Search for physics beyond the standard model in final states with two or three soft leptons and missing transverse momentum in proton-proton collisions at 13 TeV

A search for new physics is performed using events with two or three low-momentum (soft) leptons and missing transverse momentum. These signatures are motivated by theoretical models predicting new weakly interacting massive particles with nearly degenerate mass. The search results are based on data collected by the CMS experiment at the LHC from 2016 to 2018, corresponding to an integrated luminosity of up to $137~\mathrm{fb}^{-1}$. The observed event yields are in agreement with the standard model expectations. The results are interpreted in terms of pair produced electroweakinos ($\tilde{\chi}_{1}^{\pm}$,$\tilde{\chi}_{2}^{0}$) with compressed mass spectra, which are present in natural supersymmetry models with light higgsinos, as well as in terms of the pair production of top squarks nearly mass-degenerate with the lightest neutralino (LSP). For the electroweakino interpretation, two simplified models are considered: a wino-bino model and a higgsino model. In the framework of the wino-bino simplified model, the search probes $\tilde{\chi}_{2}^{0}/\tilde{\chi}_{1}^{\pm}$ masses up to $280~\mathrm{GeV}$ for a mass difference of $10~\mathrm{GeV}$ relative to the LSP, at $95\%$ confidence level. Considering the higgsino model, the probed masses reach up to $215~\mathrm{GeV}$ for a mass difference of $7.5~\mathrm{GeV}$ and $150~\mathrm{GeV}$ in the highly compressed region with a mass difference of $3~\mathrm{GeV}$. The higgsino search results are further interpreted using a phenomenological MSSM model probing the higgsino mass parameter $\mu$ up to $190~\mathrm{GeV}$ with the bino mass parameter $M_1 = 0.5 M_2$ set to $1000~\mathrm{GeV}$, where $M_2$ is the wino mass parameter. In the top squark interpretation, masses up to $550~\mathrm{GeV}$ are probed for a mass difference of $30~\mathrm{GeV}$ relative to the LSP in the scenario of four-body top squark decay and in the scenario of a chargino-mediated decay, masses up to $475~\mathrm{GeV}$ are probed for the same mass difference.