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E. Hernández

Publications and source records attributed to E. Hernández.

At least 19 recordsLinked to original sources
arXiv2026

Probing New Physics and CP Violation in $ν_τn \to Λ_c τ^- (π^- ν_τ)$ and $\barν_τp \to Λτ^+ (π^+ \barν_τ)$

We study the processes $ν_τn \to Λ_c τ^- (π^- ν_τ)$ and $\barν_τp \to Λτ^+ (π^+ \barν_τ)$, with particular emphasis on the pion energy and angular distributions, as a possible signal for lepton flavor universality violation, and in general of physics beyond the Standard Model (SM), as well as a sensitive probe of the $τ$ polarization vector. We work within an effective low-energy extension of the SM with all dimension-six four-fermion operators. In this framework, complex Wilson coefficients which encode new physics can generate CP-violating contributions. We propose an observable that provides a genuine CP-odd signal due to its sensitivity to particular transverse components of the $τ$ polarization vector. Namely, we show that the asymmetry in the azimuthal-angle distribution of the pion in the decay $τ^\pm\to π^\pm ν_τ$ constitutes a smoking-gun prediction of such a beyond the SM scenario. We estimate the strength of this effect extrapolating nucleon-hyperon form factors recently obtained from lattice QCD calculations.

hep-ph↗
arXiv2025

Inclusive quasielastic (anti-)neutrino nucleus scattering within the Standard Model and beyond

We derive fully general expressions for the inclusive (anti-)neutrino-induced nuclear quasielastic production of a strange or charmed baryon $Y$, considering all dimension-six new physics operators relevant to the semileptonic $q\to q'\ell ν_\ell$ transition with both left- and right-handed neutrino fields. We illustrate the formalism by applying it to the $Λ_c$ production in neutrino nucleus scattering. The nuclear response is computed using a state-of-the-art {\it ab initio} spectral function, based on realistic nuclear many-body wavefunctions obtained via coupled-cluster method and a nuclear Hamiltonian derived from chiral effective field theory. We also highlight the role played by the final state interactions between the produced $Y$ hyperon and the residual nuclear system, which can obscure potential new physics signals in this (anti-)neutrino nucleus reaction. Our results improve upon previous studies that neglected nuclear corrections and uncertainties -- effects we show to be comparable to, or even larger than, those expected from new physics.

hep-ph↗
arXiv2023

The High-Altitude Water Cherenkov (HAWC) Observatory in México: The Primary Detector

The High-Altitude Water Cherenkov (HAWC) observatory is a second-generation continuously operated, wide field-of-view, TeV gamma-ray observatory. The HAWC observatory and its analysis techniques build on experience of the Milagro experiment in using ground-based water Cherenkov detectors for gamma-ray astronomy. HAWC is located on the Sierra Negra volcano in México at an elevation of 4100 meters above sea level. The completed HAWC observatory principal detector (HAWC) consists of 300 closely spaced water Cherenkov detectors, each equipped with four photomultiplier tubes to provide timing and charge information to reconstruct the extensive air shower energy and arrival direction. The HAWC observatory has been optimized to observe transient and steady emission from sources of gamma rays within an energy range from several hundred GeV to several hundred TeV. However, most of the air showers detected are initiated by cosmic rays, allowing studies of cosmic rays also to be performed. This paper describes the characteristics of the HAWC main array and its hardware.

astro-ph.HE↗
arXiv2022

Tau longitudinal and transverse polarizations from visible kinematics in (anti-)neutrino nucleus scattering

