arXiv subjects

G. López Castro

Publications and source records attributed to G. López Castro.

At least 19 recordsLinked to original sources

arXiv2026

Semileptonic $D_{e4}$ decays: hadronic dynamics and the determination of $|V_{cs}|$

The four-body decays $D^+ \to K^-π^+e^+ν_e$ ($D_{e4}^+$) and $D^0\to \overline{K^0}π^-e^+ν_e$ ($D^0_{e4}$) are studied in a model where the momentum-dependence of the hadronic matrix elements are described in terms of $K^*(892)$ and $D^*(2010)$ pole contributions. From fits to the recent data of the BESIII collaboration we find that the $D^*$-pole can mimic the effect of the $S$-wave contribution of the $Kπ$ system to the branching fraction. Implications for the determination of the $|V_{cs}|$ quark mixing matrix element are discussed.

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arXiv2026

Beyond scalar QED radiative corrections: the $ρ^{\pm}-ρ^0$ width difference, FSR corrections and their impact on $Δa_μ^{\rm HVP, LO}[τ]$

In a previous paper arXiv:0708.3256 [hep-ph] we have calculated the radiative corrections to $ρ\to ππ$ decays, aiming to estimate the width difference between charged and neutral rho mesons. There, we have used the scalar QED approximation and taken the convection terms to keep the loop contributions finite in the case of charged rho meson decays. Here we compute the radiative corrections by considering the electromagnetic structure of charged mesons and we also include the full Lorentz structure of the electromagnetic vertices. We re-evaluate the width difference of $ρ^{\pm}-ρ^0$ vector mesons and calculate the structure-dependent contributions to Final State Radiation terms in the $e^+e^-\to π^+π^-$ cross section. Both effects are important inputs for evaluating the isospin breaking corrections in the dominant hadronic vacuum polarization contributions to the muon $g-2$ when using $τ$ lepton data.

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arXiv2025

The anomalous magnetic moment of the muon in the Standard Model: an update

We present the current Standard Model (SM) prediction for the muon anomalous magnetic moment, $a_μ$, updating the first White Paper (WP20) [1]. The pure QED and electroweak contributions have been further consolidated, while hadronic contributions continue to be responsible for the bulk of the uncertainty of the SM prediction. Significant progress has been achieved in the hadronic light-by-light scattering contribution using both the data-driven dispersive approach as well as lattice-QCD calculations, leading to a reduction of the uncertainty by almost a factor of two. The most important development since WP20 is the change in the estimate of the leading-order hadronic-vacuum-polarization (LO HVP) contribution. A new measurement of the $e^+e^-\toπ^+π^-$ cross section by CMD-3 has increased the tensions among data-driven dispersive evaluations of the LO HVP contribution to a level that makes it impossible to combine the results in a meaningful way. At the same time, the attainable precision of lattice-QCD calculations has increased substantially and allows for a consolidated lattice-QCD average of the LO HVP contribution with a precision of about 0.9%. Adopting the latter in this update has resulted in a major upward shift of the total SM prediction, which now reads $a_μ^\text{SM} = 116\,592\,033(62)\times 10^{-11}$ (530 ppb). When compared against the current experimental average based on the E821 experiment and runs 1-6 of E989 at Fermilab, one finds $a_μ^\text{exp} - a_μ^\text{SM} =38(63)\times 10^{-11}$, which implies that there is no tension between the SM and experiment at the current level of precision. The final precision of E989 (127 ppb) is the target of future efforts by the Theory Initiative. The resolution of the tensions among data-driven dispersive evaluations of the LO HVP contribution will be a key element in this endeavor.

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arXiv2025

Invisible decays of vector Charmonia and Bottomonia to determine the Weak Mixing Angle at quarkonia scale

We compute the branching fractions of vector quarkonia ($V_Q=J/ψ, ψ', Υ(nS)$) decays into neutrino pairs, considering both Dirac and Majorana types, within the Standard Model (SM) and beyond. The vector nature of quarkonium states yields a decay width in the SM that depends upon the weak vector coupling of the heavy quark, offering the possibility to measure the weak mixing angle at the quarkonia mass scales. If neutrinos have non-standard neutral weak couplings, this could help to distinguish the nature of neutrinos in principle.

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arXiv2022

$Δ$L=2 hyperon decays induced by Majorana neutrinos and doubly-charged scalars

Searches for $ΔL=2$ decays are crucial to disentangle the Dirac vs Majorana nature of neutrinos. While neutrinoless nuclear double beta decays are the most promising tool to look for di-electron signals of $ΔL=2$, searches of di-lepton channels with other flavors are far less limited by statistics. We revisit the calculation of $ΔL=2$ decays of hyperons $B^-_i\to B^+_f\ell^-\ell'^-$. We compute the rates of these decays by including the hyperon form factors, which yields finite results in the one-loop model mechanism involving Majorana neutrinos (long-range contributions). In addition, we study the short-range contributions to these kinds of processes in two appealing scenarios. First, we consider the effects of heavy Majorana neutrinos in low-scale seesaw models that involve a minimal parametrization with only two heavy Majorana states. Second, we study an alternative model where $ΔL=2$ decays are induced by a scalar boson coupled to di-leptons to provide additional predictions.

