Search for Pair Production of Vector-Like Quarks in Leptonic Final States in Proton-Proton Collisions at 13 TeV at the CMS Detector in the LHC

Discovery of a Higgs Boson with mass near 125 GeV in 2012 marked one of the most important milestones in particle physics. The low mass of Higgs with diverging loop corrections adds motivation to look for new physics Beyond the Standard Model (BSM). Several BSM theories introduced new heavy quark partners, called vector-like quarks (VLQ), with mass in TeV scale. In particular, the vector-like top quark ($T$) can cancel the largest correction due to top quark loops, one of the main contributions to the divergence, and stabilize the scalar Higgs mass. This analysis searches for pair production of vector-like $T$ or $B$ quark with charge $2e/3$ and $-e/3$ in proton-proton collisions at 13 TeV in the LHC. Theories predict 3 decay modes for $T$ and $B$ each : $bW$, $tZ$ , $tH$ and $tW$, $bZ$, $bH$. The branching ratios vary over different theoretical models. We focus on events where bosons decay leptonically and result in final states with multiple (3 or more) leptons or exactly two leptons with same-signed electric charge. We analyze data collected by the CMS detector in the LHC in 2017 and 2018 with integrated luminosities of 41.53 and 59.74 fb$^{-1}$. Besides Standard Model (SM) processes, lepton misidentification contributes a significant part of the background and is estimated by data-driven method. For the same-signed dilepton final state, there is also background from misidentification of electric charges. Comparing estimated background with data, and considering uncertainties/systematics, we can find the upper limit on $T\overline{T}$ or $B\overline{B}$ production cross section. We calculate limits at different mass points of $T$ and $B$ and different branching ratio combinations.