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The MEG experiment has been looking for the decay of the muon in the ultra rare lepton
flavour violating channel μ → eγ since 2009 and will conclude the data taking at the end of
summer 2013. This decay channel is forbidden by the minimal Standard Model of particle
interactions, which preserves lepton flavour. Extensions of the model predict μ → eγ, some
of them with a branching ratio close to the MEG sensitivity. The MEG collaboration recently
published the result of the analysis of the data collected in the years 2009-2011 setting an upper
limit to the branching ratio equal to 5.7 · 10−13 at 90% confidence level.
The MEG analysis consists in the reconstruction of the momentum of photons and positrons
coming from the muon decay at rest. The photons are detected by a scintillation electromagnetic
calorimeter which uses liquid xenon as a scintillating medium, while the positrons trajectories
are reconstructed through a set of drift chambers in a non homogeneous magnetic field and a
set of scintillation bar placed at the end of the tracker for the timing measurement.
The MEG upgrade will preserve the beam and target used by MEG while the calorimeter,
the timing counter and the tracking system will be subject to modification, especially the drift
chamber will have a totally new design. It will be a unique volume filled with a mixture of helium
and isobutane with more than 2000 wires almost two meters long. The required resolution of
the point of closest approach to the wire for the new drift chamber is of about 100 μm so that
a more sensitive tracking system is useful to test the effective performance of the chamber and
of the reconstruction algorithm. For this purpose a cosmic ray telescope has been set up.
The sensitive part of the telescope is made by four spare modules of the BaBar silicon vertex
tracker, the resolution of the hits on each plane depend on the pitch of the strips, 100 μm along
one direction and 50 μm along the other. The resolution at the height of the detector under
test, given by the errors on the fit parameters, is around 20 μm low enough to characterize the
MEG drift chamber prototypes.
In this thesis are described the steps needed to set the telescope in operating condition. The
programming of the FPGA and the microprocessor used to manage the communication (com-
mand and data transfer) between the PC and the SVT frontend chips. The noise analysis and
the investigation of the environmental conditions that affect the performance of the detectors.
Finally a description of the calibration, data acquisition, and data analysis software.
This cosmic ray telescope has a trigger rate around 0.3 Hertz, collecting more than 1500
reconstructed tracks per day, so that a sufficient statistic will be accumulated in a few day of
data acquisition.
The telescope will remain as a facility of the INFN section of Pisa available to test in house
any moderately compact detector.