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arXiv2021

Tests of the Atomki anomaly in lepton pair decays of heavy mesons

The anomalies recently reported in lepton pair transitions of $^8$Be$^*$ and $^4$He nuclei may be attributed to the existence of a feebly interacting light vector boson $X17$. We study the effects of this hypothetical particle in the semileptonic $H^* \to H e^+e^-$ decays ($H$ a $Q\bar{q}$ meson) in the framework of the HQET+VMD model. Using current bounds and the universality assumption of the $X17$ boson to quarks, we find that decays of $D^{*+}$ and $D_s^{*+}$ mesons can be importantly enhanced relative to the dominant photon-mediated contributions. Dedicated experimental searches at current heavy meson factories may confirm the existence of this light boson or set stronger bounds of their couplings to ordinary matter.

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arXiv2020

Lepton-pair production in di-pion $τ$ lepton decays

We study the $τ^- \to ν_τ π^{-}π^{0}\ell^{+}\ell^{-}$ ($\ell=e,\,μ$) decays, which are $O(α^2)$-suppressed with respect to the dominant di-pion tau decay channel. Both the inner-bremsstrahlung and the structure- (and model-)dependent contributions are considered. In the $\ell=e$ case, structure-dependent effects are $\mathcal{O}(1\%)$ in the decay rate, yielding a clean prediction of its branching ratio, $2.3\times10^{-5}$, measurable with BaBar or Belle(-II) data. For $\ell=μ$, both contributions have similar magnitude and we get a branching fraction of $(1.6\pm0.3)\times10^{-7}$, reachable by the end of Belle-II operation. These decays allow to study the dynamics of strong interactions with simultaneous weak and electromagnetic probes; their knowledge will contribute to reducing backgrounds in lepton flavor/number violating searches.

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arXiv2020

Effects of heavy Majorana neutrinos on lepton flavor violating processes

The observation of lepton flavor violating processes at colliders could be a clear signal of a non-minimal neutrino sector. We define a 5-parameter model with a pair of TeV fermion singlets and arbitrary mixings with the three active neutrino flavors. Then we analyze several flavor violating transitions ($\ell\to \ell^{\prime}γ,\,\ell^{\prime}\ell^{\prime\prime}\bar{\ell}^{\prime\prime\prime}$ or $μ-e$ conversions in nuclei) and $Z\to\bar{\ell}\,\ell^{\prime}$ decays induced by the presence of heavy neutrinos. In particular, we calculate all the one-loop contributions to these processes and present their analytic expressions. We focus on the genuine effects of the heavy Majorana masses, comparing the results in that case with the ones obtained when the two heavy neutrinos define a Dirac field. Finally, we use our results to update the bounds on the heavy-light mixings in the neutrino sector.

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arXiv2018

Non-standard interactions in $τ^- \to (π^-η,π^-π^0)ν_τ$ decays

Originally thought as clean processes to study the hadronization of the weak currents, semileptonic tau lepton decays can be useful to set constraints on non-standard (NS) weak interactions. In this talk we summarize our recent studies on the effects of NS interactions in $τ^- \to (π^-π^0,π^-η)ν_τ$ decays. We find that experimental data on these decays provide strong constraints on NS scalar and tensor interactions, respectively. Further improved measurements at Belle II and a better knowledge of necessary tensor and scalar form factors will allow to set limits on these NS interactions that are similar or better to contraints obtained from other low-energy processes.

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arXiv2018

Constraints on a sub-eV scale sterile neutrino from non-oscillation measurements

Anomalies in several short-baseline neutrino oscillation experiments suggest the possible existence of sterile neutrinos at about eV scale having appreciable mixing with the already known three neutrinos. We find that if such a light sterile neutrino exists, through a combined study of the leptonic decays of $μ^-$, $τ^-$, $π^-$ and $K^-$, some semi-leptonic decays of $τ^-$ and the invisible decay width of the $Z$ boson, it is possible to constrain the relevant mixing matrix elements. Furthermore, we compare the constraints, derived by using the method presented here, with the experimental results obtained from short-baseline neutrino oscillation experiments. We find that a single light sterile neutrino cannot satisfy the existing short-baseline neutrino oscillation constraints and explain the anomalies mentioned above. Along the way we provide a number of experimentally clean observables which can be used to directly study the light sterile neutrino independently of the neutrino oscillation experiments.

