arXiv subjects

A. Ramos

Publications and source records attributed to A. Ramos.

At least 109 records · Page 6Linked to original sources

arXiv2009

K- absorption in nuclei by two and three nucleons

It will be shown that the peaks in the (Lambda p) and (Lambda d) invariant mass distributions, observed in recent FINUDA experiments and claimed to be signals of deeply bound kaonic states, are naturally explained in terms of K- absorption by two or three nucleons leaving the rest of the original nuclei as spectator. For reactions on heavy nuclei, the subsequent interactions of the particles produced in the primary absorption process with the residual nucleus play an important role. Our analyses leads to the conclusion that at present there is no experimental evidence of deeply bound K- state in nuclei. Although the FINUDA experiments have been done for reasons which are not supported a posteriori, some new physics can be extracted from the data.

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arXiv2008

Liquid-gas phase transition in nuclear matter from realistic many-body approaches

The existence of a liquid-gas phase transition for hot nuclear systems at subsaturation densities is a well established prediction of finite temperature nuclear many-body theory. In this paper, we discuss for the first time the properties of such phase transition for homogeneous nuclear matter within the Self-Consistent Green's Functions approach. We find a substantial decrease of the critical temperature with respect to the Brueckner-Hartree-Fock approximation. Even within the same approximation, the use of two different realistic nucleon-nucleon interactions gives rise to large differences in the properties of the critical point.

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arXiv2008

Strange mesons in nuclear matter at finite temperature

We study the properties of $K$ and $\bar K$ mesons in nuclear matter at finite temperature from a chiral unitary approach in coupled channels which incorporates the $s$- and p-waves of the kaon-nucleon interaction. The in-medium solution accounts for Pauli blocking effects, mean-field binding on all the baryons involved, and $π$ and kaon self-energies. We calculate $K$ and $\bar K$ (off-shell) spectral functions and single particle properties. The $\bar K$ effective mass gets lowered by about -50 MeV in cold nuclear matter at saturation density and by half this reduction at T=100 MeV. The p-wave contribution to the ${\bar K}$ optical potential, due to $Λ$, $Σ$ and $Σ^*$ excitations, becomes significant for momenta larger than 200 MeV/c and reduces the attraction felt by the $\bar K$ in the nuclear medium.The $\bar K$ spectral function spreads over a wide range of energies, reflecting the melting of the $Λ(1405)$ resonance and the contribution of $YN^{-1}$ components at finite temperature. In the $KN$ sector, we find that the low-density theorem is a good approximation for the $K$ self-energy close to saturation density due to the absence of resonance-hole excitations. The $K$ potential shows a moderate repulsive behavior, whereas the quasi-particle peak is considerably broadened with increasing density and temperature. We discuss the implications for the decay of the $ϕ$ meson at SIS/GSI energies as well as in the future FAIR/GSI project.

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arXiv2008

Critical review of [K- ppn] bound states

We make a thorough study of the process of three body kaon absorption in nuclei, in connection with a recent FINUDA experiment which claims the existence of a deeply bound kaonic state from the observation of a peak in the Lambda d invariant mass distribution following K- absorption on Li6. We show that the peak is naturally explained in terms of K- absorption from three nucleons leaving the rest as spectators. We can also reproduce all the other observables measured in the same experiment and used to support the hypothesis of the deeply bound kaon state. Our study also reveals interesting aspects of kaon absorption in nuclei, a process that must be understood in order to make progress in the search for K- deeply bound states in nuclei.

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arXiv2008

Chiral SU(3) Bethe Salpeter Model: Extension to SU(6) and SU(8) Spin-Flavor Symmetries

Consistent SU(6) and SU(8) spin-flavor extensions of the SU(3) flavor Weinberg-Tomozawa (WT) meson-baryon chiral Lagrangian are constructed, which incorporate vector meson degrees of freedom. In the charmless sector, the on-shell approximation to the Bethe-Salpeter (BS) approach successfully reproduces previous SU(3) WT results for the lowest-lying s--wave negative parity baryon resonances. It also provides some information on the dynamics of heavier ones and of the lightest d-wave negative parity resonances, as e.g. the Lambda(1520). For charmed baryons the scheme is consistent with heavy quark symmetry, and our preliminary results in the strangeness-less charm C=+1 sector describe the main features of the three-star J^P=1/2^- Lambda_c(2595) and J^P=3/2^- Lambda_c(2625) resonances. We also find a second broad J^P=1/2^- state close to the Lambda_c(2595)

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arXiv2007

Resonances in chiral unitary approaches

The extension of chiral theories to the description of resonances, via the incorporation of unitarity in coupled channels, has provided us with a new theoretical perspective on the nature of some of the observed excited hadrons. In this contribution some of the early achievements in the field of baryonic resonances are reviewed, the recent evidence of the two-pole nature of the Lambda(1405) is discussed and results on charmed baryon resonances are presented.

