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Register a new userOn the Delta I = 1/2 rule in the Lambda N ----> N N reaction
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It is shown that the mass dependence of the $Lambda$-lifetime in heavy hypernuclei is sensitive to the ratio of neutron-induced to proton-induced non-mesonic decay rates R_n/R_p. A comparison of the experimental mass dependence of the lifetimes with the calculated ones for different values of R_n/R_p leads to the conclusion that this ratio is larger than 2 on the confidence level of 0.75. This suggests that the phenomenological $Delta$I=1/2 rule might be violated for the nonmesonic decay of the $Lambda$-hyperon.
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We have developed a model for the N N --> N N pi pi reaction and evaluated cross sections for the different charged channels. The low energy part of those channels where the pions can be in an isospin zero state is dominated by N* excitation, driven by an isoscalar source recently found experimentally, followed by the decay N* --> N (pi pi, T=0, s-wave). At higher energies, and in channels where the pions are not in T=0, Delta excitation mechanisms become relevant. A rough agreement with the experimental data is obtained in most channels. Repercussions of the present findings for the ABC effect and the p p --> p p pi0 reaction close to threshold are also suggested.
The complexity of threshold phenomena is exemplified on a prominent and long-known case - the structure in the $Lambda p$ cross section (invariant mass spectrum) at the opening of the $Sigma N$ channel. The mass splitting between the $Sigma$ baryons together with the angular momentum coupling in the $^3S_1$-$^3D_1$ partial wave imply that, in principle, up to six channels are involved. Utilizing hyperon-nucleon potentials that provide an excellent description of the available low-energy $Lambda p$ and $Sigma N$ scattering data, the shape of the resulting $Lambda p$ cross section is discussed and the poles near the $Sigma N$ threshold are determined. Evidence for a strangeness $S=-1$ dibaryon is provided, in the form of a deuteron-like (unstable) $Sigma N$ bound state. Predictions for level shifts and widths of $Sigma^-p$ atomic states are given.
We theoretically analyze the $K^{-} {}^{3} text{He} to Lambda p n$ reaction for the $bar{K} N N$ bound-state search in the J-PARC E15 experiment. We find that, by detecting a fast and forward neutron in the final state, an almost on-shell $bar{K}$ is guaranteed, which is essential to make a bound state with two nucleons from ${}^{3} text{He}$. Then, this almost on-shell $bar{K}$ can bring a signal of the $bar{K} N N$ bound state in the $Lambda p$ invariant-mass spectrum, although it inevitably brings a kinematic peak above the $bar{K} N N$ threshold as well. As a consequence, we predict two peaks across the $bar{K} N N$ threshold in the spectrum: the lower peak coming from the $bar{K} N N$ bound state, and the higher one originating from the kinematics.
Based on the scenario that a $bar{K} N N$ bound state is generated and it eventually decays into $Lambda p$, we calculate the cross section of the $K^{-} {}^{3} text{He} to Lambda p n$ reaction, which was recently measured in the J-PARC E15 experiment. We find that the behavior of the calculated differential cross section $d ^{2} sigma / d M_{Lambda p} d q_{Lambda p}$, where $M_{Lambda p}$ and $q_{Lambda p}$ are the $Lambda p$ invariant mass and momentum transfer in the $(K^{-} , , n)$ reaction in the laboratory frame, respectively, is consistent with the experiment. Furthermore, we can reproduce almost quantitatively the experimental data of the $Lambda p$ invariant mass spectrum in the momentum transfer window $350 text{ MeV} /c < q_{Lambda p} < 650 text{ MeV} /c$. These facts strongly suggest that the $bar{K} N N$ bound state was indeed generated in the J-PARC E15 experiment.
The near-threshold n p -> d pi0 cross section is calculated in chiral perturbation theory to next-to-leading order in the expansion parameter sqrt{M m_pi}/Lambda_chi. At this order irreducible pion loops contribute to the relevant pion-production operator. While their contribution to this operator is finite, considering initial-and final-state distortions produces a linear divergence in its matrix elements. We renormalize this divergence by introducing a counterterm, whose value we choose in order to reproduce the threshold n p -> d pi0 cross section measured at TRIUMF. The energy-dependence of this cross section is then predicted in chiral perturbation theory, being determined by the production of p-wave pions, and also by energy dependence in the amplitude for the production of s-wave pions. With an appropriate choice of the counterterm, the chiral prediction for this energy dependence converges well.
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