MEG, an international collaboration at Paul Scherrer Insitute in Switzerland, is conducting a search for the lepton-flavor violating muon decay to electron and gamma.

MEG appratus by Prof. Saoshi Mihara


It is considered in the supersymmetric grand unification theory of the particle physics that three forces acting over elementary particles are unified at very high energy and that quarks and leptons, consitutent of matter are also unified. As a result the lepton-flavor violation, mixinig of chaged leptons among different generations, is expected to occur as well even at the low energy scale. The MEG experiment is serching for one of such phenommena, the muon decay to an electron and a photon (μ->eγ decay).

The μ->eγ decay is forbidden in the framework of the standard model though flavor mixiing has been observed in neutrinos, which are also categorized as leptons. Therefore there is a possiblity that the μ->eγ decay occurs. However the expected probablity of the decay of μ->eγ in the standard model is much smaller than the limit achievable by current available technology. On the other hand the probablity can be enchanced if the supersymmetry exists owinig to contributions of other particles than starndard model partciles such as neutralinos. The MEG experiment measures the μ->eγ decay with a sensitivity of 10-13, providing information of the existence of the supersymmetry.

The proton cyclotron at Paul Scherrer Insitute has a beam power of 2MW, providing the most intence DC muon beam in the world. The MEG experiment stops 30 milion muons at every second on a target and mesures photons and positrons (positive electrons) using a liquid xenon photon detector and positron spectrometer with gradient magnetic field. The experiment started data acquision in 2009 and continues for a few year to reach to the experiment sensitivity of 1 over ten trillion (1013). The MEG group is composed of about 70 researchers from over the world; KEK and the university of Tokyo groups are mainly contributin to the exepriment from Japan.

muon: 2nd generation lepton with chage in three generations. Electron is a lepton belonging to 1st generation. In the 3rd generation there is a taw lepton. There are corresponding neutrinos in each generation.

flavour mixing: In the framwork of the starndard model, the lepton number (flavor) is convserved in any reaction. In the normal muon decay (μ->eν(anit)ν) the flavor is conserved owing to a neutrino and an anti-neutroino in the final start.

Flavor mixing means a phenomenum such as violating this conservation law.

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Related Research Group

MEG Japan Collaboration

Related Facilities

Paul Scherrer Institute