The international MEG collaboration (Italy, Japan, Switzerland, USA, Russia) searches for the decay \( \mu^+ \to e^+ \gamma \) at the Paul Scherrer Institut in Switzerland. This decay, practically forbidden in the Standard Model of particle physics, is foreseen in many extensions of this model at an experimentally accessible level. The observation of \( \mu^+ \to e^+ \gamma \) would be a clear signal of New Physics while stringents constraints allow to set limits on the parameters space of new theoretical models or even to exclude some of them. The pictures below show the historical progress of the charged Lepton Flavour Violation search highlighting the region allowed by New Physics models. As it can be seen, the MEG II result set rather stringent limits on the parameters space.
Since it is a very rare decay in the Standard Model, a very intense muon beam is necessary. At PSI the most intense continuous muon beam in the world is available, with up to 10\(^8\) muons per second, so the MEG experiment places itself at the intensity frontier of New Physics searches, complementary to the energy frontier (LHC). The MEG final result, based on the analysis of the data taken in 2009-2013 is: BR(\( \mu^+ \to e^+ \gamma \)) < 4.2 x 10\(^{-13}\) @ 90% C.L. (EPJC76,8,434 (2016)). The MEG experiment has been upgraded to the MEG II experiment which is in the data-taking phase since 2021. In a special seminar at the Paul Scherrer Institut, the MEG II Collaboration presented on Oct. 20th 2023 its first results about a new search for this process, based on data collected in 2021, which were submitted to European Journal of Physics C (link). By combining them with the previous MEG result, an improved upper limit of BR(\( \mu^+ \to e^+ \gamma \)) < 3.1 x 10\(^{-13}\) @ 90% C.L. has been obtained, which is the most stringet limit on this process to date.
The experiment is still taking data and a twenty-fold increase in statistics is foreseen by 2026, with the goal to improve the sensitivity to the decay probability by one order of magnitude with respect to MEG. Below, a scheme of the MEG-II detector is shown. The muon beam is stopped by a thin plastic target and decays at rest: a photon and a positron are searched, emitted simultaneously, back to back, with an energy equal to half of the muon mass. The positron is reconstructed in a spectrometer (drift chamber, CDCH, in a magnetic field, COBRA magnet) and then its time is measured by scintillator detectors (timing counters, TC). The photon is detected using a very large C-shaped liquid Xenon calorimeter.
