$\tau^-\to\pi^-\gamma\nu_\tau$ as a source of background on the Lepton Flavour Violating decay $\tau^-\to\mu^-\gamma$

Abstract

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 $\tau^-\to\mu^-\gamma$.