arXiv subjects

J. Nieves

Publications and source records attributed to J. Nieves.

At least 145 records · Page 8Linked to original sources

arXiv2009

Charmed hadrons in nuclear medium

We study the properties of charmed hadrons in dense matter within a coupled-channel approach which accounts for Pauli blocking effects and meson self-energies in a self-consistent manner. We analyze the behaviour in this dense environment of dynamically-generated baryonic resonances as well as the open-charm meson spectral functions. We discuss the implications of the in-medium properties of open-charm mesons on the $D_{s0}(2317)$ and the predicted X(3700) scalar resonances.

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arXiv2009

Neutrino induced coherent pion production

We discuss different parameterizations of the $C_5^A(q^2)$ $NΔ$ axial form factor, fitted to the old Argonne bubble chamber data for pion production by neutrinos, and we use coherent pion production to test their low $q^2$ behavior. We find moderate effects that will be difficult to observe with the accuracy of present experiments. We also discuss the use of the Rein-Sehgal model for low energy coherent pion production. By comparison to a microscopic calculation, we show the weaknesses of some of the approximations in that model that lead to very large cross sections as well as to the wrong shapes for differential ones. Finally we show that models based on the partial conservation of the axial current hypothesis are not fully reliable for differential cross sections that depend on the angle formed by the pion and the incident neutrino.

hep-ph↗

arXiv2009

Semileptonic bc to cc and bb to bc Baryon Decays and Heavy Quark Spin Symmetry

We study the semileptonic decays of the lowest-lying $bc$ baryons to the lowest-lying $cc$ baryons ($Ξ_{bc}^{(\prime*)}\to Ξ_{cc}^{(*)}$ and $Ω_{bc}^{(\prime*)}\to Ω_{cc}^{(*)}$), in the limit $m_b, m_c \gg Λ_\mathrm{QCD}$ and close to the zero recoil point. The separate heavy quark spin symmetries make it possible to describe all these decays using a single form factor. We also show how these constraints can be used to test the validity of different quark model calculations. $bb$ to $bc$ baryon decays are also discussed.

hep-ph↗

arXiv2009

The properties of D and D* mesons in the nuclear medium

We study the properties of $D$ and $D^*$ mesons in nuclear matter within a simultaneous self-consistent coupled-channel unitary approach that implements heavy-quark symmetry. The in-medium solution accounts for Pauli blocking effects, and for the $D$ and $D^*$ self-energies in a self-consistent manner. We pay a special attention to the renormalization of the intermediate propagators in the medium beyond the usual cutoff scheme. We analyze the behavior in the nuclear medium of the rich spectrum of dynamically-generated baryonic resonances in the C=1 and S=0 sector, and their influence in the self-energy and, hence, the spectral function of the $D$ and $D^*$ mesons. The $D$ meson quasiparticle peak mixes with $Σ_c(2823)N^{-1}$ and $Σ_c(2868)N^{-1}$ states while the $Λ_c(2595)N^{-1}$ mode is present in the low-energy tail of the spectral function. The $D^*$ spectral function incorporates $J=3/2$ resonances, and $Σ_c(2902)N^{-1}$ and $Λ_c(2941)N^{-1}$ fully determine the behavior of $D^*$ meson spectral function at the quasiparticle peak. As density increases, these resonant-hole modes tend to smear out and the spectral functions get broad. We also obtain the $D$ and $D^*$ scattering lengths, and optical potentials for different density regimes. The $D$ meson potential stays attractive while the $D^*$ meson one is repulsive with increasing densities up to twice that of the normal nuclear matter. Compared to previous in-medium SU(4) models, we obtain similar values for the real part of the $D$ meson potential but much smaller imaginary parts. This result can have important implications for the observation of $D^0$-nucleus bound states.

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arXiv2009

Strange and charm mesons at FAIR

We study the properties of strange and charm mesons in hot and dense matter within a self-consistent coupled-channel approach for the experimental conditions of density and temperature expected for the CBM experiment at FAIR/GSI. The in-medium solution at finite temperature accounts for Pauli blocking effects, mean-field binding of all the baryons involved, and meson self-energies. We analyze the behaviour in this hot and dense environment of dynamically-generated baryonic resonances together with the evolution with density and temperature of the strange and open-charm meson spectral functions. We test the spectral functions for strange mesons using energy-weighted sum rules and finally discuss the implications of the properties of charm mesons on the D_{s0}(2317) and the predicted X(3700) scalar resonances.

hep-ph↗

arXiv2009

The role of hyperfine mixing in $b\to c$ semileptonic decays of doubly-heavy baryons

