arXiv subjects

A. Ramos

Publications and source records attributed to A. Ramos.

At least 127 records · Page 7Linked to original sources

arXiv2005

Evidence for two Lambda(1405) resonance states

The $K^- p \to π^0 π^0 Σ^0$ reaction is studied within a chiral unitary model. The distribution of $π^0 Σ^0$ states forming the $Λ(1405)$ shows, in agreement with a recent experiment, a peak at 1420 MeV and a relatively narrow width of $Γ= 38$ MeV. We use these data in combination with those of the $π^- p \to K^0 πΣ$ reaction and elements of chiral unitary theory to prove that there are two $Λ(1405)$ states instead of one as so far assumed.

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arXiv2005

Two pole structure of the Lambda(1405) resonance

The K^- p --> pi^0 pi^0 Sigma^0 reaction is studied within a chiral unitary model. The distribution of pi^0 Sigma^0 states forming the Lambda(1405) shows, in agreement with a recent experiment, a peak at 1420 MeV and a relatively narrow width of 38 MeV. This reaction gives maximal possible weight to the second Lambda(1405) state found in chiral unitary theories, which is narrow and of higher energy than the nominal Lambda(1405). In contrast, the pi^- p --> K^0 pi Sigma reaction gives more weight to the pole at lower energy and with a larger width. The data of these two experiments, together with the present theoretical analysis, provides for the first time a clear experimental evidence of the two pole structure of the Lambda(1405).

hep-ph↗

arXiv2005

Microscopic calculations of spin polarized neutron matter at finite temperature

The properties of spin polarized neutron matter are studied both at zero and finite temperature within the framework of the Brueckner--Hartree--Fock formalism, using the Argonne v18 nucleon-nucleon interaction. The free energy, energy and entropy per particle are calculated for several values of the spin polarization, densities and temperatures together with the magnetic susceptibility of the system. The results show no indication of a ferromagnetic transition at any density and temperature.

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arXiv2005

Chiral dynamics of baryon resonances and hadrons in a nuclear medium

In these lectures I make an introduction to chiral unitary theory applied to the meson baryon interaction and show how several well known resonances are dynamically generated, and others are predicted. Two very recent experiments are analyzed, one of them showing the existence of two $Λ(1405)$ states and the other one providing support for the $Λ(1520)$ resonance as a quasibound state of $Σ(1385) π$. The use of chiral Lagrangians to account for the hadronic interaction at the elementary level introduces a new approach to deal with the modification of meson and baryon properties in a nuclear medium. Examples of it for $\bar{K}$, $η$ and $ϕ$ modification in the nuclear medium are presented.

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arXiv2005

Evidence for the two pole structure of the Lambda(1405) resonance

The K^- p --> pi^0 pi^0 Sigma^0 reaction is studied within a chiral unitary model. The distribution of pi^0 Sigma^0 states forming the Lambda(1405) shows, in agreement with a recent experiment, a peak at 1420 MeV and a relatively narrow width of Gamma = 38 MeV. The mechanism for the reaction is largely dominated by the emission of a pi^0 prior to the K^- p interaction leading to the Lambda(1405). This ensures the coupling of the Lambda(1405) to the K^- p channel, thus maximizing the contribution of the second state found in chiral unitary theories, which is narrow and of higher energy than the nominal Lambda(1405). This is unlike the pi^- p --> K^0 πSigma reaction, which gives more weight to the pole at lower energy and with a larger width. The data of these two experiments, together with the present theoretical analysis, provides a firm evidence of the two pole structure of the Lambda(1405).

hep-ph↗

arXiv2005

Dynamical baryon resonances from chiral unitarity

We report on the latests developments in the field of baryonic resonances generated from the meson-baryon interaction in coupled channels using a chiral unitary approach. The collection of resonances found in different strangeness and isospin sectors can be classified into SU(3) multiplets. The Lambda(1405) emerges as containing the effect of two poles of the scattering amplitude and various reactions that might preferentially select one or the other pole are discussed.

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arXiv2005

In-medium effects on the $K^-/K^+$ ratio at GSI

The in-medium modifications on the $K^-/K^+$ ratio produced at GSI are studied. Particular attention is paid to the properties of antikaons, which determine the chemical potential and temperature at freeze-out conditions. Different approaches have been considered: non-interacting $K^-$, on-shell self-energy and single-particle spectral density. We observe that the full off-shell approach reproduces the "broad-band equilibration" advocated by Brown being the spectral density crucial to explain an enhanced $K^-/K^+$ ratio.

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arXiv2005

Chiral dynamics of hadrons in nuclei

In this talk I report on selected topics of hadron modification in the nuclear medium using the chiral unitary approach to describe the dynamics of the problems. I shall mention how antikaons, $η$, and $ϕ$ are modified in the medium and will report upon different experiments done or planned to measure the $ϕ$ width in the medium.

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arXiv2005

Structure of Lambda(1405) and chiral dynamics

We report on a recent theoretical work on the structure of the Lambda(1405) resonance within a chiral unitary approach, in which the resonance is dynamically generated in meson-baryon scattering. Studying the analytic structure of the scattering amplitude, we have found that there are two poles lying around energies of Lambda(1405) with different widths and couplings to the meson-baryon states. We discuss reactions to conform the double pole structure in experiment and elastic K^- p scattering at low energies.

hep-ph↗

arXiv2004

Asymmetries in the Non-Mesonic Weak Decay of Polarized Lambda-Hypernuclei

The non-mesonic weak decay of polarized Lambda-hypernuclei is studied for the first time by taking into account, with a Monte Carlo intranuclear cascade code, the nucleon final state interactions. A one-meson-exchange model is employed to describe the Lambda N-> n N processes in a finite nucleus framework. The relationship between the intrinsic Lambda asymmetry parameter a_Λand the asymmetry a^M_Λaccessible in experiments is discussed. A strong dependence of a^M_Λon nucleon final state interactions and detection threshold is obtained. Our results for a^M_Λare consistent with ^{11}_ΛB and ^{12}_ΛC data but disagree with observations in ^5_ΛHe.

