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Tesi etd-06202016-113900

Thesis type
Tesi di laurea magistrale
The new drift chamber of the MEG-II experiment, in search for lepton flavor violations
Corso di studi
relatore Prof. Baldini, Alessandro
relatore Prof. Cei, Fabrizio
Parole chiave
  • Lepton Flavor Violation
  • Particle Track Fitting
  • Detector Design and Construction
  • Pattern Recognition
  • High Energy Physics
  • Drift Chambers
  • Gaseous Detectors
  • Particle Tracking Detectors
  • MEG experiment
Data inizio appello
Riassunto analitico

The MEG experiment represents the state of the art in the search for the possible existence of the Charged Lepton Flavor Violating (CLFV) decay $\mu^+ \rightarrow e^+ \gamma$. This process is completely prohibited in the framework of the Minimal Standard Model (SM) of Particle Physics and practically forbidden also in the SM extensions including neutrino masses and oscillations, but is foreseen in many other extensions. The predicted Branching Ratios (BR) are not far from the current experimental sensitivities and the discovery of such a decay would be an indisputable proof of the existence of New Physics Beyond the Standard Model (BSM).

The MEG collaboration has been presenting the final results of the experiment, exploiting the full statistics collected during the 2009-2013 data taking period at the Paul Scherrer Institut (PSI), which provides the most intense continuous low-energy muon beam in the world, up to $10^8$ $\mu^+$/s. The analysis of the complete data set, corresponding to a total number of $\approx 7.5 \times 10^{14}$ muons stopped in the target at $3 \times 10^{7}$ $\mu^+$/s, has resulted in the new best upper limit on the BR$(\mu^+ \rightarrow e^+ \gamma) \leq 4.2 \times 10^{-13}$ at $90\%$ Confidence Level (C.L.). This value is a factor of $\approx 30$ better than the previously published limit set by the MEGA experiment and lowers the former MEG limit on the BR$(\mu^+ \rightarrow e^+ \gamma) \leq 5.7 \times 10^{-13}$ at $90\%$ C.L. with the analysis of 2009-2011 data, corresponding to $\approx 3.6 \times 10^{14}$ stopped muons (half statistics).

Although the MEG experiment detains the most stringent constraint on the $\mu^+ \rightarrow e^+ \gamma$ BR to date, it has reached its ultimate level of sensitivity, limited by the resolutions on the measurement of the kinematic variables of the two decay products. Therefore an upgrade (MEG-II) of the current experimental apparatus has been approved and is presently under construction. It aims at reaching a sensitivity enhancement of at least one order of magnitude compared to the final MEG results, in three years of data taking, by improving the detector figures of merit and the muon stopping rate.

The items of the thesis are summarized in the following. After a brief theoretical introduction to Flavor Physics, with particular emphasis on the Lepton Flavor Violation (LFV) in the muon sector, an overview of the MEG experiment is presented. Starting from the experimental apparatus, the final MEG data analysis is described, ending with the presentation of the current best upper limit on the $\mu^+ \rightarrow e^+ \gamma$ BR. Afterwards, a detailed description of the MEG detectors upgrade is provided. It consists in improving the performance of the Liquid Xenon (LXe) calorimeter by enhancing its geometric acceptance and granularity through new photo-detectors (SiPM) and in developing and installing a new drift chamber, as well as a new positrons time detector (Timing Counter) featuring a scintillating multi-tiles configuration. The new drift chamber is designed to overcome the limitations of the MEG $e^+$ tracker and guarantee the proper operation at high rates with big detector stability. In fact, during the MEG-II data taking the muon rate on the stopping target will be more than doubled, up to $7 \times 10^7$ Hz. This also requires the design of a new trigger and DAQ electronics, capable of handling the increased number of readout channels, while maintaining a high bandwidth.

A whole Chapter is dedicated to the main theme of the thesis: the MEG-II CYLindrical Drift CHamber (CYLDCH). It features a unique volume covering the whole azimuthal angle around the muon stopping target and a total length of $\approx 2$ meters which improves the geometric acceptance for signal positrons and allows the use of a new tracking procedure. It is designed to follow the $e^+$ tracks up to Timing Counter and to match the information provided by the two individual detectors in order to determine the positrons kinematic variables more accurately and minimize the background sources. The high-granularity of the new MEG-II CYLDCH is ensured by ten layers of drift cells defined by 13056 wires arranged in a stereo configuration and filled with a low-mass 85:15 Helium:Isobutane gas mixture. This minimizes the material budget in the sensitive volume, thus reducing the total radiation length down to $1.5 \times 10^{-3}$ X$_0$ and keeping the Multiple Coulomb Scattering (MCS) contribution to a minimum. Particular emphasis is put on the drift chamber geometry, focusing on the design and detector construction, which is a direct responsability of the MEG-II Pisa group. The CYLDCH assembly is performed in the San Piero a Grado (Pisa) facility of the Istituto Nazionale di Fisica Nucleare (INFN).

In the next Chapter a description of a new Pattern Recognition (PR) algorithm for the new MEG-II CYLDCH is reported with the analysis of its performance in simulated experimental conditions near to that of the MEG-II data taking. Finally, in the following Chapter, the Track Fitting toolkit which will be used during the second phase of the MEG experiment is described, together with a study of the momentum and angular resolutions expected for MEG-II. At the end, possible further improvements and a brief review of the MEG-II experiment potentialities and expected time schedule will be presented in the conclusions.