NEPAL

Individual lepton flavours (and total lepton number) are strictly conserved in the SM. The observation of neutrino oscillations constitute evidence of lepton flavour violation in the neutral lepton sector and suggests that the SM should be extended to account for non-massless neutrinos and their mixing. Additionally, the lepton flavour symmetries are accidental in the SM and there is no reason why physics beyond the SM (BSM) should conserve lepton flavour. Charged lepton flavour violating (LFV) decays have thus been searched for since several decades. The presence of neutrino mixing makes their branching fractions non zero within the SM, but there are still tens of orders of magnitude below the current experimental sensitivity (\(<\mathcal{O}(10^{-40})\)). The observation of LFV decays would therefore be an unquestionable proof of physics beyond the SM.

B LFV decays win have been studied by Babar, LHCb and Belle (see HFLAV report for complete status). While the most stringent limits are at the order of 10\(^{-8}\) or 10\(^{-9}\) for final states with electron and muons, decays into tau are much more experimentally challenging and the best limits are of the order of 10\(^{-5}\). The NEPAL team is focusing on decays such as \( B\to K^*\tau\ell\) and \( B\to \rho\tau\ell\), where \( \ell \) is an electron or a muon.