Since the $ν_τ(\barν_τ) A_Z \to τ^\mp X$ reaction is notoriously difficult to be directly measured, the information on the dynamics of this nuclear process should be extracted from the analysis of the energy and angular distributions of the tau decay visible products. These distributions depend on the components of the tau-polarization vector. We give, for the first time, the general expression for the outgoing hadron (pion or rho meson) energy and angular differential cross section for the sequential $ν_τA_Z \to τ^-(π^- ν_τ, ρ^-ν_τ) X$ and $\barν_τA_Z \to τ^+(π^+ \barν_τ, ρ^+ \barν_τ) X$ reactions. Though all possible nuclear reaction mechanisms contribute to the distribution, it may be possible to isolate/enhance one of them by implementing appropriate selection criteria. For the case of the quasi-elastic reaction off oxygen and neutrino energies below 6 GeV, we show that the pion distributions are sensitive to the details of the tau-polarization components. We find significant differences between the full calculation, where the longitudinal and transverse components of the tau polarization vector vary with the energy and the scattering angle of the produced tau, and the simplified scheme in which the polarizations are set to one and zero (respective asymptotic values in the high energy regime). In addition to its potential impact on neutrino oscillation analyses, this result can be used to further test different nuclear models, since these observables provide complementary information to that obtained by means of the inclusive nuclear weak charged-current differential cross section. We also study the effects on the cross section of the $W_4$, $W_5$ nuclear structure functions, which contributions are proportional to the charged lepton mass, and therefore difficult to constrain in muon and electron neutrino experiments.

hep-ph↗
arXiv2022

Visible energy and angular distributions of the charged particle from the $τ-$decay in $b\to c τ\, (μ\bar ν_μν_τ,πν_τ,ρν_τ) \barν_τ$ reactions

We study the $d^2 Γ_d /(dωd\cosθ_d) $, $dΓ_d /d\cosθ_d$ and $ dΓ_d /dE_d $ distributions, which are defined in terms of the visible energy and polar angle of the charged particle from the $τ-$decay in $b\to c τ\, (μ\bar ν_μν_τ,πν_τ,ρν_τ) \barν_τ$ reactions. The first two contain information on the transverse tau-spin, tau-angular and tau-angular-spin asymmetries of the $H_b\to H_cτ\barν_τ$ parent decay and, from a dynamical point of view, they are richer than the commonly used one, $d^2 Γ_d /(dωdE_d) $, since the latter only depends on the tau longitudinal polarization. We pay attention to the deviations with respect to the predictions of the standard model (SM) for these new observables, considering new physics (NP) operators constructed using both right- and left-handed neutrino fields, within an effective field-theory approach. We present results for $Λ_b\toΛ_cτ\, (μ\bar ν_μν_τ,πν_τ,ρν_τ)\barν_τ$ and $\bar B \to D^{(*)}τ\, (μ\bar ν_μν_τ,πν_τ,ρν_τ) \barν_τ$ sequential decays and discuss their use to disentangle between different NP models. In this respect, we show that $dΓ_d /d\cosθ_d$, which should be measured with sufficiently good statistics, becomes quite useful, especially in the $τ\to πν_τ$ mode. The study carried out in this work could be of special relevance due to the recent LHCb measurement of the lepton flavor universality ratio ${\cal R}_{Λ_c}$ in agreement with the SM. The experiment identified the $τ$ using its hadron decay into $π^-π^+π^-ν_τ$, and this result for ${\cal R}_{Λ_c}$, which is in conflict with the phenomenology from the $b$-meson sector, needs confirmation from other tau reconstruction channels.

hep-ph↗
arXiv2021

The role of right-handed neutrinos in $b\to c τ\, (πν_τ, ρν_τ, μ\bar ν_μν_τ) \barν_τ$ from visible final-state kinematics

In the context of lepton flavor universality violation (LFUV) studies, we fully derive a general tensor formalism to investigate the role that left- and right-handed neutrino new-physics (NP) terms may have in $b\to c τ\barν_τ$ transitions. We present, for several extensions of the Standard Model (SM), numerical results for the $Λ_b\toΛ_cτ\barν_τ$ semileptonic decay, which is expected to be measured with precision at the LHCb. This reaction can be a new source of experimental information that can help to confirm, or maybe rule out, LFUV presently seen in $\bar B$ meson decays. The present study analyzes observables that can help in distinguishing between different NP scenarios that otherwise provide very similar results for the branching ratios, which are our currently best hints for LFUV. Since the $τ$ lepton is very short-lived, we consider three subsequent $τ$-decay modes, two hadronic $πν_τ$ and $ρν_τ$ and one leptonic $μ\barν_μν_τ$, which have been previously studied for $\bar B\to D^{(*)}$ decays. Within the tensor formalism that we have developed in previous works, we re-obtain the expressions for the differential decay width written in terms of visible (experimentally accessible) variables of the massive particle created in the $τ$ decay. There are seven different $τ$ angular and spin asymmetries that are defined in this way and that can be extracted from experiment. Those asymmetries provide observables that can help in constraining possible SM extensions.

