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

The standard model production of four top quarks ($\mathrm{t\overline{t}t\overline{t}}$) is studied by the CMS Collaboration using events containing at least three leptons ($e, \mu$) or a same-sign pair. The events are produced in proton-proton collisions with a center-of-mass energy of $13~\mathrm{TeV}$ at the LHC, and the data sample corresponds to an integrated luminosity of $35.9~\mathrm{fb}^{-1}$, recorded in 2016. Jet multiplicity and flavor are used to enhance signal sensitivity, and dedicated control regions are used to constrain the dominant standard model backgrounds. The observed (expected) significance is 1.6 (1.0) standard deviations, and the $\mathrm{t\overline{t}t\overline{t}}$ cross section is measured to be $16.9^{+13.8}_{-11.4}~\mathrm{fb}$, in agreement with next-to-leading-order standard model predictions. These results are also used to constrain the Yukawa coupling of the top quark.