Sélection des électrons et recherche du boson de Higgs se désintégrant en paires de leptons tau avec l'expérience CMS au LHC

This thesis fits into the first operating years of the Large Hadron Collider. Thismonumental machine was built to explore the infinitesimal structure of matter at themulti-TeV scale. The LHC aimed primarily at searching for the Higgs boson, the discoveryof which would confirm the electroweak symmetry breaking model. This mechanism, whichprovides W and Z bosons with a mass, describes the transition from a unified electroweakinteraction to a weak interaction (short range) and an electromagnetic interaction (infiniterange). The LHC’s proton collisions, operated at a 50 ns period, are analysed by 4 largedetectors, including the Compact Muon Solenoid (CMS). This small period allows toobserve very rare phenomena, such as the Higgs boson production and decay, but it requiresa fast online selection of the interesting collisions : the trigger system. The computingresources available for the data’s storage and analysis set a limit to the trigger rate.Therefore the bandwidth, which is split into several physics signals, must be optimised.Firstly, I studied the electron trigger : electrons are a clear signature in the intensehadronic environment within the LHC and allow a high measurement accuracy, as well asa search for rare signals. Besides, they are part of the final states investigated by a largenumber of analyses (Higgs, electroweak, etc).From the first collisions in 2010, anomalous signals in the CMS electromagneticcalorimeter (ECAL) were a source of uncontrolled trigger rate increase. Indeed, theirproduction rate increased along with the collisions’ energy and intensity : they were likelyto saturate the bandwidth as early as 2011, crippling drastically the CMS physics performances. I optimised the anomalous signal rejection algorithm, while conserving anexcellent electron triggering efficiency, as regards the data collected in 2011. Moreover,the increasing intensity of the LHC collisions causes a loss of transparency in the ECALcrystals. The setting-up of weekly corrections to the ECAL trigger calibration helpedmake up for the inefficiency caused by this loss of transparency.Secondly, I contributed to the search for the Higgs boson decaying to 2 tau leptons. Sofar, this analysis proved to be the only possible method to check the coupling of the Higgsboson to leptons. The tau lepton decays either into lighter leptons (electron or muon),or into hadrons : hence the study of six final states. I focused on the semileptonic finalstates, in which the expected signal is the most statistically significant.The trigger algorithms dedicated to this analysis select a lepton and a hadronic tau,with high transverse momenta. However, this selection removes half of the signal, whichmotivated the elaboration of new algorithms selecting low momenta leptons, including acut on the missing transverse energy. This cut helps controlling the trigger rate and selectsevents containing neutrinos, which are a distinguishing feature of the tau lepton decay.The invariant mass distributions for all background and signal processes allow toquantify the compatibility between the acquired data and the presence of a signal. Thecombination of all final states leads to the observation of an excess of events over a largemass range. Its statistical significance is 3,2 standard deviations at 125 GeV ; the bosonmass measured in the H → τ τ channel is 122 ± 7 GeV. This measurement is the firstevidence for a coupling between the Higgs boson and the tau lepton.Keywords : LHC, CMS, Higgs, tau lepton, electron, trigger.