Search for a Light Charged Higgs Boson in the CMS Experiment in pp Collisions at $\sqrt{s)}$= 7 TeV

In the standard model of particle physics (SM) the masses of the elementary particles aregenerated by the Higgs mechanism predicting the existence of the Higgs boson. Recently theCMS and ATLAS experiments at the CERN LHC reported evidence of a new particle compatiblewith the SM Higgs boson. The Higgs sector of the SM suffers, however, from the so-calledhierarchy problem, for which one possible solution is supersymmetry. Supersymmetric modelsthat extend the SM to contain at least two Higgs doublets, like the minimal supersymmetricstandard model (MSSM), predict the existence of a charged Higgs boson. Therefore the searchfor the charged Higgs boson has an important role for further studies of the Higgs sector of thefundamental theory of the elementary particles.This thesis describes a search for a light (𝑚H+ < 𝑚t ) charged Higgs boson, which can be producedin the decay of the top quark, in the CMS experiment. The study considers the fully hadronic¯final state of top quark pair decay t¯ → H± W∓ bb, where the charged Higgs boson decays astH+ → τντ , τ → hadrons + ντ with the branching fraction ℬ (H+ → τν) ≈ 1, and the associatedW boson decays to hadrons. This process is one of the most important ones for the discoveryof the light charged Higgs boson. The analysis is also sensitive to the top quark pair decay¯t¯ → H± H∓ bb. A feature of the fully hadronic final state is that it allows the reconstruction oftthe transverse mass of the tau-neutrino system. The challenge is, however, the identification ofthe hadronic τ decays amongst the copious production of hadronic jets in QCD multijet eventsat the LHC.Results from the first 2.3 fb−1 of the proton-proton collisions at the centre-of-mass energy of7 TeV recorded by the CMS detector during the 2011 data gathering are presented. The analysedevents were triggered requiring a hadronically decaying τ and missing transverse energy. Atoffline, the events had to have in addition at least three jets, of which at least one was identifiedas originating from a bottom quark. The main backgrounds were measured from data.The existence of a possible signal was statistically investigated with a frequentist method employing the distribution of the transverse mass of the tau-neutrino system. The results werecombined with the other final states analysed in CMS. No significant evidence of signal wasobserved, and an upper limit of 2–3 % was established on the branching fraction ℬ (t → H+ b) atthe 95 % confidence level for charged Higgs boson masses between 80 and 160 GeV/𝑐2 , with theassumption that ℬ (H+ → τν) = 1. The upper limit was interpreted in the MSSM 𝑚max scenario,hexcluding a significant region of the (tan 𝛽, 𝑚H+ ) parameter space.