Calibrazioni e Monitoraggio dell'Esperimento MEG

\documentclass[a4paper,11pt]{article} \usepackage[latin1]{inputenc} \usepackage{graphicx} \usepackage[italian]{babel} \usepackage[centertags]{amsmath} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsthm} \usepackage{newlfont} \usepackage[usenames,dvipsnames]{color} \title {Calibrazioni e Monitoraggio dell'Esperimento MEG} \author{ Francesco Tenchini } \begin{document} %\maketitle \begin{center} \makebox[\textwidth]{ \raisebox{5cm}{ \begin{minipage}[t]{50truecm}\begin{center}{ \hspace*{0.2cm}{\Large \textbf{Universit\`a di Pisa}}\\ \vspace{0.3cm} \hspace*{0.2cm}{\textbf{Facolt\`a di Scienze Matematiche Fisiche e Naturali}}\\ \vspace{0.2cm} \hspace*{0.2cm}{ \textbf{Corso di Laurea Specialistica in Scienze Fisiche}}\\ \vspace{0.2cm} \hspace*{0.2cm}{\small{Anno Accademico 2009/2010}}\\ \vspace{0.4cm} \hspace*{0.2cm}{ Riassunto dell'Elaborato Finale}\\ \vspace{0.4cm} \hspace*{0.2cm}{\large {\bf Calibrazioni e Monitoraggio dell'Esperimento MEG}}} \end{center} \end{minipage}}} \end{center} \noindent\hspace*{0.2cm}{Candidato: \bf Francesco Tenchini} \vspace{0.2cm} \noindent\hspace*{0.2cm}{Relatore: \bf Prof. Alessandro Baldini} \vspace{0.7cm} \noindent L'esperimento MEG si propone di misurare il rapporto di decadimento $BR(\mu\rightarrow\ e^+\gamma)=\frac{\mu^{+}\rightarrow e^{+}\gamma}{\mu^{+}\rightarrow TOT}$ con una sensibilit\`a pari a $BR(\mu\rightarrow\ e+\gamma)\approx10^{-13}$, due ordini di grandezza superiore ai risultati ottenuti dal precedente esperimento MEGA. Il Modello Standard (MS), per il quale la massa dei neutrini \`e nulla, prevede una completa conservazione del numero leptonico, quindi $BR(\mu\rightarrow\ e^+ \gamma) = 0$. I recenti risultati sulle oscillazioni di neutrini mostrano che esiste almeno un autostato di massa di neutrino non nullo. Tuttavia il Modello Standard, opportunamente modificato, prevede un valore di $BR(\mu\rightarrow\ e^+\gamma)$ attualmente non misurabile ($BR(\mu\rightarrow\ e^+ \gamma) <10^{-50}$). I processi di violazione del sapore leptonico (LFV) sono invece previsti in numerose teorie di unificazione supersimmetriche con valori del rapporto di decadimento molto pi\`u elevati, attorno a $10^{-12}\div10^{-14}$. Osservare il processo $\mu^+\rightarrow e^+ \gamma$ sarebbe quindi un'evidenza importante di fisica oltre il Modello Standard. Una mancata osservazione risulterebbe comunque utile ad imporre limiti pi\`u stringenti alle nuove teorie. Il decadimento del muone viene studiato in MEG a riposo. La segnatura associata al processo $\mu^+\rightarrow e^+\gamma$ \`e data dall'emissione in coincidenza di un gamma ed un positrone, con un angolo relativo di $180^o$ ed un'energia di $E_{\gamma}=E_{e^+}=m_\mu/2=52.8$ MeV. Il fascio di muoni utilizzato per l'esperimento \`e fornito dal Paul Scherrer Institut (PSI, Villigen, Svizzera) ed \`e il pi\`u intenso fascio continuo di muoni attualmente esistente ($\sim10^8\mu/s$). La misura del quadri-impulso del $\gamma$ viene eseguita usando il calorimetro a Xenon liquido; la misura del quadri-impulso del positrone viene eseguita usando lo spettrometro COBRA, costituito da un sistema di camere a deriva ed un magnete superconduttore, ed un sistema di rapidi contatori plastici. A causa dell'elevata precisione richiesta dall'esperimento risulta fondamentale avere sotto controllo tutti i parametri dei rivelatori, in modo da minimizzare le incertezze sistematiche legate alla strumentazione. Per questo motivo \`e necessaria un'accurata calibrazione ed un preciso monitoraggio di tutto l'apparato. L'elaborato discute i diversi metodi di calibrazione e monitoraggio dell'esperimento, con particolare enfasi su quelli a cui il candidato ha contribuito durante il periodo di tesi. In particolare si descrive lo sviluppo del sistema di monitoraggio del calorimetro a Xenon mediante $\gamma$ da 9 MeV, ottenuti per cattura di neutroni termici nel Nickel. La caratteristica, unica di questo metodo, \`e quella di permettere il monitoraggio del calorimetro in presenza del fascio di muoni, usando un generatore impulsato di neutroni. Vengono descritti gli studi preliminari di Monte Carlo che hanno portato alla scelta della configurazione del sistema moderatore-nickel; il test di accettazione del generatore di neutroni utilizzato al PSI; e l'osservazione del segnale dei $\gamma$ da 9 MeV prodotti. \`E descritta l'installazione ed il primo test di un nuovo metodo di calibrazione sviluppato per una migliore comprensione dello spettrometro, utilizzato per eseguire la misura dell'impulso e della direzione del positrone. Il metodo si basa sulla diffusione elastica Mott di un fascio monocromatico di positroni su nuclei leggeri. Infine \`e descritto il montaggio di un sistema pneumatico per il movimento di una sorgente di Americio-Berillio (ulteriore metodo di calibrazione dell'esperimento) e la programmazione del sistema di controllo remoto. Le attivit\`a sopra riportate sono state svolte al PSI, durante una lunga permanenza del candidato presso il laboratorio. \end{document

Search for Lepton Flavor Violation in the MEG Experiment and its Upgrade

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

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