• violazione numero leptonico meg charged lepton fla

The MEG experiment, located at the Paul Scherrer Institut near Zurich (Switzerland),
searches for the lepton flavour violating decay
μ+ → e+ + γ
(1)
The MEG collaboration has recently published a new upper limit on the branching ratio
BR(μ+ → e+ + γ) < 5.7 · 10−13 (90% C.L.)
The Standard Model does not predict violation of the flavour of charged-leptons, that
means the decay (1) is forbidden by this theory, unless the recently discovered neu-
trino oscillations are incorporated in the Standard Model: in this case, an experimentally
unattainable branching ratio ∼ 10−55 is expected. Some Super-Symmetric extensions to
the Standard Model predict the decay (1) in the range 10−11 − 10−14 , making the search
worthwhile with a new upgraded apparatus.
The MEG II experiment aims to reach a ten times lower limit on the branching ratio
BR(μ+ → e+ + γ) < 6.0 · 10−14
in 3 years of data taking; studies on the upgrade have already started and will continue
until 2015, then a first test run is expected to be done at the end of 2015.
In order to achieve this result, an improvement of the detectors resolutions is mandatory:
the two main detectors involved in the upgrade process are the magnetic spectrometer
and the LXe calorimeter which, respectively, measure momentum and emission point of
the positron and the photon energy, as well as the position of the photon conversion.
The new improved drift chamber is designed with more than a thousand wires in a stereo
configuration, immersed in a helium-isobutane gas mixture as active medium. The cham-
ber allows for a 3-dimensional reconstruction of the positron trajectory.
An increase by a factor 3 of the MEG muon-stopping rate is also foreseen in the MEG II
upgrade, but it will affect the drift chamber performances during the detector operation.
Fragmentation of the gas molecules in the avalanches causes formation of deposits along
the wires, with a reduction of the gas gain or, in the extreme case, continous discharge of
the chamber: this phenomenon is called ‘ageing’. The collected charge density on wires is
of the order of a few tenth of Coulombs per centimetre during the whole experiment life-
time, so a study of how much the chamber performances change due to ageing is crucial.
This thesis analyzes different aspects that must be taken into account in the construction
of the final MEG II drift chamber, through realization and characterization of several
different prototypes at the Pisa INFN laboratories.
The ageing effects have been studied in a series of prototypes, based on a single drift cell
configuration, some of them built using a preliminary set of wires and one with the final
chamber wires; also the insertion of contaminant material inside the gas volume has been
studied.
Inspections of the aged wires with the SEM (Scanning Electron Microscope) are presentedalong with an EDX (Energy Dispersive X-ray spectroscopy) analysis.
The main feature of a drift chamber is the capability of tracking a charged particle po-
sition by using drift time information, which is the time between the positron crossing
and the avalanche formation on the anode wire: a basic configuration of three-cell ar-
rangement has been simulated through Monte Carlo techniques, then built and tested on
a prototype. Starting from three anode signals, a simple algorithm provides the track
parameters of a charged ionizing particle passing through the detector volume and the
single-hit resolution, defined as the uncertainty on the three impact parameters, is esti-
mated to be ∼ 100 μm.
In a stereo wires disposition, a drift cell mantains its shape but the transverse size changes,
in addition it is twisted along the z−axis direction: if the longitudinal cell extension is
∼ 2 m, the cell transverse size variation is non-negligible and gain variations effects arise.
Gain variations are studied in a 180 cm long single-cell prototype, built with the prelim-
inary set of wires: the experimental results are compared to Monte Carlo simulations.
Results obtained with these prototypes have been already implemented in the MEG II
drift chamber design: when the detector will be operative, they can be a valuable start-
ing point in achieving optimal features, both in terms of experimental resolutions and
functional stability.