hep-ph↗
arXiv2021

New physics and the tau polarization vector in $b\to c τ\barν_τ$ decays

For a general $H_b\to H_cτ\barν_τ$ decay we analyze the role of the $τ$ polarization vector ${\cal P}^μ$ in the context of lepton flavor universality violation studies. We use a general phenomenological approach that includes, in addition to the Standard Model (SM) contribution, vector, axial, scalar, pseudoscalar and tensor new physics (NP) terms which strength is governed by, complex in general, Wilson coefficients. We show that both in the laboratory frame, where the initial hadron is at rest, and in the center of mass of the two final leptons, a $\vec {\cal P}$ component perpendicular to the plane defined by the three-momenta of the final hadron and the $τ$ lepton is only possible for complex Wilson coefficients, being a clear signal for physics beyond the SM as well as time reversal (or CP-symmetry) violation. We make specific evaluations of the different polarization vector components for the $Λ_b\toΛ_c$, $\bar B_c\toη_c,J/ψ$ and $\bar B\to D^{(*)}$ semileptonic decays, and describe NP effects in the complete two-dimensional space associated with the independent kinematic variables on which the polarization vector depends. We find that the detailed study of ${\cal P}^μ$ has great potential to discriminate between different NP scenarios for $0^-\to 0^-$ decays, but also for $Λ_b \to Λ_c$ transitions. For this latter reaction, we pay special attention to corrections to the SM predictions derived from complex Wilson coefficients contributions.

hep-ph↗
arXiv2020

$\bar B_c \to η_c$, $\bar B_c \to J/ψ$ and $\bar B \to D^{(*)}$ semileptonic decays including new physics

We apply the general formalism derived in N. Penalva et al. [Phys. Rev. D 101, 113004 (2020)] to the semileptonic decay of pseudoscalar mesons containing a $b$ quark. While present $B\to D^{(*)}$ data give the strongest evidence in favor of lepton flavor universality violation, the observables that are normally considered are not able to distinguish between different new physics (NP) scenarios. In the above reference we discussed the relevant role that the various contributions to the double differential decay widths $d^2Γ/(dωd\cosθ_\ell)$ and $d^2Γ/(dωdE_\ell)$ could play to this end. Here $ω$ is the product of the two hadron four-velocities, $θ_\ell$ is the angle made by the final lepton and final hadron three-momenta in the center of mass of the final two-lepton system, and $E_\ell$ is the final charged lepton energy in the laboratory system. The formalism was applied there to the analysis of the $Λ_b\toΛ_c$ semileptonic decay showing the new observables were able to tell apart different NP scenarios. Here we analyze the $\bar B_c\to η_c τ\barν_τ$, $\bar B_c\to J/ψτ\barν_τ$, $\bar B\to Dτ\barν_τ$ and $\bar B\to D^*τ\barν_τ$ semileptonic decays. We show that, as a general rule, the $\bar B_c \to J/ψ$ observables, even including $τ$ polarization, are less optimal for distinguishing between NP scenarios than those obtained from $\bar B_c \to η_c$ decays, or those presented in N. Penalva et al. for the related $Λ_b \to Λ_c$ semileptonic decay. Finally, we show that $\bar B\to D$ and $\bar B_c\to η_c$, and $\bar B\to D^*$ and $\bar B_c\to J/ψ$ decay observables exhibit similar behaviors.

