arXiv subjects

S. L. Tostado

Publications and source records attributed to S. L. Tostado.

9 recordsLinked to original sources

arXiv2025

Approximate magic symmetry in a cobimaximal scenario of Majorana neutrinos

The searches for an underlying pattern in neutrino masses have motivated different proposals for textures in the neutrino mass matrix, which is also related to particular arrangements of the mixing matrix. Current experimental determinations of neutrino mixings have restricted some of the most studied proposals. In this work, we propose a new combined constraint on the neutrino mass matrix arising from the cobimaximal mixing and the magic symmetry in a model-independent fashion. We show that both conditions cannot be fulfilled simultaneously in an exact but rather in an approximate way. We have found that the Majorana CP phases are tightly restricted within this approach, leading to well-defined regions in the neutrinoless double beta decay amplitude. The small symmetry breaking requirement allows us to determine the Majorana phases for specific values of the lightest neutrino mass, consistent with current bounds on the mixing angles, neutrinoless double-beta decay, and cosmology results.

hep-ph↗

arXiv2023

Deviations to $μ-τ$ symmetry in a cobimaximal scenario

Within the different patterns of the neutrino mixing matrix, the cobimaximal mixing remains a plausible possibility for understanding the flavor structure of neutrinos as it is consistent with current experimental data. Such a pattern is related to a concrete form of the mass matrix, displaying a $μ-τ$ reflection symmetry, which has motivated many theoretical investigations in recent years. In this paper, we discuss the effects of the Majorana phases over a $μ-τ$ symmetric mass matrix obtained from a cobimaximal mixing matrix. We investigate how these phases could be restricted through deviations from the symmetric scenario and the most precise determinations of the mixing parameters. Some of our relations could be tested with future results.

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arXiv2020

Restricting Majorana phases in the Tri-Bi-Maximal limit

The Tri-Bi-Maximal pattern has been long investigated as the symmetric scenario that lies behind the neutrino mixing matrix. It predicts a null reactor angle and hence forbids $CP$ violation in the lepton sector, which is in contrast to the current experimental determinations. We explore different deviations from this pattern to restore the compatibility with the latest fits of neutrino mixing parameters. We consider two unitary matrices to correct the symmetric pattern, each of them is written in terms of one single angle and one complex phase, which will be constrained by the experimental mixings and from symmetry restrictions in the mass matrix. We note that these correction parameters would allow us to obtain simultaneous information about the Dirac and Majorana $CP$ phases in some specific scenarios. We show that the predicted values lead to sharped regions for the neutrinoless double beta decay amplitude, in the selected cases, that could be tested with forthcoming results.

hep-ph↗

arXiv2019

Deviations to Tri-Bi-Maximal mixing in the limit of $μ-τ$ symmetry

In the limit of an approximate $μ-τ$ symmetry in the neutrino mass matrix, we explore deviations to the Tri-Bi-Maximal mixing pattern in the neutrino sector. We consider two different ansatzes for the corrected pattern to predict the current values of neutrino mixing parameters. We show that it is possible to constrain the Majorana $CP$ phases by studying their correlation to the mixing parameters and we study their effects on neutrinoless double beta decay observables. These predictions are sharp for the quasi-degenerate ordering and can be tested in upcoming experiments.

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arXiv2017

Evaluation of CKM matrix elements from exclusive $P_{\ell 4}$ decays

We consider the exclusive $P_{\ell 4}$ decays, $P\to (P_1P_2)_V\ell ν_{\ell}$, where the subindex $V$ means that the invariant mass of the pseudoscalar pair is taken within a small window around the mass of the vector meson $V$. Pole contributions beyond the dominant $P\to V(\to P_1P_2)\ellν_{\ell}$ amplitude of $P_{\ell 4}$ decays are identified, which, in turn, affects the determination of the CKM matrix elements $|V_{qq'}|$. We evaluate the effects of those contributions in the extraction of bottom and charm quark mixings. An application to $B\to (ππ)_ρ\ell ν_{\ell}$ data from Belle collaboration, shows an increase in the extracted value of $|V_{ub}|$ in better agreement with determinations based on $B\to π\ellν_{\ell}$ decays. The effect of the $ρ$ and $D^*$ pole contributions in the determination of $|V_{cd}|$ from the decay $D\to ππ\ell^- \barν_{\ell}$, has been also investigated.

