arXiv subjects

J. Nieves

Publications and source records attributed to J. Nieves.

225 records · Page 13Linked to original sources

arXiv2000

Unitarized Heavy Baryon Chiral Perturbation Theory and the $Δ$ Resonance in $πN $ Scattering

Based on the recent phenomenological analysis done by M. Mojzis, Eur. Phys. Jour. {\bf C2} (1998) 182, of $πN $ scattering within the framework of Heavy Baryon Chiral Perturbation Theory we predict, after a suitable unitarization of the amplitude, the $Δ$ resonance at $\sqrt{s} = M_Δ= 1238 \er{22}{18}{\rm MeV} $ and with a width of $ Γ_Δ= 150 \er{43}{31} {\rm MeV} $ in excellent agreement with the experimental numbers. The error bars reflect only the uncertainties in the low energy parameters as determined from an extrapolation to threshold of experimental data. We also describe satisfactorily, within errors, the phase shifts up to $\sqrt{s} \sim 1300 {\rm MeV} $.

hep-ph↗

arXiv1999

An Investigation of the Soft Pion Relation in Quenched Lattice QCD

A lattice determination of the form factor and decay constants for the semileptonic decay of heavy pseudoscalar (PS) mesons at zero recoil is presented from which the soft pion relation is satisfied. Chiral extrapolation of the form factor is performed at constant $q^2$. Pole dominance is used to extrapolate the form factor in heavy quark mass. At the B mass, the form factor at zero recoil lies somewhat below the ratio of decay constants; the relation remains satisfied within error.

hep-lat↗

arXiv1998

Double $Λ$ Hypernuclei and the Nuclear Medium Effective $ΛΛ$ Interaction

We fit the $^1 S_0 ΛΛ$ interaction in the nuclear medium to the masses of the experimentally known double-$Λ$ hypernuclei: $^{\phantom{6}6}_{ΛΛ}$He, $^{10}_{ΛΛ}$Be and $^{13}_{ΛΛ}$B. We derive this effective interaction from OBE Jülich $ΛΛ$-type potentials and using both Hartree-Fock and variational approaches. We find that the inclusion of $ΛΛ$ correlations in the variational scheme leads to significant differences and a better understanding of the dynamical features of the system. We investigate the sensitivity of the binding energies and the mesonic decay widths of the above double-$Λ$ hypernuclei to the $ωΛΛ$ coupling and the form factor at the $σΛΛ$ vertex. We also use this effective interaction to predict binding energies and pionic decay widths of heavier double-$Λ$ hypernuclei, not discovered yet. Finally, we discard the existence of $^1 S_0$ $ΛΛ$ bound states provided the $ΛΛ-ΞN$ coupling can be neglected.

nucl-th↗

arXiv1997

Many Body approach to the inclusive $(e,e')$ reaction from the quasielastic to the $Δ$ excitation region

We have performed a many body calculation of the inclusive (e,e') cross section which runs over the three traditional regions at intermediate energies: the quasielastic peak, the dip region and the delta region. The longitudinal and transverse response functions in the quasielastic peak have also been evaluated. Traditional effects like polarization, meson exchange currents, final state interaction and delta renormalization in the nuclear medium have been included. Meson exchange currents are generated from a model of pion electroproduction on the nucleon which reproduces accurately the experimental data. The inclusive cross section accounts for 1N,2N,3N mechanisms of virtual photon absorption and one pion production. Meson exchange currents associated to the (γ^*,2π) reaction are also accounted for. We obtain good results for the (e,e') cross sections in the whole energy range and for different nuclei. The response functions are also in good agreement with the latest experimental analysis. On the other hand, the method provides the separation of the contribution to the inclusive cross section from different physical channels which is a necessary input to evaluate cross sections like (e,e' N), (e,e' NN), (e,e' π) etc.

nucl-th↗

arXiv1997

Inclusive $(e,e^\prime N)$, $(e,e^\prime NN)$, $(e,e^\prime π)$ ... reactions in nuclei

We study the inclusive $(e,e^\prime N)$, $(e,e^\prime NN)$, $(e,e^\prime π)$, $(e,e^\prime πN)$ reactions in nuclei using a Monte Carlo simulation method to treat the multichannel problem of the final state. The input consists of reaction probabilities for the different steps evaluated using microscopical many body methods. We obtain a good agreement with experiment in some channels where there is data and make predictions for other channels which are presently under investigation in several electron laboratories. The comparison of the theoretical results with experiment for several kinematical conditions and diverse channels can serve to learn about different physical processes ocurring in the reaction. The potential of this theoretical tool to make prospections for possible experiments, aiming at pinning down certain reaction probabilities, is also emphasized.

nucl-th↗

arXiv1997

First Lattice Study of Semileptonic Decays of Lambda_b and Xi_b Baryons

We present the results of the first lattice study of semileptonic decays of baryons containing a b-quark. Predictions for the decay distributions are given and the Isgur-Wise functions for heavy baryons are computed, for values of the velocity transfer up to about omega=1.2. The computations are performed on a 24^3 x 48 lattice at beta=6.2 using the Sheikholeslami-Wohlert action in the quenched approximation.

hep-lat↗

arXiv1997

Radiative pion capture in nuclei: a continuum shell-model approach

The radiative pion capture process in nuclei is approached by using a continuum shell-model description of the nucleus, together with a phenomenological treatment of the two particle-two hole effects. It is found that these effects play an important role to reproduce the observed experimental photon energy distribution. This distribution as well as the integrated one depends significantly on the details of the mean field potential. This makes this process interesting to investigate the nuclear structure dynamics.

nucl-th↗

arXiv1995

Form Factors for B -> pi l nu-bar_l and B -> K* gamma Decays on the Lattice

We present a unified method for analysing form factors in B -> pi l nu-bar_l and B -> K* gamma decays. The analysis provides consistency checks on the q^2 and 1/M extrapolations necessary to obtain the physical decay rates. For the first time the q^2 dependence of the form factors is obtained at the B scale. In the B -> pi l nu-bar_l case, we show that pole fits to f^+ may not be consistent with the q^2 behaviour of f^0, leading to a possible factor of two uncertainty in the decay rate and hence in the value of |V_{ub}|^2 deduced from it. For B -> K* gamma, from the combined analysis of form factors T_1 and T_2, we find the hadronisation ratio R_{K^*} of the exclusive B -> K* gamma to the inclusive b -> s gamma rates is of order 35% or 15% for constant and pole-type behaviour of T_2 respectively.

hep-lat↗

arXiv1994

Pion Cloud Contribution to K+ Nucleus Scattering

A careful reanalysis is done of the contribution to $K^{+}$ nucleus scattering from the interaction of the kaon with the virtual pion cloud. The usual approximations made in the evaluation of the related kaon selfenergy are shown to fail badly. We also find new interaction mechanisms which provide appreciable corrections to the kaon selfenergy. Some of these contribute to the imaginary part below pion creation threshold. The inclusion of these new mechanisms in the inelastic part of the optical potential produces a significant improvement in the differential and total $K^{+}$ nuclear cross sections. Uncertainties remain in the dispersive part of the optical potential.

nucl-th↗