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Register a new userStudy of the $Lambda p$ Interaction Close to the $Sigma^+n$ and $Sigma^0p$ Thresholds
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The $Lambda p$ interaction close to the $Sigma N$ threshold is considered. Specifically, the pronounced structure seen in production reactions like $K^-d to pi^- Lambda p$ and $ppto K^+ Lambda p$ around the $Sigma N$ threshold is analyzed. Modern interaction models of the coupled $Lambda N - Sigma N$ systems generate such a structure either due to the presence of a (deuteron-like) unstable bound state or of an inelastic virtual state. % A determination of the position of the prominent peak as observed in various experiments for the two aforementioned reactions leads to values that agree quite well with each other. Furthermore, the deduced mean value of $2128.7pm 0.3$ MeV for the peak position coincides practically with the threshold energy of the $Sigma^+ n$ channel. This supports the interpretation of the structure as a genuine cusp, signaling an inelastic virtual state in the $^3S_1-^3D_1$ partial wave of the $Sigma N$ isospin 1/2 channel. % There is also evidence for a second peak (or shoulder) in the data sets considered which appears at roughly 10-15 MeV above the $Sigma N$ threshold. However, its concrete position varies significantly from data set to data set and, thus, a theoretical interpretation is difficult.
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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.
We present results of an exclusive measurement of the first excited state of the Sigma hyperon, Sigma(1385)^+, produced in p+p -> Sigma^+ + K^+ + n at 3.5 GeV beam energy. The extracted data allow to study in detail the invariant mass distribution of the Sigma(1385)^+. The mass distribution is well described by a relativistic Breit-Wigner function with a maximum at m_0 = 1383.2 +- 0.9 MeV/c^2 and a width of 40.2 +- 2.1 MeV/c^2. The exclusive production cross-section comes out to be 22.27 +- 0.89 +- 1.56 +3.07 -2.10 mu b. Angular distributions of the Sigma(1385)^+ in different reference frames are found to be compatible with the hypothesis that 33 % of Sigma(1385)^+ result from the decay of an intermediate Delta^{++} resonance.
We study the photoproduction of the $Lambda(1405)$ and $Sigma(1400)$ hyperon resonances, the latter of which is not a well established state. We evaluate the $s$-, $t$- and $u$-channel diagrams in the Born approximation by employing the effective Lagrangians. A new ingredient is the inclusion of a nucleon resonance $N^*(1895)$ that is dynamically generated with predictions for its coupling to the $KLambda(1405)$ and $KSigma(1400)$ channels. To extend the applicability of the model to energies beyond the threshold region, we consider a Regge model for the $t$-channel $K$- and $K^*$-exchanges. Our results are in good agreement with the CLAS data available on $Lambda(1405)$, while for $Sigma(1400)$ we predict observables for its production. We also provide polarization observables for both hyperon productions, which can be useful in future experimental investigations. The present study provides new information on the nucleon resonance $N^*(1895)$ which can be an alternative source for generating the hyperon resonances $Lambda(1405)$ and $Sigma(1400)$.
We report measurements of cross sections for the reaction p(e,eK)Y, for both the Lambda and Sigma_0 hyperon states, at an invariant mass of W=1.84 GeV and four-momentum transfers 0.5<Q2<2 (GeV/c)2. Data were taken for three values of virtual photon polarization, allowing the decomposition of the cross sections into longitudinal and transverse components. The Lambda data is a revised analysis of prior work, whereas the Sigma_0 results have not been previously reported.
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