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arXiv2018

Flavor violating leptonic decays of $τ$ and $μ$ leptons in the Standard Model with massive neutrinos

We have revisited the computations of the flavor violating leptonic decays of the $τ$ and $μ$ leptons into three lighter charged leptons in the Standard Model with non-vanishing neutrino masses. We were driven by a claimed unnaturally large branching ratio predicted for the $τ^-\to μ^- \ell^+ \ell^-$ ($\ell=μ, e$) decays \cite{Pham:1998fq}, which was at odds with the corresponding predictions for the $μ^-\to e^- e^-e^+$ processes \cite{Petcov:1976ff}. In contrast with the prediction in \cite{Pham:1998fq}, our results are strongly suppressed and in good agreement with the approximation done in ref.~\cite{Petcov:1976ff}, where masses and momenta of the external particles were neglected in order to deal with the loop integrals. However, as a result of keeping external momenta and masses in the computation of the dominant penguin and box diagrams- we even find slightly smaller branching fractions. Therefore, we confirm that any future observation of such processes would be an unambiguous manifestation of new physics beyond the Standard Model

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arXiv2017

Effective-field theory analysis of the $τ^- \to η^{(\prime)} π^- ν_τ$ decays

The rare $τ^- \to η^{(\prime)} π^- ν_τ$ decays, which are suppressed by $G$-parity in the Standard Model (SM), can be sensitive to the effects of new interactions. We study the sensitivity of different observables of these decays in the framework of an effective field theory that includes the most general interactions between SM fields up to dimension six, assuming massless neutrinos. Owing to the strong suppression of the SM isospin breaking amplitudes, we find that the different observables would allow to set constraints on scalar interactions that are stronger than those coming from other low-energy observables.

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arXiv2017

Discovering intermediate mass sterile neutrinos through $τ^- \to π^- μ^- e^+ ν$ (or $\barν$) decay

Distinguishing the Dirac and Majorana nature of neutrinos remains one of the most important tasks in neutrino physics. By assuming that the $τ^- \to π^- μ^- e^+ ν$ (or $\barν$) decay is resonantly enhanced by the exchange of an intermediate mass sterile neutrino $N$, we show that the energy spectrum of emitted pions and muons can be used to easily distinguish between the Dirac and Majorana nature of $N$. This method takes advantage of the fact that the flavor of light neutrinos is not identified in the tau decay under consideration. We find that it is particularly advantageous, because of no competing background events, to search for $N$ in the mass range $m_e + m_μ \leqslant m_N \leqslant m_μ + m_π$, where $m_X$ denotes the mass of particle $X \in \{ e, μ, π, N \}$.

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arXiv2017

G-parity breaking in $τ^- \to η^{(\prime)} π^- ν_τ$ decays induced by the $η^{(\prime)}γγ$ form factor

Breaking of G-parity or new weak (second class) currents can be responsible for $τ^- \to η^{(\prime)} π^- ν_τ$ decays. Forthcoming measurements of $τ$ lepton properties at the Belle II experiment will be able to measure this decay channel for the first time. Isolating new physics contributions from the measured rates will require a careful evaluation of G-parity breaking contributions. Here we evaluate the one-loop contribution to $τ^- \to η^{(\prime)} π^- ν_τ$ decays induced by the emission of two virtual photons and its later conversion into an $η^{(\prime)}$ meson. As expected, this contribution is very small and may be relevant only for new physics searches contributing at the $10^{-4}$ level to the decay rate of $τ^- \to η^{(\prime)} π^- ν_τ$ .

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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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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.

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arXiv2016

Five-body leptonic decays of muon and tau leptons

We study the five-body decays $μ^- \to e^- e^+ e^- ν_μ \barν_{e}$ and $τ^- \to \ell^- \ell'^+\ell'^-ν_τ\barν_{\ell}$ for $\ell, \ell'=e, μ$ within the Standard Model (SM) and in a general effective field theory description of the weak interactions at low energies. We compute the branching ratios and compare our results with two previous, mutually discrepant, SM calculations. By assuming a general structure for the weak currents we derive the expressions for the energy and angular distributions of the three charged leptons when the decaying lepton is polarized, which will be useful in precise tests of the weak charged current at Belle II. In these decays, leptonic $\mathbf{T}$-odd correlations in triple products of spin and momenta --which may signal time reversal violation in the leptonic sector-- are suppressed by the tiny neutrino masses. Therefore, a measurement of such $\mathbf{T}$-violating observables would be associated to neutrinoless lepton flavor violating (LFV) decays, where this effect is not extremely suppressed. We also study the backgrounds that the SM five-lepton lepton decays constitute to searches of LFV $L^- \to \ell^- \ell'^+\ell'^-$ decays. Searches at high values of the invariant mass of the $\ell'^+\ell'^-$ pair look the most convenient way to overcome the background.

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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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