Lepton flavor violation in the muon sector: MEG II results and future horizons

by Lorenzo Ferrari Barusso (INFN Genova)

Thursday 21 May 2026, 14:00 → 15:00 Europe/Rome

Aula 602 (Genova)

In this journal club, I will discuss how rare muon decays can be used as a powerful window on physics beyond the Standard Model. The search for the decay $\mu^+ \to e^+ \gamma$ has a long and fascinating history. It was already discussed in the 1930s, when the nature of weak interactions was still far from clear. The fact that this decay was not observed in early experiments played an important role in shaping our understanding of leptons, eventually contributing to the idea that different lepton families exist and that lepton flavor is conserved in ordinary weak processes. Today, the same decay remains one of the cleanest and most powerful ways to search for physics beyond the Standard Model.

The MEG II experiment at the Paul Scherrer Institut in Switzerland focuses on searching for this very rare decay. In the Standard Model, the decay is so suppressed that it is essentially unobservable, but many theories extending the Standard Model predict much higher probabilities. Therefore, observing it would constitute a clear signal of new physics.

I will introduce the basic physics idea behind charged lepton flavor violation, emphasizing why muons are particularly useful particles for such studies. I will then describe the MEG II experiment, its main detector components, and the experimental challenge of identifying a rare two-body decay against a large background of ordinary muon decays.

The latest MEG II result, based on approximately one quarter of the expected final dataset, sets an upper limit

$$BR(\mu^+ \to e^+ \gamma) < 1.5 \times 10^{-13}$$

at 90% confidence level, improving the previous MEG limit by nearly a factor of three. I will discuss what this result implies and the prospects for the full dataset.

Particular attention will be given to the positron timing detector, developed with significant contributions from the INFN Genova and the University of Tokyo. This detector employs hundreds of small scintillating counters read out by thousands of silicon photomultipliers, achieving a timing resolution of approximately 30 picoseconds. Such precision is crucial for rejecting background and improving the sensitivity of the search.

Finally, I will briefly outline future perspectives at PSI, where more intense muon beams could enable even more sensitive searches for rare muon decays, and I will place these studies in the broader context of low-energy precision experiments, including searches for light or hidden particles, which complement high-energy collider approaches.