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A. Ramos

Publications and source records attributed to A. Ramos.

At least 145 records · Page 8Linked to original sources
arXiv2003

$Λ(1405)$ production in the $π^-p\to K^0πΣ$ reaction

We discuss the mechanisms that lead to $Λ(1405)$ production in the $π^-p\to K^0πΣ$ reaction. The problem has gained renewed interest after different works converge to the conclusion that there are two resonances around the region of 1400 MeV, rather than one, and that they couple differently to the $πΣ$ and $\bar{K}N$ channels. We look at the dynamics of that reaction and find two mechanisms which eventually filter each one of the resonances, leading to very different shapes of the $πΣ$ invariant mass distributions. The combination of the two mechanisms leads to a shape of this distribution compatible with the experimental measurements.

nucl-th↗
arXiv2003

Isotope-selective photo-ionization for calcium ion trapping

We present studies of resonance-enhanced photo-ionization for isotope-selective loading of Ca+ into a Paul trap. The 4s^2 ^1S_0 <-> 4s4p ^1P_1 transition of neutral calcium is driven by a 423nm laser and the atoms are photo-ionized by a second laser at 389nm. Isotope-selectivity is achieved by using crossed atomic and laser beams to reduce the Doppler width significantly below the isotope shifts in the 423nm transition. The loading rate of ions into the trap is studied under a range of experimental parameters for the abundant isotope 40Ca+. Using the fluorescence of the atomic beam at 423nm as a measure of the Ca number density, we estimate a lower limit for the absolute photo-ionization cross-section. We achieve loading and laser-cooling of all the naturally occurring isotopes, without the need for enriched sources. Laser-heating/cooling is observed to enhance the isotope-selectivity. In the case of the rare species 43Ca+ and 46Ca+, which have not previously been laser-cooled, the loading is not fully isotope-selective but we show that pure crystals of 43Ca+ may nevertheless be obtained. We find that for loading 40Ca+ the 389nm laser may be replaced by an incoherent source.

physics.atom-ph↗
arXiv2003

Microscopic study of neutrino trapping in hyperon stars

Employing the most recent parametrization of the baryon-baryon interaction of the Nijmegen group, we investigate, in the framework of the Brueckner--Bethe--Goldstone many-body theory at zero temperature, the influence of neutrino trapping on the composition, equation of state, and structure of neutron stars, relevant to describe the physical conditions of a neutron star immediately after birth (protoneutron star). We find that the presence of neutrinos changes significantly the composition of matter delaying the appearance of hyperons and making the equation of state stiffer. We explore the consequences of neutrino trapping on the early evolution of a neutron star and on the nature of the final compact remnant left by the supernova explosion.

astro-ph↗
arXiv2003

Antikaon production in A+A collisions at SIS energies within an off-shell G-matrix approach

The production and propagation of antikaons -- described by dynamical spectral functions $A_h(X,\vec{P},M^2)$ as evaluated from a coupled channel $G$-matrix approach -- is studied for nucleus-nucleus collisions at SIS energies in comparison to the conventional quasi-particle limit and the available experimental data using off-shell transport theory. We find that the $K^-$ spectra for $^{12}C + ^{12}C$ and $^{58}Ni + ^{58}Ni$ at 1.8 A$\cdot$GeV remain underestimated in the $G$-matrix approach as in the on-shell quasi-particle approximation whereas the preliminary spectra for $Au + Au$ at 1.5 A$\cdot$GeV are well described in both limits. This also holds approximately for the $K^-$ rapidity distributions in semi-central collisions of $Ni+Ni$ at 1.93 A$\cdot$GeV. However, in all limits considered there is no convincing description of all spectra simultaneously. Our off-shell transport calculations, furthermore, demonstrate that the strongest in-medium effects should be found for low antikaon momenta in the center-of-mass frame, since the deceleration of the antikaons in the attractive Coulomb and nuclear potentials and the propagation to the on-shell mass induces a net shift and squeezing of the $K^-$ spectra to the low momentum regime.