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arXiv2007

Two-pion-exchange in the non-mesonic weak decay of $Λ$-hypernuclei

The non-mesonic weak decay of $Λ$--hypernuclei is studied within a one-meson-exchange potential supplemented by a chirally motivated two-pion-exchange mechanism. The effects of final state interactions on the outgoing nucleons are also taken into account. In view of the severe discrepancies between theoretical expectations and experimental data, particular attention is payed to the asymmetry of the protons emitted by polarized hypernuclei. The one-meson-exchange model describes the non-mesonic rates and the neutron-to-proton ratio satisfactorily but predicts a too large and negative asymmetry parameter. The uncorrelated and correlated two-pion mechanisms change the rates moderately, thus maintaining the agreement with experiment. The modification in the strength and sign of some decay amplitudes becomes crucial and produces asymmetry parameters which lie well within the experimental observations.

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arXiv2007

The Delta N -> NN transition in finite nuclei

We perform a direct finite nucleus calculation of the partial width of a bound Delta isobar decaying through the non-mesonic decay mode, Delta N -> NN. This transition is modeled by the exchange of the long ranged pi meson and the shorter ranged rho meson. The contribution of this decay channel is found to be approximately 60 % of the decay width of the Delta particle in free space. Considering the additional pionic decay mode, we conclude that the total decay width of a bound Delta resonance in nuclei is of the order of 100 MeV and, consequently, no narrow Delta nuclear states exist, contrary to recent claims in the literature. Our results are in complete agreement with microscopic many-body calculations and phenomenological approaches performed in nuclear matter.

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arXiv2007

Recent developments in chiral dynamics of hadrons and hadrons in nuclei

In this talk I present recent developments in the field of hadronic physics and hadrons in the nuclear medium. I review the unitary chiral approach to meson baryon interaction and address the topics of the two dynamically generated $Λ(1405)$ resonances, with experiments testing it, the $Λ(1520)$ and $Δ(1700)$ resonances, plus the $Λ(1520)$, $Σ(1385)$ and $ω$ in the nuclear medium.

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arXiv2007

A theoretical view on bound antikaon-nuclear states

We present an overview of the latest theoretical studies on the antikaon properties in the nuclear medium, in connection with the recent experimental claims of very deeply bound antikaon-nuclear states. We argue that proper many-body formulations using modern realistic antikaon-nucleon interactions are not able to generate such systems. Instead, a simple two-nucleon antikaon absorption mechanism where the remaining nucleus acts as spectator explains the peak in the semi-inclusive proton momentum spectrum, observed on a 4He target at KEK (but later not confirmed in an inclusive experiment) and on a 6Li target at FINUDA. This signal is clearly seen in another FINUDA experiment measuring the invariant mass of Lambda-proton pairs after two-nucleon kaon absorption. We show that another peak of this experiment, seen at lower invariant masses and interpreted as a bound K-pp state, is simply generated by the same two-nucleon absorption mechanism followed by final-state interactions of the produced particles with the residual nucleus. Our conclusion is that all the experimental claims for the formation of very deeply bound antikaonic nuclear systems receive an alternative explanation in terms of conventional nuclear processes.

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arXiv2007

Further considerations concerning claims for deeply bound kaon atoms and reply to criticisms

We briefly review the experiments of KEK and FINUDA, that claim evidence for deeply bound kaon states, from the perspective of recent theoretical papers and experiments that provide an alternative explanation of the peaks seen. At the same time we show that recent criticisms raised by Akaishi and Yamazaki, and exposed by Akaishi in this Conference, have no base.

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arXiv2006

D Mesons in Nuclear Matter: A DN Coupled-Channel Equations Approach

A set of coupled two-body scattering equations is solved for the DN system embedded in an iso-symmetric nuclear matter. The in-medium behavior of charmed D mesons: (D^+,D^0), is investigated from the self-consistent solution within this scheme. The effective meson-baryon Lagrangian in charm quantum number one sector, the key ingredient in the present study, is adopted from a recent model by Hofmann and Lutz which has aimed at combining the charmed meson degree of freedom in a consistent manner with chiral unitary models. After a critical examination, the original model is modified in several important aspects, such as the method of regularization, in order to be more consistent and practical for our objective. The resultant interaction is used to reproduce the position and width of the s-wave Λ_c(2593) resonance in the isospin zero DN channel. In the isospin one channel, it generates a rather wide resonance at \~2770 MeV. The corresponding in-medium solution is then sought by incorporating Pauli blocking and the D- and π-meson dressing self-consistently. At normal nuclear matter density, the resultant Λ_c (2593) is found to stay narrow and shifted at a lower energy, while the I=1 resonance is lowered in position as well and broadened considerably. The possible implication of our findings on the J/Ψsuppression, etc. in relativistic heavy ion collisions is briefly discussed.

