Hadronic recoil in the W boson production at LHC for a W mass measurement with the CMS experiment

In the first chapter of this work, an overall picture of the theoretical basis is presented. Starting from the foundations of the Standard Model, Higgs mechanism and electroweak symmetry breaking are introduced, focusing on their role of providing SM gauge boson with masses. The important facts of electroweak precision test are also introduced in the last part of the first chapter. After an overview of the Large Hadron Collider (LHC), which is currently operating at CERN, the second part of this work describes the Compact Muon Solenoid (CMS) experiment, aimed to explore in depth particle physics up to the TeV scale: the main features of the subdetectors are briefly described, together with the reconstruction algorithms; focus has been put mostly on those features of interest for W mass physics. The third chapter is devoted to discuss the past and the on going efforts for the W boson mass measurement. The original work developed during the thesis is fully discussed in chapters four, five and six. Two are the main objectives: to deepen the knowledge of the variables used in the W mass measurement, with particular attention on the event-by-event experimental estimator of the boson transverse momentum; to define and calibrate an experimental definition of the recoiling system to the W, suitable for the real measurement process at the CMS experiment. Transverse momentum of the recoil and of the boson are two faces of the same coin: both are crucial in the extraction of the mass value. Events in which the W boson decays into a muon and a neutrino are considered. The W mass is extracted from the distributions of the modulus of the lepton trans- verse momentum and of the transverse mass (MT ), a scalar quantity function of the lepton momentum and the recoil, which is the vectorial sum of the momenta of all reconstructed particles excluding the lepton. With the purpose to maximize the performance in terms of systematic uncertainty on the final measurement, a new experimental definition of the recoil, based on machine learning algorithms, is discussed. As support to the remarkable impact of my work, there are reported cross checks and performances of the new definition, in terms of resolution and uncertainty improvement for a W mass measurement. In the last chapter, recoil-related systematic uncertainties on the W mass measurement are presented. Furthermore, it is reported a new method, based on multi-dimensional morphing, used to calibrated the Monte Carlo simulation using collision data. The systematic uncertainties of the W mass measurement before and after this calibration are studied. Finally, conclusions summarize the main results, underlining the importance of the work, and suggesting possible future developments.