Physics with the tau lepton

In the standard model of particle physics, the fundamental building blocks of matter are divided into six quarks and six leptons (and their respective antiparticles). The tau lepton is the heaviest of the six leptons and, the only lepton that can also decay into quarks. Decays of tau leptons into quarks or lighter leptons can be used to perform precision tests of the standard model, such as tests of lepton flavour universality, also called LFU.

The standard model, in fact, accidentally predicts the couplings of the weak bosons to different lepton generations (the electron, the muon, the tau and their respective neutrino species) to be the same. Since within the standard model, leptonic decays of tau leptons are charged weak currents mediated by the W boson, they are supposed to happen (nearly, due to mass effects) with the same probability.

In the Belle II experiment, tau leptons are copiously produced in particle/anti-particle pairs, but their decays contain at least one neutrino, two in the case of leptonic decays, and these do not interact with the Belle II detector, making tau leptons and their decays very difficult to identify. 

I have recently coordinated the world's most precise test of lepton flavour universality in leptonic tau decays at the Belle II experiment, a very complex measurement in which, using a neural network and the so-called 1x1 (one-by-one) tau decay topology, in which one tau decays to one charged particle, and the other to a lepton plus neutrino. In this measurement we compared the decay rates of the tau lepton to muon and neutrinos with the decay to electron and neutrinos.

At the current level of precision, this measurement strikingly confirms the standard model predictions but non-conclusively. New measurements, which I am also coordinating at the Belle II experiment and that use a different tau decay topology, will tell. Lepton flavour universality can also be tested in hadronic decays of the tau leptons, and this measurement is now being prepared.

One of my phenomenology studies (a review) investigating lepton flavour universality in tau and other decays can be found here.