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Register a new userOn the structure in the $Lambda N$ cross section at the $Sigma N$ threshold
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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.
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We reexamine the spin-orbit splitting of 9 Lambda Be excited states in terms of the SU_6 quark-model baryon-baryon interaction. The previous folding procedure to generate the Lambda alpha spin-orbit potential from the quark-model Lambda N LS interaction kernel predicted three to five times larger values for Delta E_{ell s}=E_x(3/2^+)-E_x(5/2^+) in the model FSS and fss2. This time, we calculate Lambda alpha LS Born kernel, starting from the LS components of the nuclear-matter G-matrix for the Lambda hyperon. This framework makes it possible to take full account of an important P-wave Lambda N - Sigma N coupling through the antisymmetric LS^{(-)} force involved in the Fermi-Breit interaction. We find that the experimental value, Delta E^{exp}_{ell s}=43 pm 5 keV, is reproduced by the quark-model G-matrix LS interaction with a Fermi-momentum around k_F=1.0 fm^{-1}, when the model FSS is used in the energy-independent renormalized RGM formalism.
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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The $gamma + p rightarrow K^0 + Sigma^+$ photoproduction reaction is investigated in the energy region from threshold to $E_gamma = 2250$,MeV. The differential cross section exhibits increasing forward-peaking with energy, but only up to the $K^*$ threshold. Beyond, it suddenly returns to a flat distribution with the forward cross section dropping by a factor of four. In the total cross section a pronounced structure is observed between the $K^*Lambda$ and $K^*Sigma$ thresholds. It is speculated whether this signals the turnover of the reaction mechanism from t-channel exchange below the $K^*$ production threshold to an s-channel mechanism associated with the formation of a dynamically generated $K^*$-hyperon intermediate state.
The $^{12}$C(n, 2n)$^{11}$C cross section was measured from just below threshold to 26.5 MeV using the Pelletron accelerator at Ohio University. Monoenergetic neutrons, produced via the $^3$H(d,n)$^4$He reaction, were allowed to strike targets of polyethylene and graphite. Activation of both targets was measured by counting positron annihilations resulting from the $beta^+$ decay of $^{11}$C. Annihilation gamma rays were detected, both in coincidence and singly, using back-to-back NaI detectors. The incident neutron flux was determined indirectly via $^{1}$H(n,p) protons elastically scattered from the polyethylene target. Previous measurements fall into upper and lower bands, the results of the present measurement are consistent with the upper band.
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