arXiv subjects

A. Parreno

Publications and source records attributed to A. Parreno.

29 records · Page 2Linked to original sources

arXiv2003

Towards the solution of the Gn/Gp Puzzle in the Non-Mesonic Weak Decay of Lambda-Hypernuclei

One of the main open problems in the physics of Lambda-hypernuclei is the lack of a sound theoretical interpretation of the large experimental values for the ratio Gn/Gp=G(Lambda n -> nn)/G(Lambda p -> np). To approach the problem, we have incorporated a one-meson-exchange model for the Lambda N -> nN transition in finite nuclei in an intranuclear cascade code for the calculation of single and double-coincidence nucleon distributions corresponding to the non-mesonic weak decay of 5_Lambda-He and 12_Lambda-C. Due to the elimination of interferences, two-nucleon coincidences are expected to give a cleaner determination of Gn/Gp than single-nucleon observables. Single-nucleon distributions are found to be less sensitive to variations of Gn/Gp than double-coincidence spectra. The comparison of our results with preliminary KEK coincidence data allows us to extract a Gn/Gp ratio for 5_Lambda-He of 0.39+-0.11 when multinucleon induced channels are omitted.

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arXiv2003

Exploring Hyperons and Hypernuclei with Lattice QCD

In this work we outline a program for lattice QCD that would provide a first step toward understanding the strong and weak interactions of strange baryons. The study of hypernuclear physics has provided a significant amount of information regarding the structure and weak decays of light nuclei containing one or two Lambda's, and Sigma's. From a theoretical standpoint, little is known about the hyperon-nucleon interaction, which is required input for systematic calculations of hypernuclear structure. Furthermore, the long-standing discrepancies in the P-wave amplitudes for nonleptonic hyperon decays remain to be understood, and their resolution is central to a better understanding of the weak decays of hypernuclei. We present a framework that utilizes Luscher's finite-volume techniques in lattice QCD to extract the scattering length and effective range for Lambda-N scattering in both QCD and partially-quenched QCD. The effective theory describing the nonleptonic decays of hyperons using isospin symmetry alone, appropriate for lattice calculations, is constructed.

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arXiv2003

Lowest Order Effective Field Theory for the weak $ΛN$ interaction

The $|ΔS|=1$ $ΛN$ interaction, responsible for the decay of hypernuclei, is studied by means of an Effective Field Theory where the long range physics is described by pion and kaon exchange mechanisms, and its short range counterpart is obtained from the most general non-derivative local four-fermion interaction. We show that, including the Lowest Order Parity Conserving contact terms, allows us to reproduce the total decay rates for $^5_Λ{\rm He}$, $^{11}_Λ{\rm B}$ and $^{12}_Λ{\rm C}$ with a reasonable value of ${\hat χ^2}$, while in order to get a prediction for the Parity Violating (PV) asymmetry compatible with experiments, we have to include the Lowest Order PV contact pieces.

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arXiv2003

Nucleon-Nucleon Coincidence Spectra in the Non-Mesonic Weak Decay of Lambda-Hypernuclei and the n/p Puzzle

The main open problem in the physics of Lambda-hypernuclei is the lack of a sound theoretical interpretation of the large experimental values for the ratio G_n/G_p. To approach the problem, we have incorporated a Lambda N-> nN one-meson-exchange model in an intranuclear cascade code for a finite nucleus calculation of the nucleon-nucleon (angular and energy) coincidence distributions in the non-mesonic weak decay of 5_Lambda-He and 12_Lambda-C hypernuclei. The two-nucleon induced decay mechanism, Lambda np-> nnp, has been taken into account by means of the polarization propagator formalism in local density approximation. With respect to single nucleon spectra studies, the treatment of two-nucleon correlations allow a cleaner and more direct determination, from data, of G_n/G_p. Our results agree with the preliminary coincidence data of KEK-E462 for 5_Lambda-He. This allows us to conclude that G_n/G_p for 5_Lambda-He should be close to 0.46, the value predicted by our one-meson-exchange model for this hypernucleus. Unfortunately, to disentangle one- and two-body induced decay channels from experimental non-mesonic decay events, three-nucleon coincidences should be measured.

