• MEG Experiment

The Standard Model (SM) of electroweak interactions assumes complete conservation of the
lepton flavor. Even introducing corrections due to neutrinos masses, the SM predicts an unmeasurable branching ratio for this decay (B< 10−55). Supersymmetric grand unification models
(SUSY-GUT), in which the SM is incorporated as a low energy subgroup, provide estimates of
μ -> e + g decay just below the current experimental limit (< 1.2 × 10−11): searching for lepton flavor violation in this channel will therefore lead to the first observation of physics beyond the
standard model, or set strong constraints on those theories.
The MEG experiment at PSI searches for the μ -> e + g decay with a sensitivity around 10−13,
thus improving the present best experimental limit of roughly two orders of magnitude. The
experiment is in operation since 2007, while physics data taking started officially in 2008 and
will last until 2012.
The first chapter of this thesis begins by showing a short summary of theoretical motivations
supporting the search of μ -> e + g
(principles of SM and SUSY-GUT models). After a historical introduction to the μ-> e + g
decay searches, the event signature, backgrounds and experimentalsensitivity are discussed.
In the second chapter we describe the MEG apparatus: the beam line setup, the magnetic
spectrometer, the data acquisition system and the analysis software.
In the third, fourth and fifth chapters we discuss in detail the innovative liquid xenon photon detector, its calibration methods and reconstruction algorithms.
The performances of the calorimeter and the other MEG detectors during 2008 MEG run are presented in chapters 6 and 7.
Chapter 8 shows the physics analysis procedure and the final result from 2008 data.
Finally in chapter 9 a short look at preliminary 2009 MEG run results are shown.