arXiv subjects
Publications and source records attributed to A. Ramos.
The basic concepts of factorizable problems in one-dimensional Quantum Mechanics, as well as the theory of Shape Invariant potentials are reviewed. The relation of this last theory with a generalization of the classical Factorization Method presented by Infeld and Hull is analyzed in detail. By the use of some properties of the Riccati equation the solutions of Infeld and Hull are greatly generalized in a simple way.
We have developed a formalism for microscopic Brueckner-type calculations of dense nuclear matter that includes all types of baryon-baryon interactions and allows to treat any asymmetry on the fractions of the different species (n, p, $Λ$, $Σ^0$, $Σ^+$, $Σ^-$, $Ξ^-$ and $Ξ^0$). We present results for the different single-particle potentials focussing on situations that can be relevant in future microscopic studies of beta-stable neutron star matter with strangeness. We find the both the hyperon-nucleon and hyperon-hyperon interactions play a non-negligible role in determining the chemical potentials of the different species.
We report on recent progress on the chiral unitary approach, which is shown to have a much larger convergence radius than ordinary chiral perturbation theory, allowing one to reproduce data for meson meson interaction up to 1.2 GeV and meson baryon interaction up to the first baryonic resonances. Applications to physical processes so far unsuited for a standard chiral perturbative approach are presented, concretely the K^- p\toΛ(1405)γreaction and the N^\ast (1535)N^\ast(1535)πand ηcouplings.
We report on recent progress on the chiral unitary approach, analogous to the effective range expansion in Quantum Mechanics, which is shown to have a much larger convergence radius than ordinary chiral perturbation theory, allowing one to reproduce data for meson meson interaction up to 1.2 GeV. Applications to physical processes so far unsuited for a standard chiral perturbative approach are presented, in which two hadrons, either two mesons or a meson and a baryon, in a resonant state are produced in photoproduction processes.
The self-energy of the K^- meson in nuclear matter is calculated in a self-consistent microscopic approach, using a \bar{K}N interaction obtained from the lowest-order meson-baryon chiral Lagrangian. The effective \bar{K}N interaction in the medium is derived by solving the coupled-channel Bethe-Salpeter equation including Pauli blocking on the nucleons, mean-field binding potentials for the baryons and the self-energy of the πand \bar{K} mesons. The incorporation of the self-consistent {\bar K} self-energy in the description, in addition to the Pauli blocking effects, yields a weaker attractive in-medium {\bar K}N interaction and a Λ(1405) which dissolves faster with increasing matter density, as a result of the {\bar K} spectral function being spread out over a wide range of energies. These effects are further magnified when the intermediate pions are dressed.
We have carried a theoretical study of the K^- p\to M B γreaction with M B = K^-p, \bar{K}^0 n, π^- Σ^+, π^+ Σ^-, π^0 Σ^0, π^0 Λ, for K^- lab. momenta between 200 and 500 MeV/c, using a chiral unitary approach for the strong K^-p interaction with its coupled channels. The Λ(1405) resonance, which is generated dynamically in this approach, shows up clearly in the dσ/dM_I spectrum, providing new tests for chiral symmetry and the unitary approach, as well as information regarding the nature of the resonance. The photon detection alone, summing all channels, is shown to reproduce quite accurately the strength and shape of the Λ(1405) resonance. Analogous reactions in nuclei can provide much information on the properties of this resonance in a nuclear medium.
We present a nonrelativistic transition potential for the weak strangeness-changing reaction $ΛN \to NN$. The potential is based on a one meson exchange model (OME), where, in addition to the long-ranged pion, the exchange of the pseudoscalar $K, η$, as well as the vector $ρ, ω, K^*$ mesons is considered. Results obtained for different hypernuclear decay observables are compared to the available experimental data.
We have made a new evaluation of the Lambda decay width in nuclear matter within the Propagator Method. Through the Local Density Approximation it is possible to obtain results in finite nuclei. We have also studied the dependence of the widths on the N-N and Lambda-N short range correlations. Using reasonable values for the parameters that control these correlations, as well as realistic nuclear densities and Lambda wave functions, we reproduce, for the first time, the experimental non-mesonic widths in a wide range of mass numbers (from medium to heavy hypernuclei).
We report on recent progress on the chiral unitary approach, analogous to the effective range expansion in Quantum Mechanics, which is shown to have a much larger convergence radius than ordinary chiral perturbation theory, allowing one to reproduce data for meson meson interaction up to 1.2 GeV. Applications to physical processes so far unsuited for a standard chiral perturbative approach are presented. Results for the extension of these ideas to the meson baryon sector are discussed, together with applications to kaons in a nuclear medium and $K^-$ atoms.
