Kazunori Itonaga

Hypernuclear Weak Decays

The recent theoretical studies of Λ-hypernuclear weak decays of the nonmesonic and π-mesonic ones are developed with the aim to disclose the link between the experimental decay observables and the underlying basic weak decay interactions and the weak decay mechanisms. The expressions of the nonmesonic decay rates Γnm and the decay asymmetry parameter α1 of protons from the polarized hypernuclei are presented in the shell model framework. We then introduce the meson theoretical ΛN → NN interactions which include the one-meson exchanges, the correlated-2π exchanges, and the chiral-pair-meson exchanges. The features of meson exchange potentials and their roles on the nonmesonic decays are discussed. With the adoption of the π + 2π/ρ + 2π/σ + ω + K + ρπ/a1 + σπ/a1 exchange potentials, we have carried out the systematic calculations of the nonmesonic decay observables for light-to-heavy hypernuclei. The present model can account for the available experimental data of the decay rates, Γn/Γp ratios, and the intrinsic asymmetry parameters αΛ (αΛ is related to α1) of emitted protons well and consistently within the error bars. The hypernuclear lifetimes are evaluated by converting the total weak decay rates Γtot = Γπ+Γnm to τ , which exhibit saturation property for the hypernuclear mass A ≥ 30 and agree grossly well with experimental data for the mass range from light to heavy hypernuclei except for the very light ones. Future extensions of the model and the remaining problems are also mentioned. The π-mesonic weak processes are briefly surveyed, and the calculations and predictions are compared and confirmed by the recent high precision FINUDA π-mesonic decay data. This shows that the theoretical basis seems to be firmly grounded.

Spectroscopy of Electro- and Photo-Productions of Hypernuclei

Recently hypernuclear electroproduction experiments have been successfully carried out at the Thomas Jefferson Laboratory, providing high-resolution spectroscopic data. This article discusses several facets inherent to this reaction process: First, numerous theoretical attempts in describing the elementary kaon photoproduction are summarized so as to choose appropriate elementary amplitudes to be used for hypernuclear production estimates. The reaction process is characterized by both the sizable momentum transfer and the spin-flip dominace involved in the transition operator. The DWIA theoretical excitation spectra of producing p-shell hypernucli are discussed together with the experimental ones obtained recently. Encouraged with the successful theory-experiment comparison, the calculational framework has been applied beyond p-shell to typical medium-heavy hypernuclear production. The hypernuclear photo-production spectra predicted with the F, Si, and Ca targets suggest that the unique characters of the electroproduction are more clearly appeared in the medium-mass region, providing interesting opportunities of spectroscopic study beyond p-shell.

A new ΛN → NN weak potential and the hypernuclear nonmesonic decay

Abstract A new ΛN → N N weak potential which incorporates the isoscalar ″2 π / σ ″ meson exchange is proposed. This potential contains central, LS and parity-violating vector parts and it has no tensor interaction. The nonmesonic decay rates of Λ 5 He, Λ 4 He and Λ 4 H are evaluated with use of V π and V 2 π / σ potentials and are compared with the data.

NONMESONIC WEAK DECAYS OF SINGLE-Λ AND DOUBLE-Λ HYPERNUCLEI OF LIGHT MASS SYSTEMS

The nucleon-induced and hyperon-induced nonmesonic weak decay rates of the single-Λ and double-Λ hypernuclei are evaluated in the meson-exchange potential model. The roles of K-meson exchange are discussed. The ΛΛ → nΛ decay rate of is estimated to be fair and might be detectable in a future experiment.

Track Following of Ξ− Hyperons in Nuclear Emulsion for the E07 Experiment

The E07 experiment is expected to provide knowledge of S=-2 systems with ten times more statistics than that of the past E373 experiment. To achieve this in a reasonable time, an automated track following system is very important. This system consists of three techniques, “emulsion surface detection”, ”alignment of plate by plate connection with K beams” and “followed track recognition in nuclear emulsion”. Ξ hyperon candidate tracks are followed from the entrance to the end point in the emulsion. If the system operates properly such that one track is processed in each plate within one minute, all Ξ candidate tracks can be followed successfully to their stopping points in a year. The development of softwares for the system is ongoing.

Single Charged Particle Identification in Nuclear Emulsion Using Multiple Coulomb Scattering Method

In J-PARC E07 experiment, emulsion experiment for double strangeness system, particle identification is an important aspect to purify Ξ stop events. Through the experiment, we expect Ξ stop event with ten time higher statistics than that of previous experiment (KEK PS-E373). Ξ stop events which include doubleΛ hypernucleus, twinΛ hypernuclei and so on give us important information such as Λ-Λ and Ξ-N interaction. Besides, Ξ stop events are interesting itself because experimental data aboutΞ stop is very few. We are developing a PID technique for single charged particles having low momentum using by “Second difference” reflecting Multiple Coulomb Scattering. A charged particle moving in nuclear emulsion is deflected by many small-angle scatters due to Coulomb interaction, depends on the momentum of the particle. It is possible to discriminate the mass of a particle by evaluating the behavior of scattering near stopping point. We introduce “Second difference”, i.e. δ plane, defined as in Fig. (1), and intend to use its distribution as Probability Distribution Functions for likelihood method. We performed Geant4 simulation and obtained the second differences of several hundred of tracks for various single charged particles in emulsion. Fig. (2) shows their geometric mean distributions of among 0.2mm-2.0mm range. It is promising in the separation of single charged unknown particles in emulsion.

Event Analysis in Nuclear Emulsion for the E07 Experiment at J-PARC

Hammer track events in nuclear emulsion were analyzed to measure the excitation energy of 8Be* (2+) nucleus. The kinetic energies of two alpha particles of hammer track events were obtained from their ranges with use of range-energy relation. The range-energy relation was calibrated by measuring the alpha particle tracks emitted from 212Po of Thorium decay series in the emulsion. From this calibration, we obtained density and shrinkage factor of the emulsion. The excitation energy and width of 8Be* (2+) nucleus were measured to be 3.39 ± 0.24 MeV and  = 1.22 ± 0.60 MeV respectively.

Spectroscopy of medium-mass hypernuclear production

Theoretical hypernuclear photoproduction spectra are demonstrated by choosing typical medium-heavy nuclear targets such as 28Si, 40Ca, and 52Cr. The predictions provide us with interesting opportunities of spectroscopic study beyond p-shell on the dynamical coupling between a hyperon and nuclear core excitations.