arXiv subjects

Neus Penalva

Publications and source records attributed to Neus Penalva.

4 recordsLinked to original sources

arXiv2024

The $\Lambda_c\to \Lambda\, l^+\nu_\ell$ weak decay including new physics

We investigate the $\Lambda_c \to \Lambda \ell^{+} \nu_\ell$ decay with a focus on potential new physics (NP) effects in the $\ell = \mu$ channel. We employ an effective Hamiltonian within the framework of the Standard Model Effective Field Theory (SMEFT) to consider generalized dimension-6 semileptonic $c\to s$ operators of scalar, pseudoscalar, vector, axial-vector and tensor types. We rely on Lattice QCD (LQCD) for the hadronic transition form factors, using heavy quark spin symmetry (HQSS) to determine those that have not yet been obtained on the lattice. Uncertainties due to the truncation of the NP Hamiltonian and different implementations of HQSS are taken into account. As a result, we unravel the NP discovery potential of the $\Lambda_c\to \Lambda$ semileptonic decay in different observables. Our findings indicate high sensitivity to NP in lepton flavour universality ratios, probing multi-TeV scales in some cases. On the theoretical side, we identify LQCD uncertainties in axial and vector form factors as critical for improving NP sensitivity, alongside better SMEFT uncertainty estimations.

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arXiv2023

Study of new physics effects in $\bar B_s\to D^{(*)}_sτ^-\barν_τ$ semileptonic decays using lattice QCD form factors and heavy quark effective theory

We benefit from the lattice QCD determination by the HPQCD of the Standard Model (SM) form factors for the $\bar B_s\to D_s$ [Phys. Rev. D 101, 074513 (2020)] and the SM and tensor ones for the $\bar B_s\to D_s^{*}$ (arXiv:2304.03137 [hep-lat]) semileptonic decays, and the heavy quark effective theory (HQET) relations for the analogous $B\to D^{(*)}$ decays obtained by F.U. Bernlochner et al. in Phys. Rev. D 95, 115008 (2017), to extract the leading and sub-leading Isgur-Wise functions for the $\bar B_s\to D_s^{(*)}$ decays. Further use of the HQET relations allows us to evaluate the corresponding scalar, pseudoscalar and tensor form factors needed for a phenomenological study of new physics (NP) effects on the $\bar B_s\to D_s^{(*)}$ semileptonic decay. At present, the experimental values for the ratios ${\cal R}_{D^{(*)}}=Γ[\bar B\to D^{(*)}τ^-\barν_τ]/Γ[\bar B\to D^{(*)}e^-(μ^-)\barν_{e(μ)}]$ are the best signal in favor of lepton flavor universality violation (LFUV) seen in charged current (CC) $b\to c$ decays. In this work we conduct a study of NP effects on the $\bar B_s\to D_s^{(*)}τ^-\barν_τ$ semileptonic decays by comparing tau spin, angular and spin-angular asymmetry distributions obtained within the SM and three different NP scenarios. As expected from SU(3) light-flavor symmetry, we get results close to the ones found in a similar analysis of the $\bar B\to D^{(*)}$ case. The measurement of the $\bar B_s\to D_s^{(*)}\ell\barν_\ell$ semileptonic decays, which is within reach of present experiments, could then be of relevance in helping to establish or rule out LFUV in CC $b\to c$ transitions.

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arXiv2022

New physics effects on $Λ_b\to Λ^*_cτ\barν_τ$ decays

We benefit from a recent lattice determination of the full set of vector, axial and tensor form factors for the $Λ_b\to Λ^*_c(2595)τ\barν_τ$ and $Λ^*_c(2625)τ\barν_τ$ semileptonic decays to study the possible role of these two reactions in lepton flavor universality violation studies. Using an effective theory approach, we analyze different observables that can be accessed through the visible kinematics of the charged particles produced in the tau decay, for which we consider the $π^-ν_τ,ρ^-ν_τ$ and $μ^-\barν_μν_τ$ channels. We compare the results obtained in the Standard Model and other schemes containing new physics (NP) interactions, with either left-handed or right-handed neutrino operators. We find a discriminating power between models similar to the one of the $Λ_b\to Λ_c$ decay, although somewhat hindered in this case by the larger errors of the $Λ_b\toΛ^*_c$ lattice form factors. Notwithstanding this, the analysis of these reactions is already able to discriminate between some of the NP scenarios and its potentiality will certainly improve when more precise form factors are available.

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arXiv2019

Further tests of lepton flavour universality from the charged lepton energy distribution in $b\to c$ semileptonic decays: The case of $Λ_b\to Λ_c \ell \barν_\ell$

In a general framework, valid for any $H\to H'\ell^-\barν_\ell$ semileptonic decay, we analyze the $d^2Γ/(dωd\cosθ_\ell)$ and $d^2Γ/ (dωdE_\ell)$ distributions, with $ω$ being the product of the hadron four-velocities, $θ_\ell$ the angle made by the three-momenta of the charged lepton and the final hadron in the $W^-$ center of mass frame and $E_\ell$ the charged lepton energy in the decaying hadron rest frame. Within the Standard Model (SM), $ d^2Γ/(dωdE_\ell)\propto \left(c_0(ω)+c_1(ω)E_\ell/M+c_2(ω)E^2_\ell/M^2\right)$, with $M$ the initial hadron mass. We find that $c_2(ω)$ is independent of the lepton flavor and thus it is an ideal candidate to look for lepton flavor universality (LFU) violations. We also find a correlation between the $a_2(ω)$ structure function, that governs the $(\cosθ_\ell)^2$ dependence of $d^2Γ/(dωd\cosθ_\ell)$, and $c_2(ω)$. Apart from trivial kinematical and mass factors, the ratio of $a_2(ω)/c_2(ω)$ is a universal function that can be measured in any semileptonic decay, involving not only $b\to c$ transitions. These two SM predictions can be used as new tests in the present search for signatures of LFU violations. We also generalize the formalism to account for some new physics (NP) terms. Finally, in order to illustrate our findings, we apply our general framework to the $Λ_b\to Λ_c \ell \barν_\ell$ decay. We show that a measurement of $c_2$ (or $a_2$) for $τ$ decay would not only be a direct measurement of the possible existence of NP, but it would also allow to distinguish from NP fits to $b\to cτ\barν_τ$ anomalies in the meson sector, that otherwise give the same total and differential $dΓ/dω$ widths.

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