Search

This work covers two searches for exotic physics in proton-proton scattering at CERN's Large Hadron Collider, utilising data from the CMS experiment taken at a centre of mass energy of 13 TeV. The first analysis involves a search for Dark Matter in association with the decay products of a new scalar coupled exclusively to the dark sector called the Dark Higgs. The other search is for a heavy Higgs boson that decays into a top-antitop pair. The analysis on the Dark Higgs signal utilises the relations between the scalar transverse momentum sum of jets, HT and the negative vector sum of jet momentum Hmiss to separate signal from background. T The result from 35.9 fb−1 of data from 2016 is presented. No deviation from the Standard Model prediction is observed, and the first experimental limits on Dark Higgs models ever are set, with Dark Matter mediator masses of up to 1700 GeV and Dark Matter particle masses up to 300 GeV can be excluded at 95% C.L. In the search for heavy scalar and pseudoscalar Higgs bosons, the invariant top-antitop mass is explored together with an angular observable sensitive to the correlation between the spin of the top quark and the spin of the antitop quark. For this search, 41.5 fb−1 of 2017 data has been used. Expected upper limits on the coupling between the heavy Higgs boson and the top quark are set for different hypotheses for the mass of the heavy Higgs boson, which are consistent with the expected limits from the same analysis on 2016 data.

The work presented in this thesis explores the experimental, phenomenological, astrophysical and cosmological constraints on SUSY models with three distinguished input scales. The SUSY SU(5) GUT model assumes the soft SUSY-breaking parameters to be universal at the GUT scale $M_{\mathrm{GUT}}$. In the Phenomenological MSSM with eleven parameters (pMSSM11), the input parameters are specified at the $\msusy \equiv \sqrt{\mst1 \mst2}$ scale, which is also the scale at which electroweak symmetry breaking conditions are imposed. In the sub-GUT MSSM, the universality of the input parameters is imposed at some input scale $M_{\mathrm{in}}$ below $M_{\mathrm{GUT}}$ but above the electroweak scale. In addition to the SUSY models, this work also includes a global analysis of Dark Matter Simplified Models (DMSMs) with leptophobic spin-one mediator particles with either vectorial or axial couplings to Standard Model (SM) particles and the dark matter (DM) particle. The {\tt MasterCode} tool is used to analyse the constraints on the parameter spaces of each model, taking into account the relevant searches from up to 139/fb of LHC data at $\sqrt{s}=$~13 TeV, and the most recent limits from the LUX, PICO, XENON1T and PandaX-II experiments. Other experimental constraints are also taken into account (when applicable), including those from flavour physics, electroweak precision observables, the anomalous magnetic dipole momentum of the muon, and cosmological constraints on the DM relic density. The DM annihilation mechanisms responsible for bringing the relic density into the allowed region are identified in the preferred regions of the parameter spaces.

An inclusive search for supersymmetry in final states with jets and missing transverse momentum is performed in proton-proton collisions at a centre-of-mass energy of 13 TeV. A data sample with an integrated luminosity of 35.9 fb-1 recorded by the CMS detector in 2016 at the CERN LHC is analysed. The observed signal candidate event counts are found to be in agreement with the standard model expectation. Within the context of simplified models, the masses of promptly decaying bottom, top and mass-degenerate light-flavour squarks are excluded at 95% confidence level up to 1050, 1000 and 1325 GeV, respectively. The gluino mass is excluded up to 1900, 1650 and 1650 GeV for a gluino decaying promptly via the aforementioned squarks. The result is also interpreted in the context of simplified models of split supersymmetry. Lower limits are placed on the mass of a long-lived gluino for a wide range of lifetimes. Gluino masses up to 1750 and 900 GeV are excluded for gluinos with a lifetime of 1 mm and metastable gluinos, respectively. These results provide complementary coverage to dedicated searches for long-lived particles at the LHC.