nucl-th↗
arXiv2003

K^-/K^+ ratio at GSI in hot and dense matter

The $K^-/K^+$ ratio in heavy-ion collisions at GSI energies is studied including the properties of the participating hadrons in hot and dense matter. 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. Three approaches for the $K^-$ self-energy are considered: non-interacting $K^-$, on-shell self-energy and single-particle spectral density. With respect to the on-shell approach, the use of an energy dependent $\bar{K}$ spectral density, including both s- and p-wave components of the $\bar{K}N$ interaction, lowers considerably the freeze-out temperature and gives rise to the "broad-band equilibration" advocated by Brown, Rho and Song.

hep-ph↗
arXiv2003

Chiral Dynamics of the Two Λ(1405) States

Using a chiral unitary approach for the meson--baryon interactions, we show that two octets of J^π=1/2^- baryon states, which are degenerate in the limit of exact SU(3) symmetry, and a singlet are generated dynamically. The SU(3) breaking produces the splitting of the two octets, resulting in the case of strangeness S=-1 in two poles of the scattering matrix close to the nominal Λ(1405) resonance. These poles are combinations of the singlet state and the octets. We show how actual experiments see just one effective resonance shape, but with properties which change from one reaction to another.

nucl-th↗
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.

nucl-th↗
arXiv2002

Chiral dynamics in systems with strangeness

In this talk a brief review of several problems involving systems with strangeness is made. In the first place one shows how the $Λ(1405)$, $Λ(1670)$ and $Σ(1620)$ states, for $S = -1$, and the $Ξ(1620)$ for $S= -2$ are generated dynamically in the context of unitarized chiral perturbation theory. The results for the $\bar{K}N$ interaction are then used to evaluate the $K^- d$ scattering length. Results obtained for the kaon selfenergy in a nuclear medium within this approach, with application to $K^-$ atoms, are also mentioned. Finally a few words are said about recent developments in the weak decay of $Λ$ hypernuclei and the puzzle of the $Γ_n/Γ_p$ ratio.

nucl-th↗
arXiv2002

Chiral unitary approach to hadron spectroscopy

The s-wave meson-baryon interaction in the $S = -1$, $S= 0$ and $S= -2$ sectors is studied by means of coupled channels, using the lowest-order chiral Lagrangian and the N/D method or equivalently the Bethe-Salpeter equation to implement unitarity. This chiral approach leads to the dynamical generation of the $Λ(1405)$, $Λ(1670)$ and $Σ(1620)$ states for $S = -1$, the $N^*(1535)$ for $S= 0$ and the $Ξ(1620)$ for $S= -2$. We look for poles in the complex plane and extract the couplings of the resonances to the different final states. This allows identifying the $Λ(1405)$ and the $Λ(1670)$ resonances with $\bar{K}N$ and $KΞ$ quasibound states, respectively. Our results are found to be incompatible with the measured properties of the $Ξ(1690)$ resonance, thus ruling this state out as the remaining member of this octet of dynamically generated resonances. We therefore assign $1/2^-$ for the spin and parity of the $Ξ(1620)$ resonance as the $S=-2$ member of the lowest-lying $1/2^-$ octet.

nucl-th↗
arXiv2002

Chiral dynamics of p-wave in K^- p and coupled states

We perform an evaluation of the p-wave amplitudes of meson-baryon scattering in the strangeness S=-1 sector starting from the lowest order chiral Lagrangians and introducing explicitly the Sigma^* field with couplings to the meson-baryon states obtained using SU(6) symmetry. The N/D method of unitarization is used, equivalent, in practice, to the use of the Bethe-Salpeter equation with a cut-off. The procedure leaves no freedom for the p-waves once the s-waves are fixed and thus one obtains genuine predictions for the p-wave scattering amplitudes, which are in good agreement with experimental results for differential cross sections, as well as for the width and partial decay widths of the Sigma^*(1385).

