Search for standard model production of four top quarks in final states with same-sign and multiple leptons in proton-proton collisions at $\sqrt{s}=13~\mathrm{TeV}$

The standard model (SM) production of four top quarks ($\mathrm{t\overline{t}t\overline{t}}$) is studied by the CMS Collaboration using proton-proton collision events collected in Run 2 of the CERN LHC, with a center-of-mass energy of $13~\mathrm{TeV}$ and corresponding to an integrated luminosity of $137~\mathrm{fb}^{-1}$. The events are required to contain a pair of same-sign leptons ($e, \mu$) or at least three leptons, and several jets. Two approaches are used to enhance signal sensitivity: first a simple classification based on the number of jets and b-tagged jets, and second a boosted decision tree (BDT) taking advantage of kinematic variables related to leptons and jets. Control regions are used to constrain the dominant SM backgrounds. The two approaches find consistent results compatible with next-to-leading-order SM predictions. The observed (expected) significance of the BDT analysis is 2.6 (2.7) standard deviations, and the $\mathrm{t\overline{t}t\overline{t}}$ cross section is measured to be $12.6^{+5.8}_{-5.2}~\mathrm{fb}$. These results are used to constrain the Yukawa coupling of the top quark, $y_{\rm{t}}$, yielding a $95\%$ confidence level limit of $|y_{\rm{t}}/y_{\rm{t}}^{\rm{SM}}| < 1.7$, where $y_{\rm{t}}^{\rm{SM}}$ is the SM value of $y_{\rm{t}}$. Limits are also set on the production of a heavy scalar or pseudoscalar in a type II 2HDM scenario, with exclusions in the mass ranges of 350-470 GeV and 350-550 GeV for heavy scalar and pseudoscalar bosons, respectively, for the 2HDM scenarios considered.