The Standard Model (SM) of elementary particle interactions is one of the best-tested theories in physics. The discovery of the Higgs boson was one of the big breakthroughs in physics of the past decade and the last missing piece of the SM. However, despite its success in describing fundamental forces, there is abundant evidence of phenomena not covered by this model, such as the existence of Dark Matter.
In the SM, lepton flavour (LF) is empirically conserved: electrons, muons and tau are produced or decay in association with their respective neutrinos, such that the total number in each family remains the same. The tau can thus decay into a tau neutrino, a muon, and a muon antineutrino; with the antiparticle counted as (-1). Meanwhile the decay τ → μγ , where a muon is produced alongside a photon, is forbidden.
The observation of neutrino oscillations, through which the three neutrinos can transform into each other, was the first direct observation of physics outside the SM, proving lepton flavor violation (LFV) is possible. However so far no such phenomenon has been observed with charged particles. Direct observation of charged LFV decays would constitute an enormous breakthrough in the field and an unambiguous, exciting proof of the existence of new physics.
This project aims to exploit the unique data set collected by the Belle II experiment at the SuperKEKB electron-positron collider in Tsukuba, Japan, to perform a high-precision measurement of LFV decays of tau. Belle II, which has started full operation in 2019, has so far observed over 900 million tau decays that can be exploited to search for LFV decays of tau leptons with world-leading precision.
This action produced a first important measurement of LFV using this data, set the basis for the measurement of a second LFV decay, and developed techniques broadly applicable to all tau research taking place at Belle II.