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arXiv2006

Critical review of deeply bound kaonic nuclear states

We critically revise the recent claims of narrow deeply bound kaonic states and show that at present there is no convincing experimental evidence for their existence. In particular, we discuss in details the claim of K- pp deeply bound state associated to a peak seen in the Lambda p invariant mass spectrum from K- nuclear absorption reactions by the FINUDA collaboration. An explicit theoretical simulation shows that the peak is simply generated from a two-nucleon absorption process, like K- pp --> Lambda p, followed by final-state interactions of the produced particles with the residual nucleus.

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arXiv2006

The kaon optical potential modified by Θ^+ Pentaquark excitation

We study the kaon nuclear optical potential including the effect of the Θ^+ pentaquark in a self-consistent approach, starting from an extension of the Juelich meson-exchange potential as bare kaon-nucleon interaction. Significant differences between a fully self-consistent calculation and the low-density Tρapproximation are observed. Whereas te influence of the KN -> Θ^+ absorption process is found to be negligible, due to the small Θ^+ KN coupling, the two-nucleon mechanism (KNN -> Θ^+ N), estimated from the two-meson coupling of the pentaquark, provides the necessary additional kaon absorption to reconcile with data the systematically low theoretical K^+ nucleus reaction cross sections.

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arXiv2006

A critical view on the deeply bound K^- pp system

We briefly review the situation around the claimed deeply bound K^- states in different recent experiments and concentrate particularly on the state K^- pp advocated by the FINUDA collaboration in nuclear K^- absorption. We perform a theoretical simulation of the process and show that the peak in the Lambda p spectrum that was interpreted as a deep K^- pp bound state corresponds mostly to the process K^- p p --> Lambda p followed by final state interactions of the produced particles with the daughter nucleus.

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arXiv2006

Recent developments in chiral dynamics of hadrons and hadrons in a nuclear medium

In this talk I present recent developments in chiral dynamics of hadrons and hadrons in a medium addressing the following points: interaction of the octet of pseudoscalar mesons with the octet of baryons of the nucleon, showing recent experimental evidence on the existence of two $Λ(1405)$ states, the interaction of the octet of pseudoscalar mesons with the decuplet of baryons of the $Δ$, with particular emphasis on the $Λ(1520)$ resonance, dynamically generated by this interaction. Then I review the interaction of kaons in a nuclear medium and briefly discuss the situation around the claims of deeply bound states in nuclei. The large renormalization of the $Λ(1520)$ in the nuclear medium is shown as another example of successful application of the chiral unitary techniques.

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arXiv2006

Chiral approach to antikaon s- and p-wave interactions in dense nuclear matter

The properties of the antikaons in nuclear matter are investigated from a chiral unitary approach which incorporates the s- and p-waves of the ${\bar K}N$ interaction. To obtain the in-medium meson-baryon amplitudes we include, in a self-consistent way, Pauli blocking effects, meson self-energies corrected by nuclear short-range correlations and baryon binding potentials. We pay special attention to investigating the validity of the on-shell factorization, showing that it cannot be applied in the evaluation of the in-medium corrections to the p-wave amplitudes. In nuclear matter at saturation energy, the $Λ$ and $Σ$ develop an attractive potential of about -30 MeV, while the $Σ^*$ pole remains at the free space value although its width gets sensibly increased to about 80 MeV. The antikaon also develops a moderate attraction that does not support the existence of very deep and narrow bound states, confirming the findings of previous self-consistent calculations.

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arXiv2006

Single and double coincidence nucleon spectra in the weak decay of Lambda hypernuclei

Recent progress has been experienced in the field of hypernuclear weak decay, especially concerning the ratio of the neutron- to proton-induced Lambda non-mesonic decay rates, G_n/G_p. Theoretical analyses of nucleon coincidence data have been performed in a finite nucleus framework. They led to the extraction of G_n/G_p values in agreement with pure theoretical estimates, thus providing an evidence for the solution of a longstanding puzzle. Here we present an alternative approach to the problem, based on a nuclear matter formalism extended to finite nuclei via the local density approximation. The work is motivated by the exigence to make the determination of G_n/G_p from data less model dependent. One-meson-exchange potentials are used for describing both the one- and two-nucleon induced decays, Lambda N -> n N and Lambda N N -> n N N. For the latter, treated within a microscopic approach, the channels Lambda n n -> n n n and Lambda p p -> n p p are included in addition to the mode Lambda n p -> n n p already considered, in a phenomenological way, in previous studies. The propagation of the final nucleons in the residual nucleus is simulated by an intranuclear cascade code. We evaluate single and double coincidence nucleon spectra for the non-mesonic decay of C-12-Lambda. Through the comparison of our predictions with KEK coincidence data we determine G_n/G_p=0.43 \pm 0.10 for this hypernucleus, confirming previous finite nucleus analyses.

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