Semileptonic τ\tau decays: powerful probes of non-standard charged current weak interactions

When looking for heavy ($\mathcal{O}$(few TeV)) New Physics, the most efficient way to benefit from both high and low-energy measurements simultaneously is the use of the Standard Model Effective Field Theory (SMEFT). In this talk I highlight the importance of semileptonic $\tau$ decays in complementing, in this respect, the traditional low-energy precision observables and high-energy measurements. This is yet another reason for considering hadronic tau decays as golden channels at Belle-II beyond the unquestionable interest of the CP violation anomaly in $\tau\to K_S\pi\nu_\tau$ decays, that I also discuss within the effective theory. A couple of new results for $\tau^-\to K^-\nu_\tau$ decays are also included.Comment: 7 pages. Contribution to the Proceedings of the International Workshop "e+e- Collisions From Phi to Psi 2019", held in Novosibirsk, Russia from February 25 to March 1, 2019. To be published in EPJ Web Of Conference

τ−→π−γντ\tau^-\to\pi^-\gamma\nu_\tau as a source of background on the Lepton Flavour Violating decay τ−→μ−γ\tau^-\to\mu^-\gamma

We calculate the decay of $\tau^-\to\pi^-\gamma\nu_\tau$ in the framework of Resonance Chiral Theory ($R\chi T$). By demanding the high energy constraints from QCD on the related form factors, we could predict the various physical observables of $\tau^-\to\pi^-\gamma\nu_\tau$ without any free parameter. Our results show that for a realistic cut on the photon energy (around $100$ MeV) this mode gives a branching ratio of roughly $0.1$ that should have already been detected at the heavy-flavour factories. Another interesting subject we have studied based on our calculation of the decay $\tau^-\to\pi^-\gamma\nu_\tau$ is the experimental background estimation of the lepton flavour violation process $\tau^- \to \mu^- \gamma$. We point out that although the description of radiation that PHOTOS provides -which has been used by BaBar and Belle collaborations to estimate this source of background- is in excellent agreement with the theoretical expectations in the low energy region in $\tau^-\to\pi^-\gamma\nu_\tau$ decay, this is not the case in the high energy region, precisely where it is easier that this decay mimics the process $\tau^-\to\mu^-\gamma$.Comment: 6 pages, 7 figures. Contribution to the Proceedings of the TAU10 Conference held in Manchester, 13-17 September 2010. To be published in Nucl. Phys. Proc. Supp

A dispersive analysis of the pion vector form factor and τ−→K−KSντ\tau^{-}\to K^{-}K_{S}\nu_{\tau} decay

We explore the potential of a combined analysis of the decays $\tau^{-}\to\pi^{-}\pi^{0}\nu_{\tau}$ and $\tau^{-}\to K^{-}K_{S}\nu_{\tau}$ in the determination of the $\rho(1450)$ and $\rho(1700)$ resonance properties in the frame of resonance Chiral Theory supplemented by dispersion relations. On the one hand, we take advantage of the very precise data on the modulus squared of the pion vector form factor $|F_{V}^{\pi}|^{2}$ obtained by Belle to carry out a very dedicated analysis of the region where these resonances come up into play. Our study provides an improved treatment of the systematic theoretical errors and, as a most important result, we conclude that they dominate over the fit uncertainties in the determination of the $\rho(1450)$ and $\rho(1700)$ pole parameters and tend to be larger than in other determinations quoted in the literature where these errors were ignored or underestimated. The results of our analysis are summarized in numerical tables for the form factor modulus and phase, including both statistical and systematic errors, that can be found as ancillary material of this paper. As a byproduct, we also determine the low-energy observables of the pion vector form factor and the $\rho$-pole position. On the other hand, we benefit from the recent experimental data for the transition $\tau^{-}\to K^{-}K_{S}\nu_{\tau}$ released by BaBar to perform a first analysis of its decay spectrum and discuss the role of these resonances in this decay. We point out that higher-quality data on the $K^{-}K_{S}$ decay channel will allow compete with the $|F_{V}^{\pi}|^{2}$ ones and improve the determination of the $\rho(1450)$ and $\rho(1700)$ resonance parameters as a result of a combined analysis. We hope our study to be of interest for present and future experimental analysis of these decays.Comment: 35 pages, 8 figures, 2 ancillary file

Precise determination of the eta_c mass and width in the radiative J/Psi -> eta_c gamma decay

We present an effective field theory based extraction of the eta_c mass and width from a recent measurement by CLEO of the photon line shape in J/Psi -> eta_c gamma decay.Comment: 3 pages, 3 figures, contribution to 'Quark Confinement and the Hadron Spectrum IX', Univ. Complutense de Madrid, August 30th-September 3rd, 201

Consistent high-energy constraints in the anomalous QCD sector

The anomalous Green function and related form-factors (pi^0 to gamma^* gamma^* and tau^- to X^- nu_tau vector form-factors, with X^-=(KKpi)^-, phi^- gamma, (phi V)^-) are analyzed in this letter in the large-N_C limit. Within the single (vector and pseudoscalar) resonance approximation and the context of Resonance Chiral Theory, we show that all these observables over-determine in a consistent way a unique set of compatible high-energy constraints for the resonance couplings. This result is in agreement with analogous relations found in the even intrinsic-parity sector of QCD like, e.g., F_V^2 = 3 F^2. The antisymmetric tensor formalism is considered for the spin-one resonance fields. Finally, we have also worked out and provide here the relation between the two bases of odd intrinsic-parity Lagrangian operators commonly employed in the literature.Comment: 11 pages, no figures. Comparison with the phenomenology extended. Matches version to be published in PL