Search for vector-like quarks using jet substructure techniques with the CMS experiment

Abstract

In this thesis, a search for pair produced vector-like $T$ quarks in $pp$ collision data from the LHC at $\sqrt{s} = 13\,\text{Te}\kern-0.06667em\text{V}$ is presented. The data were collected with the CMS detector and correspond to an integrated luminosity of up to $2.6\,\mathrm{fb}^{-1}$. Vector-like quarks are hypothetical new particles predicted by many extensions of the Standard Model in which the Higgs boson is a composite state of an unknown strong interaction.Vector-like $T$ quarks are assumed to decay via three different decay modes to either $bW$, $tZ$ or $tH$, with branching fractions that are not fixed and can vary depending on the particular model featuring vector-like quarks.This search focuses on decays of the $T{\overline T}$ system in which at least one muon or electron is present in the final state, and in which at least one of the $T$ quarks decays to a top quark and a Higgs boson.As the $T$ quarks are expected to be quite heavy, their decay products are significantly Lorentz-boosted in the reference frame of the $T{\overline T}$ system. The subsequent decay products of the Higgs boson are then emitted with a very small angle between them. This search is optimised for the main decay channel of the Higgs boson to two bottom quarks and attempts to reconstruct the two $b$ quarks within a single jet with a large cone size. Dedicated jet substructure techniques, in combination with algorithms to identify jets originating from the fragmentation of a $b$ quark, are then used to reconstruct the entire $H \rightarrow b{\overline b}$ decay.The event categories of this search are also combined with the categories of a search for pair-produced $T$ quarks that focuses on the $T \rightarrow bW$ decay. This approach provides a high sensitivity to $T{\overline T}$ production for many different combinations of branching fractions to the three possible decay modes.No excess of the data above the expected background contribution from the Standard Model is observed in any of the final event categories. Upper limits on the $T{\overline T}$ production cross section are calculated at 95% confidence level and lower mass limits for vector-like $T$ quarks are set for various assumptions of their decays: for branching fractions corresponding to an electroweak isospin singlet $T$ quark, masses below 860$\,\text{Ge}\kern-0.06667em\text{V}$ are excluded, while for an electroweak isospin doublet, $T$ quarks are excluded with masses up to 830$\,\text{Ge}\kern-0.06667em\text{V}$. For other branching fraction combinations, lower mass limits are derived ranging from 710$\,\text{Ge}\kern-0.06667em\text{V}$ to 940$\,\text{Ge}\kern-0.06667em\text{V}$.These exclusion limits are comparable to and for some branching fraction assumptions even higher than exclusion limits set by the CMS experiment using a data set corresponding to $19.7\,\mathrm{fb}^{-1}$ at $\sqrt{s} = 8\,\text{Te}\kern-0.06667em\text{V}$.