Model independent search for new particles in two-dimensional mass space using events with missing energy, two jets and two leptons with the CMS detector

The discovery of a new particle consistent with the standard model Higgs boson at the Large Hadron Collider in 2012 completed the standard model of particle physics (SM). Despite its remarkable success many questions remain unexplained. Numerous theoretical models, predicting the existence of new heavy particles, provide answers to these unresolved questions and are tested at high energy experiments such as the Compact Muon Solenoid (CMS) detector at the Large Hadron Collider (LHC). In this thesis a model independent search method for new particles in two-dimensional mass space in events with missing transverse energy is presented using 19.7 $\mbox{fb}^{-1}$ of proton-proton collision data recorded by the CMS detector at a centre of mass energy $\sqrt{s}$ = 8 TeV at the LHC. The analysis searches for signatures of pair-produced new heavy particles $\mbox{T}^\prime$ which decay further into unknown heavy particles $\mbox{W}^\prime$ and SM quarks $q$ ($\mbox{T}^\prime\overline{\mbox{T}^\prime} \rightarrow {{\mbox{W}^\prime}^+ \mbox{q} \mbox{W}^\prime}^- \overline{\mbox{q}}$), in which both $\mbox{W}^\prime$ bosons decay leptonically, $\mbox{W}^\prime$ $\rightarrow l \nu$. A mass reconstruction of the $\mbox{T}^\prime\overline{\mbox{T}^\prime}$ system is performed using the analytical solution of the kinematical equation system together with constraints from the parton distribution functions leading to an estimate of $\mbox{M}_{T^\prime}$ and $\mbox{M}_{W^\prime} $ for each event. The masses of the unknown particles $\mbox{T}^\prime$ and $\mbox{W}^\prime$ are unconstrained and reconstructed simultaneously. There are no further assumptions apart from the topology of the decay chain. The mass reconstruction method can be applied to search for any particle decaying with the same topology into the final state mentioned.For the same event selection, the search method is employed in multiple applications. Initially, as a proof of principle, the top quark and the W boson are observed simultaneously in the two-dimensional mass space in standard model top quark pair decays. A search for a new heavy top quark partner $\mbox{T}^\prime$ decaying to standard model $W$ bosons and $b$ quarks is performed in the one-dimensional projection, as well as a search for a new heavy neutral gauge boson $\mbox{Z}^\prime$ , decaying to standard model top pairs, in the invariant mass distribution of the $\mbox{t}\overline{\mbox{t}}$ system. In absence of significant excesses over standard model predictions for both observables, 95 % confidence level upper limits on the cross section times branching ratio $\sigma \cdot BR$ are calculated and presented. In order to quantify the performance of the model-independent mass reconstruction method results are compared with dedicated searches. Finally a search for pair-produced new heavy top quark partners $\mbox{T}^\prime$ decaying into new heavy charged gauge bosons $\mbox{W}^\prime$ and standard model $b$ quarks, as predicted by a simplified Littlest Higgs model, is carried out in the two-dimensional mass-plane. No significant deviation from standard model predictions is observed.The observed 95 % confidence level lower limit on the mass $\mbox{M}_{T^\prime} $ for a new heavy top-partner $\mbox{T}^\prime$ is consistent with expectations ranging from 870 GeV and 950 GeV depending on the mass $\mbox{M}_{W^\prime} $ for a new heavy charged gauge boson $\mbox{W}^\prime$ . This is the first search performed at the LHC for both a new heavy top quark partner $\mbox{T}^\prime$ and a new heavy charged gauge boson $\mbox{W}^\prime$ as predicted by the Littlest Higgs model.