Search for a standard model Higgs boson decaying to two photons in CMS at the LHC

Abstract

In part I of this thesis, I perform a search for the Standard Model Higgs boson decaying into two photons using 5.08 fb-1 of data collected by the Compact Muon Solenoid (CMS) detector in 2011 at a center-of-mass energy √(s)= 7 TeV. The result of this search is interpreted as a local excess in the mass around 123 GeV/c2 with an significance of 3.3 standard deviation. This excess was later confirmed in the data in 2012, and was an important contribution to the CMS discovery paper published in 2012. This search makes use of the excellent energy resolution of the CMS crystal electromagnetic calorimeter (ECAL). The energy intercalibration of approximately 76,000 crystals is crucial to improve the resolution performance. I describe in detail a novel intercalibration method using neutral π0(η) → γγ decays. This method was the first one in CMS to reach 0.5% calibration precision in the central part of ECAL with pseudorapidity less than 1. This calibration improved the Higgs to two photons search sensitivity by about 30%, compared to the precalibration performed before the installation of ECAL in CMS detector. In part II of this thesis, I perform a search for an excited muon decaying into one muon and one photon using 36 pb-1 of data collected in 2010 at √(s)= 7 TeV. The result of this search indicates no evidence for excited muons. I report the first upper limits on single excited muon production cross section at this collision energy, and exclude a new region of the parameter space of compositeness scale and excited muon mass. Assume the compositness scale is the same as the excited muon mass, excited muons are excluded below 1090 GeV/c2 at the 95% confidence level, representing the most stringent limits, as of the date when the analysis was first published.