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arXiv2017

Remarks on the Standard Model predictions for $R(D)$ and $R(D^*)$

Semileptonic $b \to c$ transitions, and in particular the ratios $R(D^{(*)}) = \frac{Γ(B \rightarrow D^{(*)} τν)}{Γ(B \rightarrow D^{(*)} \ell ν)}$, can be used to test the universality of the weak interactions. In light of the recent discrepancies between the experimental measurements of these observables by BaBar, Belle and LHCb and the Standard Model predicted values, we study the robustness of the latter. Our analysis reveals that $R(D)$ might be enhanced by lepton mass effects associated to the mostly unknown scalar form factor. In constrast, the Standard Model prediction for $R(D^*)$ is found to be more robust, since possible pollutions from $B^*$ contributions turn out to be negligibly small, which indicates that $R(D^*)$ is a promising observable for searches of new physics.

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arXiv2016

Radiative corrections of $O(α)$ to $B^- \rightarrow V^0 \ell^- \barν_{\ell}$ decays

The $O(α)$ electromagnetic radiative corrections to the $B^- \rightarrow V^0 \ell^- \barν_{\ell}$ ($V$ is a vector meson and $\ell$ a charged lepton) decay rates are evaluated using the cutoff method to regularize virtual corrections and incorporating intermediate resonance states in the real-photon amplitude to extend the region of validity of the soft-photon approximation. The electromagnetic and weak form factors of hadrons are assumed to vary smoothly over the energies of virtual and real photons under consideration. The cutoff dependence of radiative corrections upon the scale $Λ$ that separates the long- and short-distance regimes is found to be mild and is considered as an uncertainty of the calculation. Owing to partial cancellations of electromagnetic corrections evaluated over the three- and four-body regions of phase space, the photon-inclusive corrected rates are found to be dominated by the short-distance contribution. These corrections will be relevant for a precise determination of the $b$ quark mixing angles by testing isospin symmetry when measurements of semileptonic rates of charged and neutral $B$ mesons at the few percent level become available. For completeness, we also provide numerical values of radiative corrections in the three-body region of the Dalitz plot distributions of these decays.

hep-ph↗

arXiv2015

Long-distance weak annihilation contribution to the $B^{\pm}\to (π^{\pm},K^{\pm}) \ell^+\ell^-$ decays

We propose an alternative evaluation of the long-distance weak annihilation (WA, also called one-photon exchange in this paper) contribution to the rare semileptonic $B^{\pm}\to (π^{\pm},K^{\pm})\ell^+\ell^-$ ($\ell=e,\,μ$) decays. This hadronic description at low energies is matched at intermediate energies to its short-distance counterpart in terms of quark and gluon degrees of freedom. Although the WA contribution does not contribute to solve the possible breaking of lepton-universality observed by LHCb in the $B^{\pm}\to K^{\pm}(μ^+μ^-/e^+e^-)$ ratio, nor provides an important hadronic contamination to their decay rates, its contribution to the branching ratios (and direct CP asymmetry) of the $B^{\pm}\to π^{\pm}\ell^+\ell^-$ transitions turns out to be significant. This hadronic pollution should be taken into account when looking for new physics effects in decays into pions, which suggests to restrict these searches to squared lepton-pair invariant mass in the $(1,8)$ GeV$^2$ range. The interference of the one-photon exchange contribution with the dominant short-distance one-loop amplitude induces a sizable CP asymmetry in these rare decays, which calls for dedicated measurements.

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arXiv2015

A new long distance contribution to $B^\pm\to K^\pm/π^\pm\ell^+\ell-$ decays

We have identified a missing long-distance (one-photon exchange) contribution to the $B^\pm\to K^\pm/π^\pm\ell^+\ell^-$ decays. Although it does not help to explain the anomaly measured by the LHCb Coll. in the ratio $R_K = \mathcal{B}(B^\pm\to K^\pmμ^+μ^-)/\mathcal{B}(B^\pm\to K^\pm e^+e^-)\sim0.75$ (2.6 $σ$ deviation with respect to $R_K=1+\mathcal{O}(10^{-4})$ in the Standard Model), it provides a sizable contribution to the branching ratio of the $B^\pm\to π^\pm\ell^+\ell^-$ decays. The new decay mechanism gives rise to a measurable CP asymmetry, which is of order 1\% in both channels. These predictions can be tested in forthcoming LHCb measurements.

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