Evidence for four-top quark production in proton-proton collisions at sqrt(s) = 13 TeV

The production of four top quarks (t¯tt¯t) is studied with LHC proton-proton collision data samples collected by the CMS experiment at a center-of-mass energy of 13TeV, and corresponding to integrated luminosities of up to 138fb−1. Events that have no leptons (all-hadronic), one lepton, or two opposite-sign leptons (where lepton refers only to prompt electrons or prompt muons) are considered. This is the first t¯tt¯tmeasurement that includes the all-hadronic final state. The observed significance of the t¯tt¯tsignal in these final states of 3.9 standard deviations (1.5 expected) provides evidence for t¯tt¯tproduction, with a measured cross section of 36+12−11fb. Combined with earlier CMS results in other final states, the signal significance is 4.0 standard deviations (3.2 expected). The combination returns an observed cross section of 17 ±4 (stat)±3 (syst)fb, which is consistent with the standard model prediction.