We study the gamma p ---> K^+ Lambda(1405) reaction at energies close to threshold using a chiral unitary model where the resonance is generated dynamically from K^-p interaction with other channels constructed from the octets of baryons and mesons. Predictions are made for cross sections into several channels and it is shown that the detection of the K^+ is sufficient to determine the shape and strength of the Lambda(1405) resonance. The determination of the resonance properties in nuclei requires instead the detection of the resonance decay channels. Pauli blocking effects on the resonance, which have been shown to be very important for the resonance at rest in the nucleus, are irrelevant here where the resonance is produced with a large momentum. The nuclear modifications here would thus offer information on the resonance and K^- nucleus dynamics complementary to the one offered so far by K^- atoms.
The weak production of Lambdas in nucleon-nucleon scattering is studied in a meson-exchange framework. The weak transition operator for the $NN \to N Λ$ reaction is identical to a previously developed weak strangeness-changing transition potential $ΛN \to NN$ that describes the nonmesonic decay of hypernuclei. The initial $NN$ and final $YN$ state interaction has been included by using realistic baryon-baryon forces that describe the available elastic scattering data. The total and differential cross sections as well as the parity-violating asymmetry are studied for the reaction $pn \to pΛ$. These observables are found to be sensitive to the choice of the strong interaction potential and the structure of the weak transition potential.
The s-wave meson-nucleon interaction in the S=-1 sector is studied by means of a coupled-channel Lippmann Schwinger equation, using the lowest order chiral Lagrangian and a cut off to regularize the loop integrals. The position and width of the Lambda(1405) resonance and the K^- p scattering cross sections at low energies are well reproduced. The inclusion of the eta Lambda, eta Sigma^0 channels in the coupled system is found very important and allows a solution in terms of only the lowest order Lagrangian. The model is applied to calculate the in-medium K^- self-energy to which we add a small p-wave piece resulting from the coupling to hyperon particle-nucleon hole excitations. The K^- feels an attraction of about -100 MeV at normal nuclear density. The Lambda(1405) resonance shifts to energies above the $K^-p$ threshold and ends up dissolving as density increases. It remains to be seen how these effects persist when the dressing of the Kbar and the pi mesons is incorporated self-consistently in the calculation.
s- and p-wave $Λ$ single-particle enrgies are obtained for a variety of $Λ$ hypernuclei from the relevant self-energy constructed within the framework of a perturbative many-body approach. Results are presented for present realistic hyperon-nucleon interactions such as Jülich B and Nijmegen SC models. Effects of the non-locality and energy-dependence of the self-energy on the bound states are investigated.
With the advent of high precision proton accelerators, such as COSY in Jülich, or with medium energy accelerators like RCNP at Osaka, it may become possible to perform a direct study of the strangeness changing hadronic $np \to Λp$ process. In this work we study this reaction using a one-meson-exchange weak potential and realistic $NN$ and $YN$ strong interactions to distort both the initial $np$ and the final $Λp$ states. The calculated cross sections are very sensitive to the different mesons included in the weak transition, as well as to the strong $YN$ interaction model that generates the distorted final state.
A recently developed nonperturbative chiral approach to describe the S=-1 meson-baryon reactions has been extended to investigate the near threshold $K^- p \to γΛ, γΣ^0$ reactions. With the parameters governed by chiral SU(3) symmetry, we show that the predicted branching ratios $Γ_{K^- p \to γΛ}/Γ_{K^- p \to all}$ and $Γ_{K^-p\to γΣ^0}/Γ_{K^- p \to all}$ are close to the experimental values. The coupling with the $η$ channels, which was shown to be important in the $S=-1$ meson-baryon reactions, is also found to be significant here. Our results are consistent with the interpretation of the $Λ(1405)$ as a quasi-bound meson-baryon state as found in other similar chiral approaches.
We review recent developments concerning the weak decay of Lambda hypernuclei. New studies covering the mesonic decay channel as well as recent models for the non-mesonic one are discussed and compared with experimental data. The puzzle of the neutron- to proton-induced decay ratio, Gamma_n/Gamma_p, is addressed in connection to the two-nucleon induced decay channel and proposals for more efficient experimental analyses of this ratio are made.
A qualitative account of the meson-meson and meson-baryon interactions using chiral Lagrangians and the inverse amplitude method in coupled channels is done. The method, imposing exact unitarity, proves to be very useful tool to extend the information contained in the chiral Lagrangians at energies beyond the realm of applicability of chiral perturbation theory.
Single-particle energies of the $Λ$ and $Σ$ hyperons are obtained from the relevant self-energies. The latter are constructed within the framework of a perturbative many-body approach employing present realistic hyperon-nucleon interactions such as the models of the Juelich and Nijmegen groups. The effects of the non-locality and energy dependence of the self-energy on the bound states are investi- gated. It is also shown that, although the single-particle hyperon energies are well reproduced by local Woods-Saxon hyperon-nucleus potentials, the wave functions from the non-local self-energy are far more extended. Implications of this behavior on the mesonic weak decay of $Λ$ hypernuclei are discused.