arXiv subjects

N. Penalva

Publications and source records attributed to N. Penalva.

5 recordsLinked to original sources

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.

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