Search for new physics in multilepton final states in pp collisions at $\sqrt{s}=13~\mathrm{TeV}$

A search for new physics in events with three or more electrons or muons is presented. The data sample corresponds to $137~\mathrm{fb}^{-1}$ of integrated luminosity in pp collisions at $\sqrt{s}=13~\mathrm{TeV}$ collected by the CMS experiment at the CERN LHC in 2016, 2017, and 2018. The targeted signal models are pair production of type-III seesaw heavy fermions and associated production of a light scalar or pseudoscalar boson with a pair of top quarks, in final states with at least three leptons. The heavy fermions may produce non-resonant excesses in the tails of the transverse mass as well as the sum of leptonic transverse momenta and missing transverse energy, whereas light scalars or pseudoscalars may create resonant dilepton mass spectra in multilepton events with or without b quark jets. The observations are found to be consistent with expectations from standard model processes. The results exclude heavy fermions of the type-III seesaw model with masses below $880~\mathrm{GeV}$ for the lepton flavor democratic scenario. Assuming a Yukawa coupling of unity strength to top quarks, the branching ratio of new scalar (pseudoscalar) bosons to dielectrons and dimuons above 0.003 (0.03) and 0.04 (0.03) are excluded for masses in the range of $15-75~\mathrm{GeV}$ and $108-340~\mathrm{GeV}$, respectively.