nucl-th↗
arXiv2002

On the spin, parity and nature of the $Ξ(1620)$ resonance

Using a unitary extension of chiral perturbation theory with a lowest-order s-wave SU(3) chiral Lagrangian we study low-energy meson-baryon scattering in the strangeness $S=-2$ sector. A scattering-matrix pole is found around 1606 MeV which corresponds to an s-wave $Ξ$ resonance with $J^P=1/2^-$. We identify this resonance with the $Ξ(1620)$ state, quoted in the Particle Data Book with $I=1/2$ but with unknown spin and parity. The addition of the $S=-2$ state to the recently computed $Λ(1670)$, $Σ(1620)$ and N(1535) states completes the octet of $J^P=1/2^-$ resonances dynamically generated in this chiral unitary approach.

nucl-th↗
arXiv2002

Magnetic moments of the Lambda(1405) and Lambda(1670) resonances

By using techniques of unitarized chiral perturbation theory, where the $Λ(1405)$ and $Λ(1670)$ resonances are dynamically generated, we evaluate the magnetic moments of these resonances and their transition magnetic moment. The results obtained here differ appreciably from those obtained with existing quark models. The width for the $Λ(1670) \to Λ(1405) γ$ transition is also evaluated leading to a branching ratio of the order of $2 \times 10^{-6}$.

hep-ph↗
arXiv2002

Low lying S=-1 excited baryons and chiral symmetry

The s-wave meson-baryon interaction in the $S = -1$ sector is studied by means of coupled-channels, using the lowest-order chiral Lagrangian and the N/D method to implement unitarity. The loops are regularized using dimensional renormalization. In addition to the previously studied $Λ(1405)$, employing this chiral approach leads to the dynamical generation of two more s-wave hyperon resonances, the $Λ(1670)$ and $Σ(1620)$ states. We make comparisons with experimental data and look for poles in the complex plane obtaining the couplings of the resonances to the different final states. This allows us to identify the $Λ(1405)$ and the $Λ(1670)$ resonances with $\bar{K}N$ and $KΞ$ quasibound states, respectively.

nucl-th↗
arXiv2001

Spin polarized neutron matte and magnetic susceptibility within the Brueckner-Hartree-Fock approximation

The Brueckner--Hartree--Fock formalism is applied to study spin polarized neutron matter properties. Results of the total energy per particle as a function of the spin polarization and density are presented for two modern realistic nucleon-nucleon interactions, Nijmegen II and Reid93. We find that the dependence of the energy on the spin polarization is practically parabolic in the full range of polarizations. The magnetic susceptibility of the system is computed. Our results show no indication of a ferromagnetic transition which becomes even more difficult as the density increases.

nucl-th↗
arXiv2001

Non-localities and Fermi motion corrections in $K^-$ atoms

We evaluate the p-wave $K^-N$ amplitudes from the chiral Lagrangians and from there construct the p-wave part of the $K^-$ nucleus optical potential plus a small s-wave part induced from the elementary p-wave amplitude and the nuclear Fermi motion. Simultaneously, the momentum and energy dependence of the s-wave optical potential, previously developed, are taken into account and shown to generate a small p-wave correction to the optical potential. All the corrections considered are small compared to the leading s-wave potential, and lead to changes in the shifts and widths which are smaller than the experimental errors. A thorough study of the threshold region and low densities is conducted, revealing mathematical problems for which a physical solution is given.

nucl-th↗
arXiv2001

Optical Potentials in Kaonic Atoms

A microscopic optical potential based on a chiral model is used as a starting point for studying kaonic atoms levels. We add to this potential a phenomenological part fitted to the experimentally known shifts and widths of kaonic levels. This fitted potential is used to predict deeply bound atomic levels, as well as nuclear levels. Comparison with the predictions of other optical models found in the literature is done. Also the effects on the kaonic atoms levels of certain known non-local contributions to the optical potential are also analyzed.

nucl-th↗
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.

nucl-th↗