We analyze the effects of hyperfine mixing in $b \to c$ semileptonic decays of doubly heavy baryons. We qualitatively confirm the results by W. Roberts and M. Pervin in Int. J. Mod. Phys. A, 2009, {\bf 24}: 2401-2413, finding that mixing has a great impact on those transitions. However, predictions without mixing differ by a factor of 2 and this discrepancy translates to the mixed case where large differences in decay widths are observed between the two calculations.

hep-ph↗

arXiv2009

Hyperfine mixing in $b\to c$ semileptonic decay of doubly heavy baryons

We qualitatively corroborate the results of W. Roberts and M. Pervin in Int. J. Mod. Phys. A 24, 2401 (2009) according to which hyperfine mixing greatly affects the decay widths of $b\to c$ semileptonic decays involving doubly heavy $bc$ baryons. However, our predictions for the decay widths of the unmixed states differ from those reported in the work of Roberts and Pervin by a factor of 2, and this discrepancy translates to the mixed case. We further show that the predictions of heavy quark spin symmetry, might be used in the future to experimentally extract information on the admixtures in the actual physical $bc$ baryons, in a model independent manner.

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arXiv2009

Nucleon Emision Off Nuclei Induced By Neutrino Interactions

We make a review of the main nuclear effects that affect neutrino-nucleus cross sections. We discuss how the different models in the literature try to describe these different effects, and thus try to compare between them. We focus on the quasi-elastic reaction in the neutrino energy region of around 1 GeV, where recent data from MiniBoone are available. Among the issues discussed are the different treatment of medium corrections to initial and nal state nucleon wave functions and the problem of the rescattering of ejected nucleons.

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arXiv2009

Neutrino Interactions Importance for Nuclear Physics

We review the general interplay between Nuclear Physics and neutrino-nucleus cross sections at intermediate and high energies. The effects of different reaction mechanisms over the neutrino observables are illustrated with examples in calculations using several nuclear models and ingredients.

nucl-th↗

arXiv2009

Neutrino induced weak pion production off the nucleon and coherent pion production in nuclei at low energies

We present a microscopic model for neutrino induced one-pion production off the nucleon and its implementation for the purpose of calculating coherent pion production in nuclei. We further criticize the use of the Rein--Sehgal model for coherent pion production by low energy neutrinos. In particular, we show how the approximations in that model give rise to a much flatter differential cross section in the $η=E_π(1-\cosθ_π)$ variable. We discuss the limitations intrinsic to any approach based on the partial conservation of the axial current hypothesis and the inability of such models to properly determine the angular distribution of the outgoing pion with respect to the direction of the incoming neutrino. We show the effects of those limitation for the case of the $\frac{dσ}{dη}$ differential cross section.

hep-ph↗

arXiv2009

Charmed baryon resonances with heavy-quark symmetry

We study charmed baryon resonances that are generated dynamically from a coupled-channel unitary approach that implements heavy-quark symmetry. Some states can already be identified with experimental observations, such as $Λ_c(2595)$, $Λ_c(2660)$, $Σ_c(2902)$ or $Λ_c(2941)$, while others need a compilation of more experimental data as well as an extension of the model to include higher order contributions. We also compare our model to previous SU(4) schemes.

hep-ph↗

arXiv2009

Neutrino Induced Coherent Pion Production off Nuclei and PCAC

We review the Rein--Sehgal model and criticize its use for low energy neutrino induced coherent pion production. We have studied the validity of the main approximations implicit in that model, trying to compare with physical observables when that is possible and with microscopical calculations. Next, we have tried to elaborate a new improved model by removing the more problematic approximations, while keeping the model still reasonably simple. Last, we have discussed the limitations intrinsic to any approach based on the partial conservation of the axial current hypothesis. In particular, we have shown the inability of such models to determine the angular distribution of the outgoing pion with respect to the direction of the incoming neutrino, except for the $q^2= 0$ kinematical point.

hep-ph↗

arXiv2009

Coherent Neutrino Scattering

We present a microscopic model for coherent pion production off nuclei induced by neutrinos. This model is built upon a model for single nucleon processes that goes beyond the usual Delta dominance by including non resonant background contributions. We include nuclear medium effects: medium corrections to Delta$ properties and outgoing pion absortion via an optical potential. This results in major modifications to cross sections for low energy experiments when compared with phenomenological models like Rein-Sehgal's.

