Search for light charged Higgs bosons with the H± -- τ±ντ decay in the fully hadronic final state

This thesis describes the work conducted for the search for light charged Higgs bosons at the Large Hadron Collider (LHC) with the Compact Muon Solenoid (CMS) experiment, in tt̄ events. In particular, the fully hadronic final state of the signal processes tt̄ → bW ±bH∓ and tt̄ → bH±bH∓ was investigated, with the H± → τ±ντ decay mode. The search was based on data corresponding to an integrated luminosity of 2.3 fb−1 , recorded in the early part of 2011 with the CMS detector at a centre-of-mass-energy of √s = 7 TeV. The Minimal Supersymmetric Standard Model (MSSM) maximal mixing scenario mmax h was tested in a cut-based analysis, by fitting a background-only hypothesis to the transverse mass shape reconstructed using the missing transverse energy Emiss T and τ jet objects, after all signal selection requirements. Model-independent upper limits for the branching ratio BR(t → bH±) were calculated, and were consequently transformed to upper limits in the (tanβ,mA0) plane, under the assumption that the light charged Higgs boson decays exclusively to a τ-lepton and a neutrino, with a branching ratio BR(H± → τ±ντ ) = 1. A significant part of the present work concentrated on the determination of the dominant background contributing to the signal region, found to be attributed to Quantum Chromodynamics (QCD) multi-jet processes, whereby a hadronic jet is falsely identified as a τ jet and the presence of transverse energy imbalance is primarily caused by jet resolution or jet mis-measurement effects. The determination of the QCD multi-jet background was achieved by employing data-driven factorisation techniques, with crucial steps of the signal selection requirements being factorised out of the cut-flow and then re-introduced in the form of applied efficiencies. This factorisation procedure was conducted within selected bins of the τ jet candidate transverse momentum, to account for the fact that the probability of a quark or gluon jet to pass the chosen τ-jet isolation criteria was found to be dependent on the jet transverse momentum. An additional physics-motivated incentive for this procedure was to minimise the small correlations that were observed to exist between the τ jet candidate transverse momentum and the missing transverse energy, in a given event. Using similar techniques, the transverse mass shape of QCD multi-jet processes after all signal selection requirements was also extracted with data-driven methods. In parallel, a complete evaluation of the uncertainties associated with all the measurements was also conducted, which included all relevant systematic and statistical contributions. The transverse mass shape extracted for QCD multi-jet processes, along with all other related background processes, were subsequently employed in a binned maximum likelihood fit to the transverse mass shape observed in the data, after all signal selection requirements. G Al xa nd ro At tik is As a result, model-independent upper limits were evaluated for the branching ratio BR(t → bH±), as a function of the light charged Higgs boson mass (mH±). The observed and expected upper limits were found to be 2.2− 7.3% and 1.5− 5.2%, respectively, for the light charged Higgs mass range of 80 GeV/c2 ≤ mH± ≤ 160 GeV/c2. The corresponding upper limits were transformed to the (tanβ,mH±) and (tanβ,mA0) parameter space of the MSSM maximal mixing scenario mmax h , and were found to exclude a significant region of the parameter space that had previously remained unexplored. H Ale xa nd ros At tik is