We measured the lifetime and the mesonic and non-mesonic decay rates of the 4He-Lambda hypernucleus. The hypernuclei were created using a 750 MeV/c momentum K- beam on a liquid 4He target by the reaction 4He(K-,pi-)4He-Lambda. The 4He-Lambda lifetime was directly measured using protons from Lambda p -> n p non-mesonic decay (also referred to as proton-stimulated decay) and was found to have a value of tau = 245 +/- 24 ps. The mesonic decay rates were determined from the observed numbers of pi-'s and pi0's as Gamma_pi-/Gamma_tot = 0.270 +/- 0.024 and Gamma_pi0/Gamma_tot = 0.564 +/- 0.036, respectively, and the values of the proton- and neutron-stimulated decay rates were extracted as Gamma_p/Gamma_tot = 0.169 +/- 0.019 and Gamma_n/Gamma_tot <= 0.032 (95% CL), respectively. The effects of final-state interactions and possible 3-body Lambda N N decay contributions were studied in the context of a simple model of nucleon-stimulated decay. Nucleon-nucleon coincidence events were observed and were used in the determination of the non-mesonic branching fractions. The implications of the results of this analysis were considered for the empirical Delta I = 1/2 rule and the decay rates of the 4H-Lambda hypernucleus.

We measured the lifetime and the mesonic and nonmesonic decay rates of the 4ΛHe hypemucleus. The hypemuclei were created using a 750 MeV/c momentum K - beam on a liquid 4 He target by the reaction 4 He(K - π - )4Λ He. The 4ΛHe lifetime was directly measured using protons from Ap → np nonmesonic decay (also referred to as proton-stimulated decay) and was found to have a value of T = 245 ± 24 ps. The mesonic decay rates were determined from the observed numbers of π - 's and π°'s as Γπ/ Γ tot = 0.270 ± 0.024 and Γπ°/ Γ tot = 0.564 ± 0.036, respectively, and the values of the proton- and neutron-stimulated decay rates were extracted as Fp/ r tot = 0.169 ± 0.019 and Γ n / Γ tot ≤0.032 (95% CL), respectively. The effects of final-state interactions and possible three-body ANN decay contributions were studied in the context of a simple model of nucleon-stimulated decay. Nucleon-nucleon coincidence events were observed and were used in the determination of the nonmesonic branching fractions. The implications of the results of this analysis were considered for the empirical ΔI = 1 2 rule and the decay rates of the 4ΛH hypemucleus.

Received 11 September 2007DOI:https://doi.org/10.1103/PhysRevC.76.039904©2007 American Physical Society

J.D. Parker, ∗ M.J. Athanas, † P.D. Barnes, ‡ S. Bart, B. Bassalleck, R. Chrien, G. Diebold, G.B. Franklin, K. Johnston, § P. Pile, B. Quinn, F. Rozon, R. Sawafta, ¶ R.A. Schumacher, R. Stearns, I. Sukaton, R. Sutter, J.J. Szymanski, ‡ and V.J. Zeps ∗∗ Carnegie Mellon University, Pittsburgh, PA 15213, USA Brookhaven National Laboratory, Upton, NY 11973, USA University of New Mexico, Albuquerque, NM 87131, USA University of Houston, Houston, TX 77004, USA Vassar College, Poughkeepsie, NY 12604, USA Indiana University Cyclotron Facility, Bloomington, IN 47408, USA (Revised September 13, 2007)

A search for Sigma hypernuclear states in p-shell hypernuclei her; been performed with the Moby Dick spectrometer and the low energy separated beam (LESB-2) at the Brookhaven Alternating Gradient Synchrotron (BNL AGS). Unlike some previously published reports, no narrow states have been observed for targets of Li-6 and Be-9 in (K-, pi(+/-)) reactions, either for bound state or continuum regions, Together with the previously reported J = 0, T = 1/2 bound state in He-4(Sigma), these results demonstrate the crucial role of isospin in Sigma hypernuclei.

The attenuation cross sections measured in transmission experiments at the alternating-gradient synchrotron for K+ on Li-6, C, Si and Ca at p(L) = 488, 531, 656, and 714 MeV/c are reanalyzed in order to derive total (sigma(T)) and reaction (sigma(R)) cross sections. The effect of plural (Moliere) scattering is properly accounted for, leading to revised values of sigma(T). We demonstrate the model dependence of these values, primarily due to the choice of K+ nuclear optical potential used to generate the necessary Coulomb-nuclear and nuclear elastic corrections. Values of sigma(R) are also derived, for the first time, from the same data and exhibit a remarkable degree of model independence. The derived values of sigma(T) and sigma(R) exceed those calculated by the first-order t rho optical potential for C, Si, and Ca, but not for Li-6, particularly at 656 and 714 MeV/c where the excess is 10-25%. Relative to Li-6, this excess is found to be nearly energy independent and its magnitude of 15-25% is not reproduced by any nuclear medium effect studied so far.

The spectrum of excited states in the hypernucleus C-13(Lambda) was studied at the Brookhaven Alternate Gradient Synchrotron using the (K-, pi(-)) reaction on an enriched C-13 target. gamma rays coincident with the p(1/2 Lambda) state were observed using two large sodium iodide detectors. We interpret them as arising from the p(1/2 Lambda) --> s(1/2 Lambda) transition in C-13(Lambda). This represents the first direct observation of a gamma ray associated with the transition of a hyperon between major nuclear shells.

Elastic differential cross sections for K+ mesons scattered from natC and 6Li targets have been measured at an incident momentum of 715 MeV/c and at angles of 7° to 42° in the laboratory frame. The experimental cross sections agree, within errors, with two different parameter-free impulse approximation calculations. To reduce the effects of the systematic errors, the ratio of the experimental cross sections for natC to 6Li is compared to the theoretical values, and these ratios do not agree with theory. This discrepancy suggests either a density-dependent alteration of K+-nucleon amplitudes or a failure of the optical potential calculations to describe these nuclides adequately.

Elastic scattering of 720 and 620 MeV/c positive kaons from targets of {sup 12}C and {sup 6}Li has been measured up to laboratory angles of 42{degrees}. Since the magnitude of the cross sections is sensitive to nuclear medium effects, the K{yields}{mu}{nu} decay mode has been used to check the normalization. GEANT has been used to mimic the kaon decays over a path length of 12cm, with a correlated beam structure matching the experimental kaon beam. The corresponding muon distribution has been passed thru Monte Carlo simulations of the moby dick spectrometer. The results are compared with the experimental number of decay muons with good agreement. These results also agree with the normalization found using p-p elastic scattering. The normalized K{sup +} elastic data are compared to recent optical model predictions based on both Klein-Gordon and KDP equations in the impulse approximation.