  A search for physics beyond the standard model in events with at least three
charged leptons (electrons or muons) is presented. The data sample corresponds
to an integrated luminosity of 137 fb$^{-1}$ of proton-proton collisions at
$\sqrt{s} =$ 13 TeV, collected with the CMS detector at the LHC in 2016-2018.
The two targeted signal processes are pair production of type-III seesaw heavy
fermions and production of a light scalar or pseudoscalar boson in association
with a pair of top quarks. The heavy fermions may be manifested as an excess of
events with large values of leptonic transverse momenta or missing transverse
momentum. The light scalars or pseudoscalars may create a localized excess in
the dilepton mass spectra. The results exclude heavy fermions of the type-III
seesaw model for masses below 880 GeV at 95% confidence level in the scenario
of equal branching fractions to each lepton flavor. This is the most
restrictive limit on the flavor-democratic scenario of the type-III seesaw
model to date. Assuming a Yukawa coupling of unit strength to top quarks,
branching fractions of new scalar (pseudoscalar) bosons to dielectrons or
dimuons above 0.004 (0.03) and 0.04 (0.03) are excluded at 95% confidence level
for masses in the range 15-75 and 108-340 GeV, respectively. These are the
first limits in these channels on an extension of the standard model with
scalar or pseudoscalar particles.

High Energy Physics - Experiment