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arXiv2004

Correlations in Hot Asymmetric Nuclear Matter

The single-particle spectral functions in asymmetric nuclear matter are computed using the ladder approximation within the theory of finite temperature Green's functions. The internal energy and the momentum distributions of protons and neutrons are studied as a function of the density and the asymmetry of the system. The proton states are more strongly depleted when the asymmetry increases while the occupation of the neutron states is enhanced as compared to the symmetric case. The self-consistent Green's function approach leads to slightly smaller energies as compared to the Brueckner Hartree Fock approach. This effect increases with density and thereby modifies the saturation density and leads to smaller symmetry energies.

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arXiv2004

Towards a solution of the Gn/Gp puzzle in the weak decay of Lambda-hypernuclei

In this contribution we discuss an evaluation of double-nucleon coincidence spectra for the non-mesonic weak decay of Lambda-hypernuclei. The results have been obtained by combining a one-meson-exchange model for the Lambda N -> nN transition in finite nuclei with a Monte Carlo code accounting for the nucleon final state interactions. A weak-decay-model independent analysis of KEK coincidence data allows us to extract Gn/Gp values around 0.3-0.4 for 5_Lambda-He and 12_Lambda-C, thus representing an important progress towards the solution of the Gn/Gp puzzle. Preliminary results for the angular asymmetries in the non-mesonic weak decay of polarized hypernuclei are also presented.

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arXiv2004

In-medium properties of antikaons at finite temperature: the K^-/K^+ ratio at GSI

The $K^-/K^+$ ratio in heavy-ion collisions at GSI is studied including the properties of the participating hadrons in hot and dense matter, paying a particular attention to the in-medium properties of antikaons at finite temperature. The determination of the temperature and chemical potential at freeze-out conditions compatible with the ratio $K^-/K^+$ is very delicate, and depends on the approach adopted for the antikaon self-energy. Different approaches for the $K^-$ self-energy have been considered: non-interacting $K^-$, on-shell self-energy and single-particle spectral density. We observe that the use of an energy dependent $\bar{K}$ spectral density lowers considerably the freeze-out temperature with respect to the on-shell approach. We also conclude that the full off-shell approach gives rise to the "broad-band equilibration" advocated by Brown, Rho and Song.

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arXiv2004

Lambda-Lambda bond energy from the Nijmegen potentials

The $ΛΛ$ bond energy $ΔB_{ΛΛ}$ in $ΛΛ$ hypernuclei is obtained from a $G$-matrix calculation which includes the coupling between the $ΛΛ$, $ΞN$ and $ΣΣ$ channels, as well as the effect of Pauli blocking to all orders. The Nijmegen NSC97e model is used as bare baryon-baryon interaction in the strangeness $S=-2$ sector. The $ΛΛ$-$ΞN$ coupling increases substantially the bond energy with respect to the uncoupled $ΛΛ$ case. However, the additional incorporation of the $ΣΣ$ channel, which couples simultaneously to $ΛΛ$ and $ΞN$ states, has a surprisingly drastic effect and reduces the bond energy down to a value closer to that obtained in an uncoupled calculation. We find that a complete treatment of Pauli blocking reduces the repulsive effect on the bond energy to about half of what was claimed before.

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arXiv2004

Chiral dynamics of the two Lambda(1405) states

The Lambda(1405) resonance is studied by a chiral unitary approach, in which the resonance is dynamically generated in coupled-channel meson-baryon scattering. Investigating the analytic structure of the scattering amplitudes obtained by the chiral unitary approach, we find two poles around the energies of the Lambda(1405) coupling differently to the meson-baryon states. We reach the conclusion that the Lambda(1405) resonance seen in experiments is not just one single resonance, but a superposition of these two states.

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arXiv2003

Non-Mesonic Weak Decay of Lambda-hypernuclei: a new determination of the Gn/Gp ratio

Theoretical descriptions of the non-mesonic weak decay of Lambda-hypernuclei are unable to reproduce the experimental values of the ratio Gn/Gp=G(Lambda n -> nn)/G(Lambda p -> np). In this contribution we discuss a new approach to this 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 double-coincidence nucleon distributions corresponding to the non-mesonic decay of 5_Lambda-He and 12_Lambda-C. The two-nucleon induced decay mechanism, Lambda np -> nnp, has been taken into account within a local density approximation scheme using a one-pion-exchange model supplemented by short range correlations. A weak decay model independent analysis of preliminary KEK coincidence data for 5_Lambda-He allows us to extract Gn/Gp=0.39 \pm 0.11 when the two-nucleon induced channel is neglected (i.e., G2=0) and Gn/Gp=0.26 \pm 0.11 when G2/G1=0.2.

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arXiv2003

Dynamical Generation of Hyperon Resonances

In this talk we report on how, using a chiral unitary approach for the meson--baryon interactions, two octets of $J^π=1/2^-$ baryon states and a singlet are generated dynamically, resulting in the case of strangeness $S=-1$ in two poles of the scattering matrix close to the nominal $Λ(1405)$ resonance. We suggest experiments which could show evidence for the existence of these states.

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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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