InterLeptons News & Co

Leptonic tau decays preserve lepton flavour universality, after all

The standard model of particle physics is a theoretical framework that describes fundamental interactions and constituents of matter. Although very successful in predicting phenomena, in recent years, experiments worldwide have reported tensions arising from anomalies observed in the decays of B mesons and in the properties of leptons. Although new refined analyses didn't confirm some of these tensions, many still remain and have consolidated, even when more data were added.

These tensions point towards new interactions which might treat, unlike the SM, leptons of different generations or flavours differently, are said to violate lepton flavour universality, and might also be the link between ordinary and dark matter (DM). It is, therefore, mandatory to perform new tests that can confirm, constrain, or eventually rule out any new physics model that might be hiding behind anomalous experimental results.
Electrons, muons, taus, and their respective neutrino species are all leptons and comprise the so-called leptonic sector of the standard model.
To ascertain whether lepton flavour universality is violated in leptonic processes, we have performed a precision test using tau lepton decays, with all the data collected by the Belle II detector, located at the SuperKEKB collider in Tsukuba, Japan.

Specifically, we studied and compared the decay of the taus to the other leptons, obtaining the world's most precise determination from a single experiment of lepton flavour universality parameters from the decays of tau and observing no departure from the standard model predictions.
The result is per se very important, not only because it shows the capability of the Belle II experiment to perform precision measurements, where understanding of the detector and of its sub-components is of paramount importance, but because it sheds light on fundamental properties of the leptonic sector of the standard model. If lepton flavour universality is violated, when comparing the couplings of the tau decaying to muon and neutrinos or to electron and neutrinos, one would observe a departure from unity; we obtained |gµ/ge|= 0.9974 +/- 0.0019, in agreement with unity at the level of 1.4 sigma.
In addition, when we combine our results with those obtained by previous experiments, a striking agreement with the standard model prediction emerges, providing an even stronger constrain |gµ/ge|= 1.0005 +/- 0.0013, ruling out the possibility of lepton flavour universality violation in tau decays. This rules out a variety of models that have suggested the existence of new fundamental interactions beyond the standard model. Researchers of the Institute of High Energy Physics of the Austrian Academy of Sciences played a major leading role in this measurement, thanks to the funding received by the European Research Council under the starting grant project called InterLeptons, whose team members performed the measurement.

The results were presented at the Tau 2023 conference, which took place In Louisville in December 04-08 2023:
Paul Feichtinger Presentation
Gianluca Inguglia Presentation