arXiv subjects

G. López-Castro

Publications and source records attributed to G. López-Castro.

5 recordsLinked to original sources

arXiv2022

Contribution of exclusive $(π^0π^0, π^0η, ηη)γ$ channels to the leading order HVP of the muon $g-2$

We evaluate the contributions of $(π^0π^0, π^0η, ηη)γ$ exclusive channels to the dispersion integral of the leading order HVP of the muon anomalous magnetic moment. These channels are included in some way in previous evaluations of the $π^0ω, ηω$ and $ηϕ$ contributions to $a_μ^{\rm had, LO}$, where the vector resonances (decaying into $π^0/η+ γ$) are assumed to be on-shell. Since the separation of resonance and background contributions in a given observable is, in general, a model-dependent procedure, here we use pseudoscalar mesons and the photon as the $in$ and $out$ states of the $e^+e^- \to (π^0π^0, π^0η, ηη)γ$ $S$-matrix, such that the cross section contains the interferences among different contributing to the amplitudes. We find $a^{\rm had, LO}_μ(P^0_1P^0_2γ)=(1.13\pm 0.13 ) \times 10^{-10}$, where uncertainties stem mainly from vector meson dominance model parameters. Improved experimental studies of these exclusive channels in the whole range below 2 GeV would reduce model-dependency.

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arXiv2022

One-loop determination of $τ\to π(K) ν_τ[γ]$ branching ratios and new physics tests

We calculate the ratios $R_{τ/ P}\equiv Γ\left( τ\to P ν_τ[γ] \right) / Γ\left(P\to μν_μ[γ]\right)$ ($P=π,K$) at one loop following a large-$N_C$ expansion where Chiral Perturbation Theory is enlarged by including the lightest resonances and respecting the short-distance behavior dictated by QCD. We find $δR_{τ/π}=(0.18\pm 0.57 )\%$ and $δR_{τ/K}=(0.97\pm 0.58 )\%$, where the uncertainties are induced fundamentally by the counterterms. We test the lepton universality, obtaining $\left|g_τ/g_μ\right|_π=0.9964\pm 0.0038$ and $\left|g_τ/g_μ\right|_K=0.9857\pm 0.0078$, and analyze the CKM unitarity, getting results at $2.1σ$ and $1.5σ$ from unitarity via $|V_{us}/V_{ud}|$ and $|V_{us}|$, respectively. We also update the search for non-standard interactions in $τ$ decays. As a by-product, we report the theoretical radiative corrections to the $τ\to P ν_τ[γ]$ decay rates: $δ_{τπ} = -(0.24 \pm 0.56) \%$ and $δ_{τK} = -(0.15 \pm 0.57) \%$.} \maketitle

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arXiv2021

Radiative corrections to $τ\to π(K) ν_τ[γ]$: a reliable new physics test

The ratios $R_{τ/P}\equiv Γ(τ\to P ν_τ[γ]) / Γ(P \toμν_μ[γ])$ ($P=π, K$) provide sensitive tests of lepton universality $\left|g_τ/g_μ\right|=1$ and are a useful tool for new physics searches. The radiative corrections to $R_{τ/P}$ are computed following a large-$N_C$ expansion to deal with hadronic effects: Chiral Perturbation Theory is enlarged by including the lightest multiplets of spin-one heavy states such that the relevant Green functions are well-behaved at high energies. We find $δR_{τ/π}=(0.18\pm 0.57 )\%$ and $δR_{τ/K}=(0.97\pm 0.58 )\%$, which imply $\left|g_τ/g_μ\right|_π=0.9964\pm 0.0038$ and $\left|g_τ/g_μ\right|_K=0.9857\pm 0.0078$, compatible with and at $1.8σ$ of lepton universality, respectively. We test unitarity and bind non-standard effective interactions with the $τ\to P ν_τ[γ]$ decays.

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arXiv2017

$τ^-\toη^{(\prime)}π^-ν_τγ$ decays as backgrounds in the search for second class currents

Observation of $τ^-\toη^{(\prime)}π^-ν_τ$ decays at Belle-II would indicate either a manifestation of isospin symmetry breaking or genuine second class currents (SCC) effects. The corresponding radiative $τ^-\toη^{(\prime)}π^-ν_τγ$ decay channels are not suppressed by $G$-parity considerations and may represent a serious background in searches of SCC in the former. We compute the observables associated to these radiative decays using Resonance Chiral Lagrangians and conclude that vetoing photons with $E_γ>100$ MeV should get rid of this background in the Belle-II environment searching for the $τ^-\toηπ^-ν_τ$ channel. Similar considerations hold inconclusive for decays involving the $η^\prime$ given the theory uncertainties in the prediction of the $τ^-\toη^\primeπ^-ν_τ$ branching ratio. Still, additional kinematics-based cuts should be able to suppress this background in the $η^\prime$ case to a negligible level.

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arXiv2015

$L\,\to\, l\, l'\, l'\, ν_l\, ν_L$ in the SM and beyond

We study the $L\to l l' l' ν_l ν_L$ decays ($L=τ,μ$; $l,l'=μ,e$) in the Standard Model (SM) and in the effective field theory (EFT) description of the weak charged current at low energy, both for polarized and unpolarized $L$, keeping the $l,l'$ mass dependence. We clarify the discrepancy between two previous SM calculations of the decay rate improving their precision. The recent $3.5\,σ$ anomaly found in $τ\to e γ\barν_eν_τ$ could be checked using our precise prediction for the $τ\to e e e \barν_eν_τ$ decays, which shall be measured analyzing already existing data from the first generation B-factories. It is shown how measurements of the di-$l'$ mass distribution (with appropriate cuts) and $T$-asymmetries are able to reveal the corresponding lepton flavor violating (LFV) processes without neutrinos in the final state.

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