Search for supersymmetry in events with photons and large missing momentum and a search of the production of a standard model Higgs boson in association with a top quark pair (ttH) in the all-jet final state using large-radius jets with the CMS detector at CERN LHC.

Abstract

This dissertation describes my PhD work that was carried out at National Centre of Scientific Research (N.C.S.R) "Demokritos" and National Technical University of Athens (N.T.U.A). The work consists of two independent analyses. Both analyses use protonproton collision data at the center of mass energy √ s = 13 TeV, collected in 2016 with the Compact Muon Solenoid (CMS) detector at CERN LHC and correspond to a total integrated luminosity of 35.9 fb−1 . The first part includes a search for gauge mediated supersymmetry breaking in events that involve photons and large missing transverse momentum p miss T . For the interpretation of the results a gauge mediated supersymmetry scenario (GMSB) was assumed. Supersymmetry is a popular extension of the standard model (SM) of particle physics. For this analysis, a gauge mediated supersymmetry scenario (GMSB) was assumed. In GMSB models, the lightest supersymmetric particle is the gravitino, and the next-to-lightest supersymmetric particle is often taken to be the neutralino. The conservation of R parity implies that the gravitino is stable and thus, it can not be detected. The resulting imbalance in the total observed transverse momentum is referred to as missing transverse momentum ˜p miss T , defined as the negative vector sum of the transverse momenta of all visible particles in an event. Its magnitude is referred to as p miss T . If the NLSP is bino-like, its primary decay will be to a gravitino and a photon (γ), resulting in final states with significant p miss T and one or more photons. The results were used to set cross section limits on gluino and squark pair production in this framework. Gluino masses below 1.86 TeV and squark masses below 1.59 TeV are excluded at a 95% confidence level. The second part of the thesis concerns a search of the production of a standard model Higgs boson in association with a top quark pair (ttH) in the all jet final state. After the Higgs boson discovery, one of the main goals of the LHC program is to understand in depth its properties and in particular its couplings with the Standard Model (SM) particles. The couplings of the Higgs boson to gauge boson have been established fairly precisely. However, there is a considerable uncertainty in the couplings to fermions. The associated production of the Higgs boson is of particular importance in CMS as it is used to measure the top Yukawa coupling. Furthermore, the b b decay mode has the largest branching fraction for the 125 GeV Higgs boson and therefore contributes a large proportion of the statistics in the context of the wider ttH search. Compared to previous searches in the fully jet final state, this analysis explores a novel approach by selecting events with highly Lorentz-boosted jets. The Higgs boson and top candidates can be produced with a large Lorentz boost and hence their decay products can be reconstructed in a large radius jet. Since the decay products are merged, one can fully reconstruct the mass of the "boosted-jet" which corresponds to the mass of each candidate. Notably, as luminosity leveling will be used extensively in HL-LHC, low-pT jets will be more difficult to trigger. Jets that are reconstructed in the boosted regime will not suffer from this effect at the trigger level, making this approach favored in this high pile up environment. To successfully identify Higgs and top candidates, dedicated Multivariate Analysis Techniques (MVA) were developed. Each boosted jet can be identified as Higgs, top, or jet coming from QCD. The methods that were developed for the background estimation in order to optimize the sensitivity of the analysis are also presented. The analysis sensitivity was studied in terms of the observed (expected) limit which is found to be 9.4 (7:6 < 10:4 < 14) times the SM expectations.