hep-ph↗
arXiv2020

Hadron and lepton tensors in semileptonic decays including new physics

We extend our general framework for semileptonic decay, originally introduced in N. Penalva et al. [Phys. Rev. D100, 113007 (2019)], with the addition of new physics (NP) tensor terms. In this way, all the NP effective Hamiltonians that are considered in lepton flavour universality violation (LFUV) studies have now been included. Besides, we now also give general expressions that allow for complex Wilson coefficients. The scheme developed is totally general and it can be applied to any charged current semileptonic decay, involving any quark flavors or initial and final hadron states. We show that all the hadronic input, including NP effects, can be parametrized in terms of 16 Lorentz scalar structure functions. In the second part of this work, we use this formalism to obtain the complete NP effects in the $Λ_b\to Λ_c τ\barν_τ$ semileptonic decay, where LFUV, if finally confirmed, is also expected to be seen. We stress the relevance of the center of mass (CM) $d^2Γ/(dωd\cosθ_\ell)$ and laboratory (LAB) $d^2Γ/(dωdE_\ell)$ differential decay widths. While models with very different strengths in the NP terms give the same differential $dΓ/dω$ and total decay widths for this decay, they predict very different numerical results for some of the $\cosθ_\ell$ and $E_\ell$ coefficient-functions that determine the above two distributions. Thus, the combined analysis of the CM $d^2Γ/(dωd\cosθ_\ell)$ and LAB $d^2Γ/ (dωdE_\ell)$ differential decay widths will help clarifying what kind of NP is a better candidate in order to explain LFUV.

hep-ph↗
arXiv2020

Weak B Decays into Orbitally Excited Charmed Mesons

The BaBar Collaboration has recently reported branching fractions for semileptonic decays of the $B$ meson into final states with charged and neutral $D_{1}(2420)$ and $D_{2}^{\ast}(2460)$, two narrow orbitally excited charmed mesons. We evaluate these branching fractions within the framework of a constituent quark model in two steps, one which involves a semileptonic decay and the other one mediated by a strong process. Our results are in agreement with the experimental data.

hep-ph↗
arXiv2020

Data approximation with time-frequency invariant systems

In this paper we prove the existence of a time-frequency space that best approximates a given finite set of data. Here best approximation is in the least square sense, among all time-frequency spaces with no more than a prescribed number of generators. We provide a formula to construct the generators from the data and give the exact error of approximation. The setting is in the space of square integrable functions defined on a second countable LCA group and we use the Zak transform as the main tool.

math.FA↗
arXiv2019

Spectroscopy, lifetime and decay modes of the $T^-_{bb}$ tetraquark

We present the first full-fledged study of the flavor-exotic isoscalar $T_{bb}^-\equiv b b \bar u \bar d$ tetraquark with spin and parity $J^P=1^+$. We report accurate solutions of the four-body problem in a quark model, characterizing the structure of the state as a function of the ratio $M_Q/m_q$ of the heavy to light quark masses. For such a standard constituent model, $T_{bb}^-$ lies approximately 150 MeV below the strong decay threshold $B^-\bar {B^*}^{0}$ and 105 MeV below the electromagnetic decay threshold $B^- \bar B^0 γ$. We evaluate the lifetime of $T_{bb}^-$, identifying the promising decay modes where the tetraquark might be looked for in future experiments. Its total decay width is $Γ\approx 87 \times 10^{-15}$ GeV and therefore its lifetime $τ\approx$ 7.6 ps. The promising final states are ${B^*}^{-}\, {D^*}^{+} \, \ell^- \, \bar ν_\ell$ and $\bar {B^*}^{0} \, {D^*}^{0} \, \ell^- \, \bar ν_\ell $ among the semileptonic decays, and ${B^*}^{-} \, {D^*}^{+} \, {D_s^*}^-$, $\bar {B^*}^{0} \, {D^*}^{0} \, {D_s^*}^- $, and ${B^*}^{-} \, {D^*}^{+} \, ρ^-$ among the nonleptonic ones. The semileptonic decay to the isoscalar $J^P=0^+$ tetraquark $T_{bc}^0$ is also relevant but it is not found to be dominant. There is a broad consensus about the existence of this tetraquark, and its detection will validate our understanding of the low-energy realizations of Quantum Chromodynamics (QCD) in the multiquark sector.

hep-ph↗
arXiv2018

Angular distributions in electroweak pion production off nucleons: odd parity hadron terms, strong relative phases and model dependence