The search for BSM physics is one of the primary objectives of the LHC. This thesis describes the alphaT search for supersymmetry using a dataset of 12.9/fb collected during the 2016 proton-proton runs by the CMS detector. The search uses a final state containing significant hadronic activity in the form of jets, missing energy, and no leptons or photons. Dedicated dimensionless variables ensure that backgrounds with fake missing energy are mitigated. A detailed description is given of the selections used to mitigate backgrounds while maintaining acceptance to a wide range of signal models and of the techniques used to ensure a robust determination of the residual backgrounds and systematic uncertainties. No evidence for BSM physics is observed and simplified models are therefore used to interpret the results of the search. Stringent new limits are placed on a range of supersymmetric topologies and a procedure to allow the search to be re-interpreted for any BSM physics model is presented. Additionally, the upgrade of the Level-1 trigger jet and energy sum algorithm is studied. The upgraded trigger hardware is exploited to ensure effective performance in conditions exceeding the design specification of the LHC.

In this thesis up-to-date global fits are presented of four benchmark models within the R-parity conserving Minimal Supersymmetric extension of the Standard Model (MSSM): the constrained MSSM (CMSSM) and two Non-Universal Higgs Mass (NUHM) models, for which soft supersymmetry-breaking input parameters are defined at the Grand Unified Theory (GUT) scale, as well as a 10-parameter phenomenological MSSM model (pMSSM10) for which the input parameters are defined at the MSUSY scale. These global fits take into account experimental constraints from flavour physics, electroweak precision observables, the anomalous magnetic dipole moment of the muon, cosmological constraints on the dark matter relic density, direct detection experiments for dark matter, properties of the Higgs boson, and searches for supersymmetric particles from Run 1 of the LHC with 20 fb−1 of proton proton collisions at √s = 8 TeV. This thesis contains a careful assessment of the impact of experimental constraints on the parameter spaces of the models. Predictions for physical observables and the corresponding prospects for future runs of the LHC, as well as other experiments, are discussed in detail. Novel features of the global fits of the CMSSM, NUHM1, and NUHM2 presented in this thesis include a comprehensive characterisation of the annihilation processes that bring the dark matter relic density in the cosmologically allowed range. The global fit of the pMSSM10 is the first global fit of a pMSSMn model that fully implements searches for supersymmetric particles from Run 1 of the LHC. The validation of these implementations is discussed in detail.

A search for supersymmetric particles in events with high transverse momentum jets and a large missing transverse energy signature is con- ducted using 11.7 fb of data, collected with a center-of-mass collision energy of 8 TeV by the CMS detector. The dimensionless kinematic variable alphaT is used to select events with genuine missing transverse energy signatures. Standard Model backgrounds are estimated through the use of data driven control samples. No excess over Standard Model expectations is found. Exclusion limits on squark and gluino masses are set at the 95% confidence level in the parameter space of a range of supersymmetric simplified models. Results of benchmarking the Level-1 (the first line of the CMS trigger system) single jet and hadronic transverse energy trigger efficiencies, before and after the implementation of a change to the Level-1 jet clustering algorithm are presented. This was introduced to negate an increase in trigger cross-section, which can be attributed to soft jets from secondary interactions. Similar performance is observed for all L1 quantities before and after this change. Furthermore, a templated method to estimate the Standard Model background distribution of the number of jets originating from a b-quark within a supersymmetric search is validated in data and simulation. Applicable to searches sensitive to gluino induced third-generation signatures, this technique is utilised as a crosscheck to the results of the alphaT analysis. Standard Model background predictions from the template fits are compared to those from the alphaT search in the hadronic signal region, where good agreement between the two methods is observed.

A search for supersymmetry in final states with jets and missing transverse energy is performed in pp collisions at a centre-of-mass energy of sqrt(s)=7TeV. The data sample corresponds to 4.98fb^-1 collected by the CMS experiment at the LHC in 2011. A dimensionless kinematic variable is used as the main discriminant between genuine and misreconstructed signal events. The search is performed in a signal region binned according to the scalar sum of the transverse energy of jets and the number of jets identified as originating from a bottom quark. The limits are presented in the parameter space of the cMSSM as well as in simplified models with particular attention paid to compressed spectra and third-generation models. Global frequentist fits to the cMSSM} and a non-universal Higgs model are also performed using the Mastercode framework incorporating recent experimental constraints, similar to those those presented here. Global likelihood contours are presented in the parameter planes of both the cMSSM and NUHM1, as well as a selection of 1D likelihood functions for observables.