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J. Nieves

Publications and source records attributed to J. Nieves.

At least 163 records · Page 9Linked to original sources
arXiv2008

Static properties and semileptonic decays of doubly heavy baryons in a nonrelativistic quark model

We evaluate static properties and semileptonic decays for the ground state of doubly heavy $Ξ, Ξ', Ξ^*$ and $Ω, Ω', Ω^*$ baryons. Working in the framework of a nonrelativistic quark model, we solve the three--body problem by means of a variational ansazt made possible by heavy quark spin symmetry constraints. To check the dependence of our results on the inter-quark interaction we use five different quark-quark potentials that include a confining term plus Coulomb and hyperfine terms coming from one--gluon exchange. Our results for static properties (masses, charge and mass radii, magnetic moments...) are, with a few exceptions for the magnetic moments, in good agreement with a previous Faddeev calculation. Our much simpler wave functions are used to evaluate semileptonic decays of doubly heavy $Ξ,Ξ'(J=1/2)$ and $Ω, Ω'(J=1/2)$ baryons. Our results for the decay widths are in good agreement with calculations done within a relativistic quark model in the quark--diquark approximation.

hep-ph↗
arXiv2008

Test of the heavy quark-light diquark approximation for baryons with a heavy quark

We check a commonly used approximation in which a baryon with a heavy quark is described as a heavy quark-light diquark system. The heavy quark influences the diquark internal motion reducing the average distance between the two light quarks. Besides, we show how the average distance between the heavy quark and any of the light quarks, and that between the heavy quark and the center of mass of the light diquark, are smaller than the distance between the two light quarks, which seems to contradict the heavy quark-light diquark picture. This latter result is in agreement with expectations from QCD sum rules and lattice QCD calculations. Our results also show that the diquark approximations produces larger masses than the ones obtained in a full calculation.

hep-ph↗
arXiv2008

Neutrino induced threshold production of two pions and N^*(1440) electroweak form factors

We study the threshold production of two pions induced by neutrinos in nucleon targets. The contribution of nucleon, pion and contact terms are calculated using a chiral Lagrangian. The contribution of the Roper resonance, neglected in earlier studies, has also been taken into account. The numerical results for the cross sections are presented and compared with the available experimental data. It has been found that in the two pion channels with $π^+π^-$ and $π^0π^0$ in the final state, the contribution of the $N^*(1440)$ is quite important and could be used to determine the $N^*(1440)$ electroweak transition form factors if experimental data with better statistics become available in the future.

hep-ph↗
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)

hep-ph↗
arXiv2008

Production of Two Pions Induced by Neutrinos

We study the threshold production of two pions induced by neutrinos in nucleon targets. The contribution of nucleon pole, pion and contact terms is calculated using a chiral Lagrangian. The contribution of the Roper resonance, neglected in earlier studies, has also been taken into account.

hep-ph↗
arXiv2008

Neutrino induced weak pion production off the nucleon

We study neutrino induced one-pion production off the nucleon in and around the Delta resonance region. Apart from the Delta-pole mechanism we include background terms required by chiral symmetry. These background terms give sizeable contributions in all channels. To better reproduce the ANL $q^2$-differential cross section data, we make a new fit of the $C_5^A(q^2)$ axial nucleon to Delta form factor. The new result $C_5^A(0)=0.867\pm 0.075$ is some 30% smaller than the commonly accepted value. This correction is compatible with most quark model estimates and a recent lattice calculation

hep-ph↗
arXiv2008

Nuclear effects in neutrino induced reactions

We discuss the relevance of nuclear medium effects in the analysis of some low and medium energy neutrino reactions of current interest. In particular, we study the Quasi-Elastic (QE) process, where RPA correlations and Final State Interactions (FSI) are shown to play a crucial role. We have also investigated the neutrino induced coherent pion production. We find a strong reduction of the cross section due to the distortion of the pion wave function and the modification of the production mechanisms in the nucleus. The sensitivity of the results to the axial $NΔ$ coupling $C_5^A(0)$ has been also investigated.

nucl-th↗
arXiv2007

Elastic s-wave scattering phase shifts and |Vub| from lattice calculations of form factors for exclusive semileptonic decays

