MEASUREMENT OF MISSING TRANSVERSE MOMENTUM AND SEARCH FOR H->TAUTAU IN P-P COLLISIONS AT SQRT{S}=8 TEV WITH ATLAS.

Understanding the origin of the mass of elementary particles has been one of the main quests for high energy physics for the last decades. The theory of the mass generating mechanism was independently hypothesized by Brout, Englert and Higgs in 1964. It has been experimentally confirmed with the discovery of the particle predicted by this theory, the Higgs boson, announced by the ATLAS and CMS experiments on July 4, 2012. At the time of this announcement only Higgs couplings to bosons could directly be proven. Subsequent crucial tests of the theory require to prove that the Higgs boson also couples to fermions. The most promising channel for this search is the $H\to\tau\tau$ process that is discussed in this thesis. One of the aspects that makes the analysis of this channel challenging is the need of a proper evaluation of the missing transverse energy (etmiss) introduced by neutrinos in the final state. For this reason, a substantial part of this thesis is devoted to the etmiss reconstruction, calibration and to techniques for pile-up suppression. More specifically, this thesis reports my work in the last three years developed in the context of the ATLAS collaboration and of the Milano University group, following the full path from low-level detector signals, through etmiss reconstruction and calibration, to the $H\to\tau\tau$ physics analysis. The thesis is organized as follows. In Chapter 1, an introduction to the theoretical framework of the Standard Model is given together with a description of the mass mechanism and its phenomenology crucial for the analysis. In Chapter 2, the LHC, the ATLAS experiment, and the experimental conditions during the data taking in 2011 and 2012, which are characterized by a large amount of pile-up interactions introduced by the high luminosity reached with the LHC, are introduced. Particular attention is given to the ATLAS calorimeter system since its importance for the subjects dealt in this thesis. In Chapter 3, the algorithms for the etmiss reconstruction used in ATLAS are described. The etmiss performance, which has been extensively studied, is presented for several final state topologies, and the crucial aspects for the $H\to\tau\tau$ analysis are discussed. This chapter represents significant input from my original studies. In Chapter 4, the degradation of the etmiss performance due to pile-up is shown and the methods to mitigate these effects are described. I have devoted large part of my activity in the development of innovative methods for pile-up suppression using either tracks or calorimeter information or a combination of both, and to their implementation in the official collaboration software package. In Chapter 5, the cut-based analysis for the search of $H\to\tau\tau$ in the semileptonic final state is described in all its main aspects, from the selection and categorization of the events, the estimation of the backgrounds, the statistical analysis, to the final signal extraction, where I have been more involved. In particular, it is shown how the etmiss after pile-up suppression improves the analysis sensitivity. The same pile-up suppressed etmiss is also used in the ATLAS $H\to\tau\tau$ analysis employing multivariate techniques, which ultimately led to the observation of the Higgs boson decaying into two taus.