Search for Type-III Seesaw Heavy Fermions with Multilepton Final States using 2.3/fb of 13 TeV proton-proton Collision Data

A search for type-III seesaw signal in events with three or more electrons or muons is presented. The data sample corresponds to $2.3\,\textrm{fb}^{-1}$ of integrated luminosity in $pp$ collisions at $\sqrt{s} = 13\,$TeV collected by the CMS experiment at the LHC. Since the signal populates channels with at least three leptons and diverse kinematic properties, the data is binned in exclusive channels. The primary selection is based on the number of leptons and the invariant mass of opposite-sign dilepton systems which helps discriminate the signal against the Standard Model background. The final optimization for the type-III seesaw signal is based on the sum of leptonic transerve momenta and missing transverse energy. Control samples in data are used to check the robustness of background evaluation techniques and to minimize the reliance on simulation. The observations are consistent with expectations from Standard Model processes. The results are used to exclude heavy fermions of the type-III seesaw model with masses below 430\,GeV.