Non-mesonic Decay Modes Research Articles

An event excess observed in the deeply bound region of the 12C (K−, p) missing-mass spectrum

Abstract We have measured, for the first time, the inclusive missing-mass spectrum of the $^{12}$C$(K^-, p)$ reaction at an incident kaon momentum of 1.8 GeV/$c$ at the J-PARC K1.8 beamline. We observed a prominent quasi-elastic peak ($K^-p \rightarrow K^-p$) in this spectrum. In the quasi-elastic peak region, the effect of secondary interaction is apparently observed as a peak shift, and the peak exhibits a tail in the bound region. We compared the spectrum with a theoretical calculation based on the Green’s function method by assuming different values of the parameters for the $\bar{K}$–nucleus optical potential. We found that the spectrum shape in the binding-energy region $-300 \, \text{MeV} < B_{K} < 40$ MeV is best reproduced with the potential depths $V_0 = -80$ MeV (real part) and $W_0 = -40$ MeV (imaginary part). On the other hand, we observed a significant event excess in the deeply bound region around $B_{K} \sim 100$ MeV, where the major decay channel of $K^- NN \to \pi\Sigma N$ is energetically closed, and the non-mesonic decay modes ($K^- NN \to \Lambda N$ and $\Sigma N$) should mainly contribute. The enhancement is fitted well by a Breit–Wigner function with a kaon-binding energy of 90 MeV and width 100 MeV. A possible interpretation is a deeply bound state of a $Y^{*}$-nucleus system.

Analysis of a Single-Λ Hypernuclear Event in KEK-PS E373 Nuclear Emulsion Experiment

The purpose of this research is to identify a single-Λ hypernucleus and its decay products which support to get more information about hyperon-nucleon interaction. Before performing the analysis of a single-Λ hypernucleus, first, we have deduced the density of emulsion by calibrating the range-energy relation using alpha decay events data from thorium series. It is very important for the mass reconstruction of hypernucleus events in nuclear emulsion. And we have reconstructed a single-Λ hypernucleus event by using the kinematical reconstruction for mesonic and non-mesonic decay modes. From the result of our analysis, we can uniquely identify that single-Λ hypernucleus is $ {}_\Lambda ^9 B $ and decay products are 4He,3He, proton and neutron.

Theoretical studies on the $$\alpha $$ α decay half-lives of hyper and normal isotopes of Po

The $$\alpha $$ decay half-lives of hyper and normal isotopes of Po nuclei are studied in the present work. The inclusion of $$\Lambda $$ –N interaction changes the half-life for $$\alpha $$ decay. The theoretical predictions on the $$\alpha $$ decay half-lives of normal Po isotopes are compared with experimental results and are seen to be matching well with each other. The neutron shell closure at $$N = 126$$ is found to be the same for both normal and hypernuclei. The Geiger–Nuttal (G–N) law for $$\alpha $$ decay is unaltered in the case of hypernuclei. The hypernuclei will decay into normal nuclei by mesonic or non-mesonic decay modes. Since the half-lives of normal Po nuclei are well within the experimental limits, our theoretical results suggest experimental verification of the $$\alpha $$ emission from hyper Po nuclei in a cascade process.

First results from the FINUDA experiment

This paper provides a summary of the first results obtained by the FINUDA Collaboration in the field of hypernuclear physics. They range from the spectroscopic study of several A-hypernuclei to the observation of some non-mesonic decay modes. Hints on rare hypernuclear two-body decays, on neutron-rich hypernuclei and on deeply-bound kaonic nuclei are also reported.

The hypertriton and its decays

Ridorous Faddeev calculations for the hypertriton, based on modern baryon-baryon forces including A-Σ conversion, are described. The mesonic and nonmesonic decay modes are evaluated taking the final-state interaction among the three nucleons exactly into account. Total and differential decay rates are provided. It is shown that the total rates for neutron- and proton-induced decays in the nonmesonic mode cannot be seperated experimentally, only fractions thereof.

Non-mesonic weak decay of A hypernuclei

The non-mesonic decay modes of Λ hypernuclei are examined. The one-nucleon induced decay (ΛN → NN) is studied in a shell-model framework using a meson-exchange picture. The total decay rates and parity-violating asymmetries reproduce the available experimental data. However, the model does not yet explain the partial rates and the resulting neutron- to proton-induced ratio Γ n /Γ p. The two-nucleon-induced decay rate (ΛNN -> NNN) is evaluated using the propagator formalism and the local-density approximation and is found to contribute about 15% to the total decay width of medium to heavy hypernuclei. The influence of this channel on the experimental determination of the ratio Γ n /Γ p is examined. The spectra of protons and neutrons from each weak-decay mechanism have been obtained using a Monte Carlo simulation.

Non-mesonic decay modes of hypernuclei

The present status of theoretical descriptions of non-mesonic ( ΛN → NN) decay modes of hypernuclei is reviewed. One model, which evaluates a strangeness-changing, parity-violating weak interaction “potential” in nuclear matter, is examined in detail to exemplify the physics which can be learned and the problems which must be confronted. The importance of measuring the different isospin decay channels and separating the parity-conserving and parity-violating modes is stressed. Finally, this model is briefly compared and contrasted with other existing calculations.

Dinucleon decay in the nucleus: A quark-model analysis.

Baryon-number-violating decay of pairs of nucleons bound together in a nucleus is studied in the nonrelativistic quark model for a class of ordinary grand unified theories including minimal SU(5). Decay modes without mesons such as pp\ensuremath{\rightarrow}${\mathrm{pe}}^{+}$, and with pions such as pp\ensuremath{\rightarrow}${\mathrm{pe}}^{+}$${\ensuremath{\pi}}^{0}$, are considered. Both spectator and pole diagrams are included. The nonmesonic decay modes are negligible, but the effective p\ensuremath{\rightarrow}${e}^{+}$${\ensuremath{\pi}}^{0}$ partial lifetime in oxygen (iron) is found to be \ensuremath{\approxeq}25% (50%) longer than the free partial lifetime. Another possible lifetime increase related to an explanation of the European Muon Collaboration effect is discussed.

The two-body nonmesonic decay modes of4HeΛ and4HΛ

The reaction rates for the two-body nonmesonic decays4HΛ →3H + n (1),4HeΛ →3He + n (2) and4HeΛ →3H + p (3) are calculated using a phenomenological nonrelativistic matrix element. This matrix element describes all transitions allowed in the weak process ΛN →NN where the Λ-hyperon interacts with the nucleons in1S0 and3S1 states. The reaction rates are given as a function of the six independent transition amplitudes contained in the matrix element and are compared with the total nonmesonic decay rates for4HΛ and4HeΛ. These total rates have been expressed in terms of the same transition amplitudes from expressions given by Block and Dalitz. The ratios of the two-body decay rates on the total rates for the corresponding hypernucleus appear to be small. Taking into account only the transition3S1(ΛN) →3P1(NN) which is expected to be dominant, we get, as a rough estimate of these ratios, 3% for reaction (1) and 1.5% for reaction (2) and for reaction (3).