Search for a charged Higgs Boson at 13 TeV in the CMS experiment at the LHC, CERN

In chapter 1, we present a brief theoretical overview of the standard model of particle physics where we discuss it’s successes and failures. In chapter 2, the two Higgs doublet model is described where various properties of the charged Higgs such as it’s interaction with the particles of SM are discussed. In the same chapter, we present the current status of the charged Higgs searches and the search strategy followed in this thesis. In chapter 3, we give a brief description of the LHC and the detectors installed there. A few important parameters of the LHC have also been described along with a few physics parameters. In chapter 4, a brief overview of the CMS experiment is presented. “ The central feature of the CMS apparatus is a superconducting solenoid of 6 m internal diameter, providing a magnetic field of 3.8 T. Within the solenoid volume are a silicon pixel and strip tracker, a lead tung state crystal electromagnetic calorimeter (ECAL), and a brass and scintillator hadron calorimeter (HCAL), each composed of a barrel and two endcap sections. Silicon pixel and tracker detector identifies the trajectory of charged particles and accurately measures their trans-verse momentum up to |η|≤2.5. Forward calorimeters extend the pseudo rapidity coverage provided by the barrel and endcap calorimeter. Segmented calorimeters provide sampling of electromagnetic and hadronic showers up to|η|≤5. Muons are detected in gas-ionization chambers embedded in the steel flux-return yoke outside the solenoid in the range of |η|≤2.4”. In chapter 5, we list the collision and simulated data samples. The data used for the analysis was collected by the CMS detector in 2016 in proton-proton (pp) collisions at √s= 13 TeV, with an integrated luminosity of 35.9 fb$^{−1}$. The simulated signal and background samples are generated using the MAD-GRAPH5_aMC@NLO and POWHEG v2 generators at parton level. In all cases, these parton level events are hadronized using PYTHIA 8 with the CUETP8M1 tune, and then passed to GEANT4 for simulation of the CMS detector response. In the same chapter, we describe the reconstruction and identification of various physics objects. In chapter 6, we describe various corrections applied on simulated samples. In chapter 7, the event selection has been described. In the event topology of interest, there are four jets (two b jets and two light jets), one charged lepton, and missing transverse energy. Various selection requirements are applied to ensure the resulting events have this topology. In chapter 8, we perform kinematic fitting to select events coming from true tt decay. In this analysis, the charged Higgs boson is assumed to decay to c$\bar{s}$ . The invariant mass of the system of the two light jets (m$_{jj}$) is thus used as the final observable. If the two observed light jets come from a semileptonic t$\bar{t}$ decay, then the m$_{jj}$ distribution should have a peak at the W boson mass. However, the observed mean of the m$_{jj}$ distribution is much higher (around 128 GeV), reflecting the fact that the two light jets in each event may not necessarily come from the decay of a W boson. To select true semileptonic tt events, a kinematic fit is performed on the reconstructed objects using the top kinematic fitter package. In the output, the top kinematic fitter gives exactly four jets (two b jets, one from each of the leptonic and hadronic t decays, and two light jets from the hadronic t decay), a lepton, and a neutrino. The two light jets coming from the hadronic t decay are further used for charm tagging. In chapter 9, we describe the procedure to estimate QCD multijet background from data. The simulation of QCD multijet events is computationally intensive, resulting in a limited number of such events being available. A data-driven approach is used to make a more precise estimation of the QCD multijet background. In chapter 10, the m$_{jj}$ distribution without and with charm tagging is given. Further, events are divided exclusively into loose, medium, and tight categories, based on whether at least one of the light jets passes the loose but neither passes the medium, at least one passes the medium but neither passes the tight, or at least one passes the tight working points of the charm tagging selection requirements, respectively. The expected signal to background ratio is different in the various charm categories, so partitioning the events into categories results in an improvement in the expected upper limits on B(t→H+b). In chapter 11, a detailed description of the statistical and systematic uncertainties is given. There are various sources of systematic uncertainty which may arise due to detector calibration effects, uncertainty in the measured reconstructed efficiency, the theoretical modeling of signal events, and other effects. In chapter 12, the final results are presented. The total expected background number of events agrees with the data within uncertainties. The absence of a charged Higgs signal in the data is characterized by setting exclusion limits on the branching ratio B(t→H+b), assuming that B (H+→cs)= 100%. In the absence of any excess, an asymptotic 95% confidence level (CL) limit using the likelihood ratios on B(t→H+b) is calculated. In chapter 13, we conclude the analysis. We also discuss how the current experimental results can be interpreted in different types of the two Higgs doublet model.