ETD system

Electronic theses and dissertations repository


Tesi etd-12012014-172925

Thesis type
Tesi di dottorato di ricerca
Search for Lepton Flavor Violation in the MEG Experiment and its Upgrade
Settore scientifico disciplinare
Corso di studi
tutor Dott. Baldini, Alessandro
Parole chiave
  • rare decay
  • Muon
  • MEG
  • Lepton Flavor Violation
  • Xenon
Data inizio appello
Riassunto analitico
The Standard Model is an incredibly successful theory that encompasses the electromagnetic, weak and strong interactions of elementary particles and describes how they shape our world at the most basic level. It is supported by a large breadth of experimental evidence and boasts many successful predictions, culminating with the recent observation of the Higgs boson (or, at the very least, a Higgs-like particle) at the LHC.
The search for rare processes beyond the scope of the Standard Model has become an increasingly important element of experimental particle physics. While the recent evidence of neutrino oscillations can be assimilated into the model with little trouble, all SM extensions predict Lepton Flavor Violating phenomena also occurring in the charged sector at high branching ratios. Observation of such extremely rare processes would be irrefutable proof of new physics.
One of these pursuits, the search for the rare lepton flavor violating μ → eγ decay, is being undertaken by the MEG experiment at the Paul Scherrer Institut (PSI, Switzer- land), in a collaboration of physicists from Italy, Japan, Switzerland, USA and Russia. The current published results, based on the analysis of the first half of the collected data, provide the best upper limit for the μ → eγ branching ratio: B = 5.7 × 10−13 at 90% CL.
A second phase of the experiment (MEG II) has been studied to provide a substan- tial increase of sensitivity, down to ∼ 5 × 10^−14. The construction of a new positron spectrometer, in which the Italian contribution features prominently, is underway, along with a new timing counter and a substantial redesign of the calorimeter.
This thesis focuses on the study of the properties of the new drift chamber and of the liquid xenon calorimeter.
In the first part, the theory behind the Standard Model and its extensions is summarized, with a focus on lepton flavor violation. The μ → eγ decay kinematics are then discussed. In the second part the MEG I experiment is described and the analysis of the latest
published results are presented. The third part discusses the MEG II upgrade scheme and objectives and contains
the core of the thesis. The drift chamber prototype studies taking place within the scope of the R&D activities in Pisa are presented, followed by an in-depth investigation of the photon detector behavior showing the limits of its achievable energy resolution.
In the drift chamber studies we concern ourselves with the realization of small prototypes for aging and single hit resolution measurements, as well as the wiring and testing of the first full scale single cell prototype, showing the feasibility of a 2 m long stereo drift chamber with variable cell size and the possibility of using a double readout to achieve single cell longitudinal resolutions on the scale of 10 cm. The gain change along the cell length is measured and compared to the simulations and a method for measuring wire tension based on acustic excitation is presented.
In the photon detector investigation we attempt to understand why the energy resolution of the MEG I calorimeter, while excellent, is still lower than the predicted value. A detailed study of calibration data is presented, leading to the development of a more accurate Monte Carlo simulation and an improved knowledge of the optical parameters of the detector. Evaluation of PMT quantum efficiency is discussed in this context, and the use of data acquired in gas phase xenon is proposed for MEG II as opposed to the liquid calibration scheme currently used. Development of a new reconstruction algorithm is attempted, with some promising results which still leave some questions open.