Searches for electroweak production of supersymmetric particles with compressed mass spectra in $\sqrt{s} = 13$ TeV $pp$ collisions with the ATLAS detector

This note presents results of searches for electroweak production of supersymmetric particles in models with compressed mass spectra. The searches use 139 fb$^{-1}$ of $\sqrt{s}= 13$ TeV proton- proton collision data collected by the ATLAS experiment at the Large Hadron Collider. Events with missing transverse momentum, two same-flavour, oppositely-charged, low transverse momentum leptons, and hadronic activity from initial state radiation are selected. The data are found to be consistent with predictions from the Standard Model. The results are interpreted using simplified models of R-parity-conserving supersymmetry in which the lightest supersymmetric partner is a neutralino with a mass similar to a chargino, heavier neutralino, or slepton. Lower limits on the masses of charginos in different simplified models range from 162 GeV to 205 GeV for moderate mass splittings, and extend down to mass splittings of 2 GeV to 2.6 GeV at the LEP chargino bounds. Similar lower limits on degenerate light-flavour sleptons reach up to masses of 256 GeV and down to mass splittings of 590 MeV.