Omnes dispersion relations make a connection between form factors for exclusive semileptonic decays and phase shifts in the corresponding elastic scattering channels. We describe two applications. In the first, we use lattice form factor calculations to learn about phase shifts in elastic s-wave isospin-1/2 channels for K pi, B pi, D pi and D K scattering. The aim of the second application is to make the determination of the CKM matrix element magnitude |Vub| from exclusive semileptonic B to pi decays competitive with that from inclusive decays. Here we use many subtractions in an Omnes dispersion relation to motivate a simple fitting function, allowing data to constrain the q^2 shape of the differential decay rate and theory, primarily lattice results, to provide normalisation via form factor values.

hep-lat↗
arXiv2007

Neutrino Interaction Calculations from MeV to GeV Region

The Quasi-Elastic (QE) contribution of the nuclear inclusive electron model developed in Nucl. Phys. A627 543 (1997) is extended to the study of neutrino/antineutrino Charged Current (CC) and Neutral Current (NC) induced nuclear reactions at intermediate energies. Long range nuclear (RPA) correlations, Final State Interaction (FSI) and Coulomb corrections are included within the model. RPA correlations are shown to play a crucial role in the whole range (100--500 MeV) of studied neutrino energies. Results for inclusive muon capture for different nuclei through the Periodic Table are also discussed. In addition, and by means of a Monte Carlo cascade method to account for the rescattering of the outgoing nucleon, we also study the CC and NC inclusive one nucleon knockout reactions off nuclei.

nucl-th↗
arXiv2007

Chiral Symmetry and Neutrino Pion Production off the Nucleon

The neutrino pion production off the nucleon is traditionally described in the literature by means of the weak excitation of the Delta(1232) resonance and its subsequent decay into N pi. Here, we present results from a model that includes also some background terms required by chiral symmetry. We show that the contribution of these terms is sizeable and leads to significant effects in total and partially integrated pion production cross sections at intermediate energies of interest for neutrino oscillation experiments. Finally, we discuss parity-violating contributions to the pion angular differential cross section induced by the interference of these non-resonant terms with the Delta piece.

hep-ex↗
arXiv2007

Pion Production in Neutrino-Nucleon Reactions

We construct a model for the weak pion production off the nucleon, which in addition to the weak excitation of the $Δ(1232)$ resonance and its subsequent decay into $Nπ$, it includes also some background terms required by chiral symmetry. We re-fit the $C_5^A(q^2)$ form factor to the flux averaged $ν_μp \to μ^-pπ^+$ ANL $q^2-$differential cross section data, finding a substantially smaller contribution of the $Δ$ pole mechanism than traditionally assumed in the literature. We also show that the interference between the Delta pole and the background terms produces parity-violating contributions to the pion angular differential cross section.

nucl-th↗
arXiv2007

Masses and semileptonic decays of doubly heavy baryons in a nonrelativistic quark model

We evaluate masses and semileptonic decay widths for the ground state of doubly heavy $Ξ$ and $Ω$ baryons in the framework of a nonrelativistic quark model. We solve the three-body problem by means of a variational ansatz made possible by heavy-quark spin symmetry constraints. Our masses are comparable to the ones obtained in relativistic calculations and we get one of the best agreements with lattice data. Our simple wave functions are used to evaluate semileptonic decays of doubly heavy $Ξ, Ξ'(J=1/2)$ and $Ω, Ω'(J=1/2)$ baryons. Our results for the decay widths are in reasonable agreement with calculations done in a relativistic calculation in the quark-diquark approximation. We also check that our wave functions comply with what it is expected in the infinite heavy quark mass limit.

nucl-th↗
arXiv2007

Weak Pion Production off the Nucleon

We develop a model for the weak pion production off the nucleon, which besides the Delta pole mechanism (weak excitation of the $Δ(1232)$ resonance and its subsequent decay into $Nπ$), includes also some background terms required by chiral symmetry. We re-fit the $C_5^A(q^2)$ form factor to the flux averaged $ν_μp \to μ^-pπ^+$ ANL $q^2-$differential cross section data, finding a substantially smaller contribution of the Delta pole mechanism than traditionally assumed in the literature. Within this scheme, we calculate several differential and integrated cross sections, including pion angular distributions, induced by neutrinos and antineutrinos and driven both by charged and neutral currents. In all cases we find that the background terms produce quite significant effects and that they lead to an overall improved description of the data, as compared to the case where only the Delta pole mechanism is considered. We also show that the interference between the Delta pole and the background terms produces parity-violating contributions to the pion angular differential cross section, which are intimately linked to $T-$odd correlations in the contraction between the leptonic and hadronic tensors. However, these latter correlations do not imply a genuine violation of time reversal invariance because of the existence of strong final state interaction effects.

hep-ph↗
arXiv2007

Can One Distinguish Tau Neutrinos from Antineutrinos in Neutral-Current Pion Production Processes?

