The possible existence of the \mu \rightarrow {e} \gammaμ→eγ
decay predicted by many new physics scenarios is investigated by
stopping positive muons in a very thin target and measuring emitted
photons and positrons with the best possible resolutions. Photons are
measured by a 2.7 ton ultra pure liquid xenon detector while positron
trajectories are measured in a specially designed gradient magnetic
field by low-mass drift chambers and precisely timed by scintillation
counters. A first phase of the experiment (MEG) ended in 2016, and
excluded the existence of the decay with branching ratios larger than
4.2 \times 10^{-13}4.2×10−13
(90% C.L.). This provides approximately 30~times stronger constraints on
a variety of new physics models than previous experiments. In the second
phase (MEG II), most of the detectors have been upgraded by adopting
up-to-date technologies to improve the search sensitivity by another
order of magnitude down to \mathcal{O}(10^{-14})𝒪(10−14).
MEG II will perform a search for physics beyond the Standard Model
complementary to high energy collider experiments with compatible or
even higher sensitivity.
MEG: Muon to Electron and Gamma