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J. Nieves

Publications and source records attributed to J. Nieves.

At least 127 records · Page 7Linked to original sources
arXiv2010

New determination of the $N$-$Δ(1232)$ axial form factors from weak pion production and coherent pion production off nuclei at T2K and MiniBooNE energies revisited

We re-evaluate our model predictions in Phys. Rev. D 79, 013002 (2009) for different observables in neutrino induced coherent pion production. This comes as a result of the new improved fit to old bubble chamber data of the dominant axial C_5^A nucleon-to-Delta form factor. We find an increase of 20%-30% in the values for the total cross sections. Uncertainties induced by the errors in the determination of C_5^A are computed. Our new results turn out to be compatible within about $1σ$ with the former ones. We also stress the existing tension between the recent experimental determination of the σ(CC coh π^+)}/σ(NC coh π^0)} $ ratio by the SciBooNE Collaboration and the theoretical predictions.

hep-ph↗
arXiv2010

Heavy Quark Spin Symmetry and Heavy Baryons: Electroweak Decays

Heavy quark spin symmetry is discussed in the context of single and doubly heavy baryons. A special attention is paid to the constraints/simplifications that this symmetry imposes on the non-relativistic constituent quark model wave functions and on the b->c semileptonic decays of these hadrons.

hep-ph↗
arXiv2010

Low-lying even parity meson resonances and spin-flavor symmetry

A study is presented of the $s-$wave meson-meson interactions involving members of the $ρ-$nonet and of the $π-$octet. The starting point is an SU(6) spin-flavor extension of the SU(3) flavor Weinberg-Tomozawa Lagrangian. SU(6) symmetry breaking terms are then included to account for the physical meson masses and decay constants, while preserving partial conservation of the axial current in the light pseudoscalar sector. Next, the $T-$matrix amplitudes are obtained by solving the Bethe Salpeter equation in coupled-channel with the kernel built from the above interactions. The poles found on the first and second Riemann sheets of the amplitudes are identified with their possible Particle Data Group (PDG) counterparts. It is shown that most of the low-lying even parity PDG meson resonances, specially in the $J^P=0^+$ and $1^+$ sectors, can be classified according to multiplets of the spin-flavor symmetry group SU(6). The $f_0(1500)$, $f_1(1420)$ and some $0^+(2^{++})$ resonances cannot be accommodated within this SU(6) scheme and thus they would be clear candidates to be glueballs or hybrids. Finally, we predict the existence of five exotic resonances ($I \ge 3/2$ and/or $|Y|=2$) with masses in the range 1.4--1.6 GeV, which would complete the $27_1$, $10_3$, and $10_3^*$ multiplets of SU(3)$\otimes$SU(2).

hep-ph↗
arXiv2010

Exotic dynamically generated baryons with C$=-$1

We follow a model based on the SU(8) symmetry for the interaction of mesons with baryons. The model treats on an equal footing the pseudo-scalars and the vector mesons, as required by heavy quark symmetry. The T-matrix calculated within an unitary scheme in coupled channels has poles which are interpreted as baryonic resonances.

hep-ph↗
arXiv2010

The role of the $N^*(2080)$ resonance in the $\vecγ p \to K^+ Λ(1520)$ reaction

We investigate the $Λ(1520)$ photo-production in the $\vecγ p \to K^+ Λ(1520)$ reaction within the effective Lagrangian method near threshold. In addition to the "background" contributions from the contact, $t-$channel $K$ exchange, and $s-$channel nucleon pole terms, which were already considered in previous works, the contribution from the nucleon resonance $N^*(2080)$ (spin-parity $J^P = 3/2^-$) is also considered. We show that the inclusion of the nucleon resonance $N^*(2080)$ leads to a fairly good description of the new LEPS differential cross section data, and that these measurements can be used to determine some of the properties of this latter resonance. However, serious discrepancies appear when the predictions of the model are compared to the photon-beam asymmetry also measured by the LEPS Collaboration.

nucl-th↗
arXiv2010

Coherent pion production off nuclei at T2K and MiniBooNE energies revisited

As a result of a new improved fit to old bubble chamber data of the dominant axial C_5^A nucleon-to-Delta form factor, and due to the relevance of this form factor for neutrino induced coherent pion production, we re-evaluate our model predictions in Phys. Rev. D79, 013002 (2009) for different observables of the latter reaction. Central values for the total cross sections increase by 20% to 30%, while differential cross sections do not change their shape appreciably. Furthermore, we also compute the uncertainties on total, differential and flux averaged cross sections induced by the errors in the determination of C_5^A. Our new results turn out to be compatible within about 1 sigma with the former ones. Finally, we stress the existing tension between the recent experimental determination of the sigma(CC coh π^+)/sigma(NC coh π^0) ratio by the SciBooNE Collaboration and the theoretical predictions.

hep-ph↗
arXiv2010

Wave functions for dynamically generated resonances; the two $Λ(1405)$ and $Λ(1670)$

In this work we develop a formalism to evaluate wave functions in momentum and coordinate space for the resonant states dynamically generated in a unitary coupled channel approach. The on shell approach for the scattering matrix, commonly used, is also obtained in Quantum Mechanics with a separable potential, which allows one to write wave functions in a trivial way. We develop useful relationships among the couplings of the dynamically generated resonances to the different channels and the wave functions at the origin. The formalism provides an intuitive picture of the resonances in the coupled channel approach, as bound states of one bound channel, which decays into open ones. It also provides an insight and practical rules for evaluating couplings of the resonances to external sources and how to deal with final state interaction in production processes. As an application of the formalism we evaluate the wave functions of the two $Λ(1405)$ states in the $πΣ$, $\bar{K} N$ and other coupled channels. It also offers a practical way to study three body systems when two of them cluster into a resonance.