A potential way to distinguish tau-neutrinos from antineutrinos, below the tau-production threshold, but above the pion production one, is presented. It is based on the different behavior of the neutral current pion production off the nucleon, depending on whether it is induced by neutrinos or antineutrinos. This procedure for distinguishing tau-neutrinos from antineutrinos neither relies on any nuclear model, nor it is affected by any nuclear effect (distortion of the outgoing nucleon waves, etc...). We show that neutrino-antineutrino asymmetries occur both in the totally integrated cross sections and in the pion azimuthal differential distributions. To define the asymmetries for the latter distributions we just rely on Lorentz-invariance. All these asymmetries are independent of the lepton family and can be experimentally measured by using electron or muon neutrinos, due to the lepton family universality of the neutral current neutrino interaction. Nevertheless and to estimate their size, we have also used the chiral model of hep-ph/0701149 at intermediate energies. Results are really significant since the differences between neutrino and antineutrino induced reactions are always large in all physical channels.

hep-ph↗
arXiv2006

Resonances and the Weinberg--Tomozawa 56-baryon --35-meson interaction

Vector meson degrees of freedom are incorporated into the Weinberg-Tomozawa (WT) meson-baryon chiral Lagrangian by using a scheme which relies on spin--flavor SU(6) symmetry. The corresponding Bethe-Salpeter approximation successfully reproduces previous SU(3)--flavor WT results for the lowest-lying s--wave negative parity baryon resonances, and it also provides some information on the dynamics of the heavier ones. Moreover, it also predicts the existence of an isoscalar spin-parity $\frac32^-$ $K^*N$ bound state (strangeness +1) with a mass around 1.7--1.8 GeV, unstable through $K^*$ decay. Neglecting d-wave KN decays, this state turns out to be quite narrow ($Γ\le 15$ MeV) and it might provide clear signals in reactions like $γp \to \bar K^0 p K^+π^-$ by looking at the three body $p K^+π^-$ invariant mass.

hep-ph↗
arXiv2006

Meson-Baryon s-wave Resonances with Strangeness -3

Starting from a consistent SU(6) extension of the Weinberg-Tomozawa (WT) meson-baryon chiral Lagrangian (Phys. Rev. D74 (2006) 034025), we study the s-wave meson-baryon resonances in the strangeness S=-3 and negative parity sector. Those resonances are generated by solving the Bethe-Salpeter equation with the WT interaction used as kernel. The considered mesons are those of the 35-SU(6)-plet, which includes the pseudoscalar (PS) octet of pions and the vector (V) nonet of the rho meson. For baryons we consider the 56-SU(6)-plet, made of the 1/2+ octet of the nucleon and the 3/2+ decuplet of the Delta. Quantum numbers I(J^P)=0(3/2^-) are suggested for the experimental resonances Omega*(2250)- and Omega*(2380)-. Among other, resonances with I=1 are found, with minimal quark content sss\bar{l}l', being s the strange quark and l, l' any of the the light up or down quarks. A clear signal for such a pentaquark would be a baryonic resonance with strangeness -3 and electric charge of -2 or 0, in proton charge units. We suggest looking for K- Xi- resonances with masses around 2100 and 2240 MeV in the sector 1(1/2^-), and for pi Omega- and K- Xi*- resonances with masses around 2260 MeV in the sector 1(3/2^-).

hep-ph↗
arXiv2006

Quark model study of the semileptonic B -> pi decay

The semileptonic decay B->pi is studied starting from a simple quark model and taking into account the effect of the B* resonance. A novel, multiply subtracted, Omnès dispersion relation has been implemented to extend the predictions of the quark model to all physical q^2 values. We find |V_{ub}|=0.0034 +/- 0.0003(exp.)+/- 0.0007(theory), in good agreement with experiment.

hep-ph↗