First Search for Pair Production of Scalar Top Quarks Decaying to Top Quarks and Light-Flavor Jets with Low Missing Transverse Momentum

After the discovery of the Higgs boson in 2012, the current best theoretical model that describes all observed particles and their interactions, the standard model (SM), was considered complete. However, there are a plethora of physical phenomena that the SM does not accurately describe, which motivates particle physicists to search for evidence of new particles produced by high energy collisions at the Large Hadron Collider (LHC). In traditional searches for physics beyond the standard model, a requirement of high missing transverse momentum (\ptmiss) is often used to identify potential new particles. However, without any signs of significant deviations from standard model expectations, a search where this requirement is removed has been performed. There are many well motivated and previously unexplored physics models, including versions of supersymmetry (SUSY) characterized by $R$-parity violation or with additional hidden sectors, that predict the production of events with low \ptmiss, many jets, and top quarks. In particular, a general search is performed to look for the pair production of scalar top quarks that would decay to two top quarks and six additional light flavor jets. The search is performed using events with at least seven jets and exactly one electron or muon. No requirement on \ptmiss is imposed. With the use of a neural-network-based signal-to-background discriminator, a background estimation has been achieved where more traditional techniques would not be possible. The study is based on a sample of proton-proton collisions at $\sqrt{s} = 13$\TeV corresponding to 137.2~\fbinv of integrated luminosity collected with the Compact Muon Solenoid (CMS) detector at the LHC in 2016, 2017, and 2018. Results of the search are interpreted for stealth SUSY and SUSY with $R$-parity violation, resulting in a lower limit exclusion of scalar top production of 900 and 700\GeV, respectively.