  The standard model (SM) production of four top quarks
($\mathrm{t\bar{t}t\bar{t}}$) in proton-proton collision is studied by the CMS
Collaboration. The data sample, collected during the 2016-2018 data taking of
the LHC, corresponds to an integrated luminosity of 137 fb$^{-1}$ at a
center-of-mass energy of 13 TeV. The events are required to contain two
same-sign charged leptons (electrons or muons) or at least three leptons, and
jets. The observed and expected significances for the
$\mathrm{t\bar{t}t\bar{t}}$ signal are respectively 2.6 and 2.7 standard
deviations, and the $\mathrm{t\bar{t}t\bar{t}}$ cross section is measured to be
12.6 $^{+5.8}_{-5.2}$ fb. The results are used to constrain the Yukawa coupling
of the top quark to the Higgs boson, $y_{\mathrm{t}}$, yielding a limit of
$|y_{\mathrm{t}}$ $/$ $y_{\mathrm{t}}^{\mathrm{SM}}|$ $<$ 1.7 at 95% confidence
level, where $y_{\mathrm{t}}^{\mathrm{SM}}$ is the SM value of
$y_{\mathrm{t}}$. They are also used to constrain the oblique parameter of the
Higgs boson in an effective field theory framework, $\hat{H}$ $<$ 0.12. Limits
are set on the production of a heavy scalar or pseudoscalar boson in Type-II
two-Higgs-doublet and simplified dark matter models, with exclusion limits
reaching 350-470 GeV and 350-550 GeV for scalar and pseudoscalar bosons,
respectively. Upper bounds are also set on couplings of the top quark to new
light particles.
