Y. Okayasu

New Measurements of the European Muon Collaboration Effect in Very Light Nuclei

J. Seely, A. Daniel, D. Gaskell, J. Arrington, ∗ N. Fomin, P. Solvignon, R. Asaturyan, † F. Benmokhtar, W. Boeglin, B. Boillat, P. Bosted, A. Bruell, M.H.S. Bukhari, M.E. Christy, B. Clasie, S. Connell, ‡ M.M. Dalton, D. Day, J. Dunne, D. Dutta, 12 L. El Fassi, R. Ent, H. Fenker, B.W. Filippone, H. Gao, 12 C. Hill, R.J. Holt, T. Horn, 3 E. Hungerford, M.K. Jones, J. Jourdan, N. Kalantarians, C.E. Keppel, D. Kiselev, M. Kotulla, C. Lee, A.F. Lung, S. Malace, D.G. Meekins, T. Mertens, H. Mkrtchyan, T. Navasardyan, G. Niculescu, I. Niculescu, H. Nomura, Y. Okayasu, A.K. Opper, C. Perdrisat, D.H. Potterveld, V. Punjabi, X. Qian, P.E. Reimer, J. Roche, V.M. Rodriguez, O. Rondon, E. Schulte, E. Segbefia, K. Slifer, G.R. Smith, V. Tadevosyan, S. Tajima, L. Tang, G. Testa, R. Trojer, V. Tvaskis, W.F. Vulcan, F.R. Wesselmann, S.A. Wood, J. Wright, L. Yuan, and X. Zheng Laboratory for Nuclear Science, Massachusetts Institute of Technology, Cambridge, MA, USA University of Houston, Houston, TX, USA Thomas Jefferson National Laboratory, Newport News, VA, USA Physics Division, Argonne National Laboratory, Argonne, IL, USA University of Virginia, Charlottesville, VA, USA Yerevan Physics Institute, Armenia University of Maryland, College Park, MD, USA Florida International University, Miami, FL, USA Basel University, Basel, Switzerland Hampton University, Hampton, VA, USA Mississippi State University, Jackson, MS, USA Triangle Universities Nuclear Laboratory, Duke University, Durham, NC, USA Kellogg Radiation Laboratory, California Institute of Technology, Pasadena, CA, USA University of the Witwatersrand, Johannesburg, South Africa James Madison University, Harrisonburg, VA, USA Tohoku University, Sendai, Japan Ohio University, Athens, OH, USA College of William and Mary, Williamsburg, VA, USA Norfolk State University, Norfolk, VA, USA (Dated: October 27, 2009)

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.

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.

Measurement of the π− decay width of HeΛ5

We have precisely measured

Measurement of the

\Lambda \to p\pi^-

\pi^-

decay width of \5LHe and demonstrated significantly larger

decay width of

\alpha

^5_\Lambda

-