Search for new physics in the compressed mass spectra scenario using events with two soft opposite-sign leptons and missing transverse momentum at $\sqrt{s}=13~\mathrm{TeV}$

A search for new physics in events with two low-momentum opposite-sign leptons and missing transverse momentum is presented using $12.9~\mathrm{fb}^{-1}$ of data collected at $13~\mathrm{TeV}$ in 2016. The observed data yields are compatible with the predictions for standard model processes. The results are interpreted in the context of supersymmetry with compressed mass spectra, described in the form of simplified models. In the first model the lightest chargino ($\tilde{\chi}_1^{\pm}$) and the second-lightest neutralino ($\tilde{\chi}_2^{0}$) are pair produced and degenerate in mass, and decay to the lightest neutralino and a virtual W and Z boson, respectively. At $95\%$ confidence level, $\tilde{\chi}_1^{\pm}$/$\tilde{\chi}_2^{0}$ masses are excluded up to $165~\mathrm{GeV}$ for a mass difference of $7.5~\mathrm{GeV}$ with respect to the lightest neutralino. Previously such a scenario had only been constrained by LEP experiments. The results are also interpreted in a simplified model of top squark pair production for the case that the mass difference between the top squark and the lightest neutralino is below the mass of the W boson. $$\textit{This document has been revised with respect to the version dated August 6, 2016.}$$