPP), a huge quantity of radionuclides was released into the atmosphere and ocean. We measured anthropogenic radionuclides in surface air at Tsukuba, about 170 km from the FDNPP. On March 15, 2011, we detected the radioactivity released from the Fukushima accident in air samples at Tsukuba. The major radionuclides that we observed were radioiodine ((131)I, (132)I, (133)I) and radiocesium ((134)Cs, (136)Cs, (137)Cs). This radioiodine consisted of gaseous and particulate forms; the percentage of particulate (131)I in the total (131)I ranged from 0 to 86%. The percentage of the particulate (131)I to the total (131)I increased on the arrival of the plumes from major emissions of the FDNPP. After activities of the radionuclides attained the maximum on March 15, 2011, the FDNPP-derived radionuclides decreased rapidly in surface air. The activity median aerodynamic diameter of (131)I-bearing particles was 0.7 μm, while those of (134)Cs- and (137)Cs-bearing particles were larger than 1 μm. Large variations of ratios of (131)I/(137)Cs, (132)Te/(137)Cs, and (99)Mo ((99m)Tc)/(137)Cs (all involving different elements) suggest that the behaviors of these radionuclides in the atmosphere, including the processes of their emission, differed each other.

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.

New results on mesonic weak decay of p-shell Lambda-hypernuclei

Abstract The FINUDA experiment performed a systematic study of the charged mesonic weak decay channel of p -shell Λ -hypernuclei. Negatively charged pion spectra from mesonic decay were measured with magnetic analysis for the first time for 7 Λ Li, 9 Λ Be, 11 Λ B and 15 Λ N. The shape of the π − spectra was interpreted through a comparison with pion distorted wave calculations that take into account the structure of both hypernucleus and daughter nucleus. Branching ratios Γ π − / Γ tot were derived from the measured spectra and converted to π − decay rates Γ π − by means of known or extrapolated total decay widths Γ tot of p -shell Λ -hypernuclei. Based on these measurements, the spin-parity assignment 1 / 2 + for 7 Λ Li and 5 / 2 + for 11 Λ B ground-state are confirmed and a spin-parity 3 / 2 + for 15 Λ N ground-state is assigned for the first time.

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.

Proton spectra from Non-Mesonic Weak Decay of p-shell Λ-hypernuclei and evidence for the two-nucleon induced process

Abstract New spectra from the FINUDA experiment of the Non-Mesonic Weak Decay (NMWD) proton kinetic energy for Be Λ 9 , B Λ 11 , C Λ 12 , C Λ 13 , N Λ 15 and O Λ 16 are presented and discussed along with the published data on He Λ 5 and Li Λ 7 . Exploiting the large mass number range and the low energy threshold (15 MeV) for the proton detection of FINUDA, an evaluation of both Final State Interactions (FSI) and the two-nucleon induced NMWD contributions to the decay process has been done. Based on this evaluation, a linear dependence of FSI on the hypernuclear mass number A is found and for the two-nucleon stimulated decay rate the experimental value of Γ 2 / Γ p = 0.43 ± 0.25 is determined for the first time. A value for the two-nucleon stimulated decay rate to the total decay rate Γ 2 / Γ NMWD = 0.24 ± 0.10 is also extracted.

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.

Measurement of the proton spectra from non-mesonic weak decay of 5ΛHe, 7ΛLi and 12ΛC

Abstract The results of a measurement of the proton spectra following the non-mesonic weak decay of 5ΛHe, 7ΛLi and 12ΛC are presented and discussed. The experiment was performed at the ( e + e − ) collider DAΦNE at Laboratori Nazionale di Frascati of INFN. It is the first measurement for 7ΛLi, and for all the spectra the lower limit on the energy of the protons is 15 MeV, never reached before. All the spectra show a similar shape, namely a peak at around 80 MeV as expected for the free Λ p → n p weak reaction, with a low energy rise that should be due to final state interactions and/or two-nucleon induced weak processes. The decay spectrum of 5ΛHe is somehow similar to the ones reported by previous measurements and theoretical calculations, but the same does not happen for the 12ΛC one.

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 the deeply bound K–pp state from the semi-inclusive forward-neutron spectrum in the in-flight K– reaction on helium-3

T. Hashimoto †1,*, S. Ajimura2, G. Beer3, H. Bhang4, M. Bragadireanu5, L. Busso7,8, M. Cargnelli6, S. Choi4, C. Curceanu9, S. Enomoto2, D. Faso7,8, H. Fujioka10, Y. Fujiwara1, T. Fukuda11, C. Guaraldo9, R. S. Hayano1, T. Hiraiwa2, M. Iio12, M. Iliescu9, K. Inoue13, Y. Ishiguro10, T. Ishikawa1, S. Ishimoto12, K. Itahashi14, M. Iwai12, M. Iwasaki14,15, Y. Kato14, S. Kawasaki13, P. Kienle‡16, H. Kou15, Y. Ma14, J. Marton6, Y. Matsuda17, Y. Mizoi11, O. Morra7, T. Nagae10, H. Noumi2, H. Ohnishi14,2, S. Okada14, H. Outa14, K. Piscicchia9, M. Poli Lener9, A. Romero Vidal9, Y. Sada10, A. Sakaguchi13, F. Sakuma14, M. Sato14, A. Scordo9, M. Sekimoto12, H. Shi9, D. Sirghi9,5, F. Sirghi9,5, S. Suzuki12, T. Suzuki1, K. Tanida4, H. Tatsuno1, M. Tokuda15, D. Tomono10, A. Toyoda12, K. Tsukada18, O. Vazquez Doce9,19, E. Widmann6, T. Yamaga13, T. Yamazaki1,14, H. Yim20, Q. Zhang14, and J. Zmeskal6