hep-ph↗
arXiv2010

Heavy meson production in hot dense matter

The properties of charmed mesons in dense matter are studied using a unitary coupled-channel approach in the nuclear medium which takes into account Pauli-blocking effects and meson self-energies in a self-consistent manner. We obtain the open-charm meson spectral functions in this dense nuclear environment, and discuss their implications on hidden charm and charm scalar resonances and on the formation of $D$-mesic nuclei at FAIR energies.

nucl-th↗
arXiv2010

Hyperfine mixing in electromagnetic decay of doubly heavy $bc$ baryons

We investigate the role of hyperfine mixing in the electromagnetic decay of ground state doubly heavy $bc$ baryons. As in the case of a previous calculation on $b\to c$ semileptonic decays of doubly heavy baryons, we find large corrections to the electromagnetic decay widths due to this mixing. Contrary to the weak case just mentioned, we find here that one can not use electromagnetic width relations obtained in the infinite heavy quark mass limit to experimentally extract information on the admixtures in a model independent way.

hep-ph↗
arXiv2010

D mesic nuclei

The energies and widths of several D^0 meson bound states for different nuclei are obtained using a D-meson selfenergy in the nuclear medium, which is evaluated in a selfconsistent manner using techniques of unitarized coupled-channel theory. The kernel of the meson-baryon interaction is based on a model that treats heavy pseudoscalar and heavy vector mesons on equal footing, as required by heavy quark symmetry. We find D^0 bound states in all studied nuclei, from 12C up to 208Pb. The inclusion of vector mesons is the keystone for obtaining an attractive D-nucleus interaction that leads to the existence of D^0-nucleus bound states, as compared to previous studies based on SU(4) flavor symmetry. In some cases, the half widths are smaller than the separation of the levels, what makes possible their experimental observation by means of a nuclear reaction. This can be of particular interest for the future PANDA@FAIR physics program. We also find a D^+ bound state in 12C, but it is too broad and will have a significant overlap with the energies of the continuum.

nucl-th↗
arXiv2010

Coherent Pions From Neutrino Scattering Off Nuclei

We describe a model for pion production off nucleons and coherent pions from nuclei induced by neutrinos in the 1 GeV energy regime. Besides the dominant Delta pole contribution, it 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. Nuclear effects like medium corrections on the Delta propagator and final pion distortion are included.

nucl-th↗
arXiv2010

Exotic dynamically generated baryons with negative charm quantum number

Following a model based on the $\rm{SU}(8)$ symmetry that treats heavy pseudo-scalars and heavy vector mesons on an equal footing, as required by heavy quark symmetry, we study the interaction of baryons and mesons in coupled channels within an unitary approach that generates dynamically poles in the scattering $T$-matrix. We concentrate in the exotic channels with negative charm quantum number for which there is the experimental claim of one state.

hep-ph↗
arXiv2010

N-Delta(1232) axial form factors from weak pion production

The N-Delta axial form factors are determined from neutrino induced pion production ANL & BNL data by using a state of the art theoretical model, which accounts both for background mechanisms and deuteron effects. We find violations of the off diagonal Goldberger-Treiman relation at the level of 2 sigma which might have an impact in background calculations for T2K and MiniBooNE low energy neutrino oscillation precision experiments.

hep-ph↗
arXiv2010

Isospin breaking effects in the dynamical generation of the X(3872)

We have studied isospin breaking effects in the X(3872) resonance and found a natural explanation for the branching fraction of the X decaying to $J/ψ$ with two and three pions being close to unit. Within our framework the X(3872) is a dynamically generated resonance in coupled channels. We also study the relationship between the couplings of the resonance to the coupled channels with its wave function, which further helps us to understand the isospin structure of the resonance.

hep-ph↗
arXiv2010

Charmed mesons in nuclear matter

We obtain the properties of charmed mesons in dense matter using a coupled-channel approach which accounts for Pauli blocking effects and meson self-energies in a self-consistent manner. We study the behaviour of dynamically-generated baryonic resonances together with the open-charm meson spectral functions in this dense nuclear environment. 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, and on the formation of $D$-mesic nuclei.

nucl-th↗
arXiv2010

Couplings in coupled channels versus wave functions: application to the X(3872) resonance

We perform an analytical study of the scattering matrix and bound states in problems with many physical coupled channels. We establish the relationship of the couplings of the states to the different channels, obtained from the residues of the scattering matrix at the poles, with the wave functions for the different channels. The couplings basically reflect the value of the wave functions around the origin in coordinate space. In the concrete case of the X(3872) resonance, understood as a bound state of $\ddn$ and $\ddc$ (and $c.c.$), with the $\ddn$ loosely bound, we find that the couplings to the two channels are essentially equal leading to a state of good isospin I=0 character. This is in spite of having a probability for finding the $\ddn$ state much larger than for $\ddc$ since the loosely bound channel extends further in space. The analytical results, obtained with exact solutions of the Schrödinger equation for the wave functions, can be useful in general to interpret results found numerically in the study of problems with unitary coupled channels methods.

hep-ph↗