The study of pion production in nuclei is important for signal and background determinations in current and future neutrino oscillation experiments. The first step, however, is to understand the pion production reactions at the free nucleon level. We present an exhaustive study of the charged-current and neutral-current neutrino and antineutrino pion production off nucleons, paying a special attention to the angular distributions of the outgoing pion. We show, using general arguments, that parity violation and time-reversal odd correlations in the weak differential cross sections are generated from the interference between different contributions to the hadronic current that are not relatively real. Next, we present a detailed comparison of three, state of the art, microscopic models for electroweak pion production off nucleons, and we also confront their predictions with polarized electron data, as a test of the vector content of these models. We also illustrate the importance of carrying out a comprehensive test at the level of outgoing pion angular distributions, going beyond comparisons done for partially integrated cross sections, where model differences cancel to a certain extent. Finally, we observe that all charged and neutral current distributions show sizable anisotropies, and identify channels for which parity violating effects are clearly visible. Based on the above results, we conclude that the use of isotropic distributions for the pions in the center of mass of the final pion-nucleon system, as assumed by some of the Monte Carlo event generators, needs to be improved by incorporating the findings of microscopic calculations.

hep-ph↗
arXiv2017

Embeddings and Lebesgue-type inequalities for the greedy algorithm in Banach spaces

We obtain Lebesgue-type inequalities for the greedy algorithm for arbitrary complete seminormalized biorthogonal systems in Banach spaces. The bounds are given only in terms of the upper democracy functions of the basis and its dual. We also show that these estimates are equivalent to embeddings between the given Banach space and certain discrete weighted Lorentz spaces. Finally, the asymptotic optimality of these inequalities is illustrated in various examples of non necessarily quasi-greedy bases.

math.FA↗
arXiv2017

Neutrino-induced one-pion production revisited: the $ν_μn\toμ^- nπ^+$ channel

Understanding single pion production reactions on free nucleons is the first step towards a correct description of these processes in nuclei, which are important for signal and background contributions in current and near future accelerator neutrino oscillation experiments. In this work, we reanalyze our previous studies of neutrino-induced one-pion production on nucleons for outgoing $πN$ invariant masses below 1.4 GeV. Our motivation is to get a better description of the $ν_μn\toμ^- nπ^+$ cross section, for which current theoretical models give values significantly below data. This channel is very sensitive to the crossed $Δ(1232)$ contribution and thus to spin 1/2 components in the Rarita-Schwinger $Δ$ propagator. We show how these spin 1/2 components are nonpropagating and give rise to contact interactions. In this context, we point out that the discrepancy with experiment might be corrected by the addition of appropriate extra contact terms and argue that this procedure will provide a natural solution to the $ν_μn\toμ^- nπ^+$ puzzle. To keep our model simple, in this work we propose to change the strength of the spin 1/2 components in the $Δ$ propagator and use the $ν_μn\toμ^- nπ^+$ data to constraint its value. With this modification, we now find a good reproduction of the $ν_μn\toμ^- nπ^+$ cross section without affecting the good results previously obtained for the other channels. We also explore how this change in the $Δ$ propagator affects our predictions for pion photoproduction and find also a better agreement with experiment than with the previous model.

hep-ph↗
arXiv2017

Particular spectral singularity in the continuum energies: a manifestation as resonances

We study the coalescence of two bound energy eigenstates embedded in the continuous spectrum of a real Hamiltonian $H[4]$ and the singular point produced by this coalescence. At the singular point, the two unnormalized Jost eigenfunctions are no longer linearly independent but coalesce to give rise to a bound state eigenfunction embedded in the continuum. We disturb the potential $V[4]$ by means of a truncation, this perturbation breaks the singular point in two resonances. The phase shift shows a jump of magnitude $2π$ and the shape of the cross section shows two inverted peaks, this behaviour is due to the interference between the two nearly degenerate resonances and the background component of the Jost function.

quant-ph↗
arXiv2016

Watson's theorem and the $NΔ(1232)$ axial transition

We present a new determination of the $NΔ$ axial form factors from neutrino induced pion production data. For this purpose, the model of Hernandez {\it et al.} [Phys. Rev. D76, 033005 (2007)] is improved by partially restoring unitarity. This is accomplished by imposing Watson's theorem on the dominant vector and axial multipoles. As a consequence, a larger $C_5^A(0)$, in good agreement with the prediction from the off-diagonal Goldberger-Treiman relation, is now obtained.

hep-ph↗