Combined search for electroweak production of charginos and neutralinos in pp collisions at $\sqrt{s} = 13~\mathrm{TeV}$

A statistical combination of several searches for the electroweak production of charginos and neutralinos is presented. All searches use $35.9~\mathrm{fb}^{-1}$ of proton-proton collisions at $\sqrt{s}=13~\mathrm{TeV}$ recorded by CMS in 2016. In addition to previously performed searches, an optimized analysis requiring three or more charged leptons (electrons or muons) is presented, targeting the challenging scenario where the difference in mass between the two least massive neutralinos is approximately equal to the mass of the Z boson. The results are interpreted in simplified models of supersymmetric chargino-neutralino or neutralino pair production. When the lightest neutralino is massless in the chargino-neutralino model, the combined result obtains an observed (expected) limit at the $95\%$ confidence level in the chargino mass of up to $650~(570)~\mathrm{GeV}$ in the most favorable scenario, improving upon the exclusion limits from individual analyses by up to $40~\mathrm{GeV}$. If the mass difference between the two least massive neutralinos is approximately equal to the mass of the Z boson in the chargino-neutralino model, the optimized search for three or more leptons obtains observed and expected exclusion limits of around $225~\mathrm{GeV}$ in the second neutralino mass and $125~\mathrm{GeV}$ in the lightest neutalino mass, improving the observed limit by as much as $60~\mathrm{GeV}$ compared to the previously published result. In the neutralino pair production model, the observed (expected) exclusion limits from the combined result extends up to $650-750~(550-750)~\mathrm{GeV}$ depending on the branching fraction assumed, extending the observed exclusion achieved by individual analyses by up to $200~\mathrm{GeV}$. The combined result additionally excludes some intermediate gaps in the mass coverage of individual analyses. $$\textit{This document has been revised with respect to the version dated July 11, 2017.}$$