Observation of four top quark production in proton-proton collisions at $\sqrt{s}=13\,\mathrm{TeV}$

The first observation of the production of four top quarks in proton-proton collisions is reported, based on a data sample collected by the CMS experiment at a center-of-mass energy of $13\,\mathrm{TeV}$ in 2016 to 2018 at the CERN LHC and corresponding to an integrated luminosity of $138\,\mathrm{fb}^{-1}$. Events with two same-sign, three, and four charged leptons (electrons and muons) and additional jets are analyzed with multivariate discriminants to distinguish the signal process from the main backgrounds. The signal cross section is measured with a profile likelihood fit to be $17.9\,^{+3.7}_{-3.5}\,\mathrm{(stat)}\,^{+2.4}_{-2.1}\,\mathrm{(syst)}\,\mathrm{fb}$, in agreement with the best available theoretical predictions. The observed (expected) significance of the signal is 5.5 (4.9) standard deviations above the background-only hypothesis.