Search for new phenomena at CMS in the multilepton final state

The standard model of particle physics has been extremely successful in describing the fundamental particles and their interactions via electroweak and strong forces. Despite this, there are various experimental observations that can’t be explained using the standard model, such as small non-zero neutrino mass and the presence of dark matter in the universe. Various extensions of SM are proposed to fix these inadequacies of the standard model. Searches for the standard model extensions, vectorlike leptons, and type-III seesaw fermions are carried out in multilepton final states using the 13 TeV proton-proton collision data collected by CMS experiment at LHC in 2016 and 2017. The events are categorized based on the multiplicity of leptons (electrons, muons, and taus) and the invariant mass of the oppositely charged lepton pairs. The missing transverse momenta and the scalar sum of lepton transverse momentum are used to distinguish a signal from the standard model background. The observations are consistent with the standard model background. The results are used to place limits on the production of vectorlike leptons and heavy fermions of the type-III seesaw model. The vectorlike leptons with a mass between 130-690 GeV are excluded using 41.4 fb$^{−1}$ of data collected in 2017, while the type-III seesaw fermions are excluded with a mass below 840 GeV using 35.9 fb$^{−1}$ of data collected in 2016.