Search for Electroweakinos in Compressed-Spectrum Scenarios with Low-Momentum Isolated Tracks in Proton-Proton Collisions at sqrt[s]=13  TeV

A search for supersymmetric electroweakinos is performed using events with a low-momentum (soft) isolated track and large missing transverse momentum, targeting nearly mass-degenerate higgsinolike charginos and neutralinos. For mass splittings of 0.3-1 GeV, the chargino decays to the lightest neutralino and a low-momentum pion, which can produce a soft, potentially displaced track. A parametrized neural network separates signal from background using kinematic and impact parameter information. The analysis uses 138  fb^{-1} of proton-proton collision data at a center-of-mass energy of 13 TeV recorded with the CMS detector. No significant excess above the standard model expectation is observed. At 95% confidence level, the considered higgsino model is excluded for mass splittings in the range 0.28-1.15 GeV and for chargino masses up to 185 GeV, setting stringent constraints on natural supersymmetry scenarios.