Search for new resonances and rare top decays to Higgs bosons at the LHC

Many searches targeting the discovery of Beyond the Standard Model Physics (BSM) take place at the different Large Hadron Collider (LHC) experiments. This thesis describes in detail two of these analyses performed within the Atlas experiment. One analysis consists in a search for localised excesses in dijet mass distributions of low dijet mass events produced in association with a high transverse energy photon. The search uses 76.6–79.8 fb$^{-1}$ of LHC pp collisions collected by Atlas at a centre-of-mass- energy of 13 TeV during 2015-2017 period. Two variants are presented: one which makes no jet flavour requirements and one which requires both jets to be tagged as b-jets. Since the observed mass distributions are consistent with multi-jet processes in the Standard Model (SM), the data are used to set upper limits on the production cross-section for a benchmark Z′ model and, separately, on generic Gaussian-shaped contributions, extending the current Atlas constraints on dijet resonances to the mass range between 225 and 1100 GeV. The other one targets a search for a light charged Higgs boson in $t \rightarrow H^{+}b$ decays, with $H \rightarrow cb$. The analysis searches for top quark pair events in which one top quark decays to Wb, with the W boson decaying leptonically, and the other top quark decays to $H^{+}b$. The search is based on pp collisions at $\sqrt{s}$=13 TeV recorded between 2015 and 2018 with the Atlas detector at the LHC and uses an integrated luminosity of 138.9 fb$^{−1}$. Data are analysed in the lepton-plus-jets final state, characterised by an isolated electron or muon and at least four jets. The search exploits the high multiplicity of b-quark jets characteristic of signal events, and employs a neural network discriminant that uses the kinematic differences between the signal and the background, which is dominated by $t\bar{t} \rightarrow W bW b$ decays. No significant excess of events above the background expectation is found, and expected 95% CL upper limits are derived for $t \rightarrow H^{+}b$ branching ratio as a function of the mass of the charged Higgs boson, ranging between 80 to 160 GeV.