Inclusive nonresonant multilepton probes of new phenomena at $\sqrt{s}=13~\mathrm{TeV}$

An inclusive search for nonresonant signatures of beyond the standard model phenomena in events with three or more charged leptons, including hadronic decays of tau leptons, is presented. The analysis is based on a data sample corresponding to an integrated luminosity of $138~\mathrm{fb}^{-1}$ of proton-proton collisions at $\sqrt{s}=13~\mathrm{TeV}$, collected with the CMS detector at the LHC in $2016-2018$. Events are categorized based on the charge and flavor multiplicities of the leptons, the multiplicity of b tagged jets, and various kinematic variables. Three scenarios of physics beyond the standard model are probed, viz. the pair production of type-III seesaw heavy fermions with arbitrary couplings to leptons of any flavor, vector-like leptons with couplings to tau leptons, and third generation leptoquarks with flavor-diagonal or cross-generational couplings involving top quarks and charged leptons. Optimal separation between signal and standard model background processes is achieved by the use of signal-specific boosted decision trees. No significant deviations from the background expectations are observed. Lower limits are set at $95\%$ confidence level on the masses of type-III seesaw heavy fermions at $890~\mathrm{GeV}$ for any branching fraction combination to standard model leptons. Scalar leptoquarks decaying exclusively to a top quark and a lepton of any flavor are excluded for masses below $1120~\mathrm{GeV}$. Doublet and singlet vector-like tau lepton extensions of standard model are excluded for masses below $1045~\mathrm{GeV}$ and in mass range of $125-170~\mathrm{GeV}$, respectively. Detailed results are also presented to facilitate alternative theoretical interpretations.