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arXiv2009

Meson Resonances at large Nc: Complex Poles vs Breit-Wigner Masses

The rigorous quantum mechanical definition of a resonance requires determining the pole position in the second Riemann sheet of the analytically continued partial wave scattering amplitude in the complex Mandelstam s-variable plane. For meson resonances we investigate the alternative Breit-Wigner (BW) definition within the large Nc expansion. By assuming that the pole position is ${\cal O} (N_C^{0})$ and exploiting unitarity, we show that the BW determination of the resonance mass differs from the pole position by ${\cal O} (N_C^{-2})$ terms, which can be extracted from pi-pi scattering data. For the case of the f0(600) pole, the BW scalar mass is predicted to occur at about 700 MeV while the true value is located at about 800 MeV.

hep-ph↗

arXiv2009

Properties of the rho and sigma Mesons from Unitary Chiral Dynamics

The chiral limit of the rho and sigma masses and widths is discussed. We work within the inverse amplitude method to one loop in SU(2) ChPT and analyze the consequences that all chiral logarithms cancel out in the rho-channel, while they do not cancel for the sigma case, and how they strongly influence the properties of this latter resonance. Our results confirm and explain the different behavior of the sigma and rho poles for NC not far from 3, but we extend the analysis to very large NC, where the behavior of these two resonances is re-analyzed. We note that the rather natural requirement of consistency between resonance saturation and unitarization imposes useful constraints. By looking only at the rho-channel, and within the single resonance approximation, we find that the masses of the first vector and scalar meson nonets, invoked in the single resonance approximation, turn out to be degenerated in the large NC limit. On the contrary we show that, for sufficiently large NC, the scalar meson evolution lies beyond the applicability reach of the one-loop inverse amplitude method and if the scalar channel is also incorporated in the analysis, it may lead, in some cases, to phenomenologically inconsistent results.

hep-ph↗

arXiv2009

The s-wave charmed baryon resonances from a coupled-channel approach with heavy quark symmetry

We study charmed baryon resonances which are generated dynamically within a unitary meson-baryon coupled channel model that treats the heavy pseudoscalar and vector mesons on equal footing as required by heavy-quark symmetry. It is an extension of recent SU(4) models with t-channel vector meson exchanges to a SU(8) spin-flavor scheme, but differs considerably from the SU(4) approach in how the strong breaking of the flavor symmetry is implemented. Some of our dynamically generated states can be readily assigned to recently observed baryon resonances, while others do not have a straightforward identification and require the compilation of more data as well as an extension of the model to d-wave meson-baryon interactions and p-wave coupling in the neglected s- and u-channel diagrams. Of several novelties, we find that the Lambda_c(2595), which emerged as a ND quasi-bound state within the SU(4) approaches, becomes predominantly a ND* quasi-bound state in the present SU(8) scheme.

hep-ph↗

arXiv2008

Theoretical study of neutrino-induced coherent pion production off nuclei at T2K and MiniBooNE energies

We have developed a model for neutrino-induced coherent pion production off nuclei in the energy regime of interest for present and forthcoming neutrino oscillation experiments. It is based on a microscopic model for pion production off the nucleon that, besides the dominant Delta pole contribution, takes into account the effect of background terms required by chiral symmetry. Moreover, the model uses a reduced nucleon-to-Delta resonance axial coupling, which leads to coherent pion production cross sections around a factor two smaller than most of the previous theoretical estimates. In the coherent production, the main nuclear effects, namely medium corrections on the Delta propagator and the final pion distortion, are included. We have improved on previous similar models by taking into account the nucleon motion and employing a more sophisticated optical potential. As found in previous calculations the modification of the Delta self-energy inside the nuclear medium strongly reduces the cross section, while the final pion distortion mainly shifts the peak position to lower pion energies. The angular distribution profiles are not much affected by nuclear effects. Nucleon motion increases the cross section by 15% at neutrino energies of 650 MeV, while Coulomb effects on charged pions are estimated to be small. Finally, we discuss at length the deficiencies of the Rein-Sehgal pion coherent production model for neutrino energies below 2 GeV, and in particular for the MiniBooNE and T2K experiments. We also predict flux averaged cross sections for these two latter experiments and K2K.

hep-ph↗

arXiv2008

Theoretical update on low energy neutrino-nucleus reactions

We review Quasielastic (QE) inclusive and semi--inclusive neutrino/antineutrino Charged Current (CC) and Neutral Current (NC) induced nuclear reactions at intermediate energies. We pay special attention to nuclear corrections besides Pauli blocking: Long and Short range nuclear correlations (RPA and SRC) and particle and hole Spectral Functions (SF). We also critically review the use of the Plane and Distorted Wave Impulse approximations (PWIA and DWIA) to describe inclusive one nucleon knockout reactions off nuclei. In this context, we present results from a Monte Carlo cascade method to account for the rescattering of the outgoing nucleon. Finally, we examine the effects of chiral non-resonant terms in neutrino pion production off the nucleon, and present some preliminary results on nuclear coherent pion production induced by neutrinos.

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