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arXiv2001

Recent results on the nonmesonic weak decay of hypernuclei within a one-meson-exchange model

We update our previous results for the nonmesonic decay of $^{12}_Λ$C and $^5_Λ$He. We pay special attention to the role played by Final State Intreractions on the decay observables. We follow a One-Meson-Exchange model which includes the exchange of the $π, ρ, K, K^*, η$ and $ω$ mesons. We also present recent predictions for different observables concerning the decay of the doubly strange $^6_{ΛΛ}$He hypernucleus.

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arXiv2001

Novel Weak Decays in Doubly Strange Systems

The strangeness-changing ($ΔS = 1$) weak baryon-baryon interaction is studied through the nonmesonic weak decay of double-$Λ$ hypernuclei. Besides the usual nucleon-induced decay $ΛN \to N N$ we discuss novel hyperon-induced decay modes $ΛΛ\to ΛN$ and $ΛΛ\to ΣN$. These reactions provide unique access to the exotic $ΛΛ$K and $ΛΣ$K vertices which place new constraints on Chiral Pertubation Theory ($χ$PT) in the weak SU(3) sector. Within a meson-exchange framework, we use the pseudoscalar $π,η,K$ octet for the long-range part while parametrizing the short-range part through the vector mesons $ρ, ω, K^*$. Realistic baryon-baryon forces for the $S=0,-1$ and -2 sectors account for the strong interaction in the initial and final states. For $^6_{ΛΛ}$He the new hyperon-induced decay modes account for up to 4% of the total nonmesonic decay rate. Predictions are made for all possible nonmesonic decay modes.

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arXiv2001

Dynamical meson-baryon resonances with chiral Lagrangians

The s-wave meson-baryon interaction is studied using the lowest-order chiral Lagrangian in a unitary coupled-channels Bethe-Salpeter equation. In the strangeness $S=-1$ sector the low-energy $K^- p$ dynamics leads to the dynamical generation of the $Λ(1405)$ as a ${\bar K}N$ state, along with a good description of the $K^- p$ scattering observables. At higher energies, the $Λ(1670)$ is also found to be generated dynamically as a $K Ξ$ quasibound state for the first time. For strangeness S=0, it is the $S_{11}(1535)$ resonance that emerges from the coupled-channels equations, leading to a satisfactory description of meson-baryon scattering observables in the energy region around the $S_{11}(1535)$. We speculate on the possible dynamical generation of $Ξ$ resonances within the chiral $S=-2$ sector as ${\bar K} Λ$ or ${\bar K} Σ$ quasibound states.

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arXiv2000

Role of deformation in the nonmesonic decay of light hypernuclei

We discuss the nonmesonic decay of deformed p-shell hypernuclei. The Nilsson model with angular momentum projection is employed in order to take into account the deformation effects. The nonmesonic decay rate and the intrinsic $Λ$ asymmetry parameter decrease as a function of the deformation parameter, while the ratio of the neutron- to proton-induced decay rates increases. We find that the deformation effects change these observables by about 10 % for $^9_Λ$Be from the spherical limit.

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arXiv2000

Chiral Unitary Approach To The NNpi, NNeta Couplings For The N*(1535) Resonance

Using a chiral unitary model in which the negative parity nucleon resonance $N^\ast \equiv N^\ast(1535)$ is generated dynamically by means of the Bethe Salpeter equation with coupled meson baryon channels in the $S = 0$ sector, we have obtained the $π^0 N^\ast N^\ast$ and $ηN^\ast N^\ast$ couplings. The $π^0 N^\ast N^\ast$ coupling has smaller strength and the same sign as the $π^0 N N$ coupling. This rules out the mirror assignment of chiral symmetry where the ground state nucleon $N$ and the negative parity resonance $N^\ast$ are envisaged as chiral partners in the baryon sector.

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arXiv1999

SU(3) Chiral approach to meson and baryon dynamics

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.

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arXiv1999

Recent progress on the chiral unitary approach to meson meson and meson baryon interactions

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.

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