Monojet plus soft dilepton signal from light higgsino pair production at LHC14

Abstract: Naturalness arguments imply the existence of higgsinos lighter than 200-300 GeV. However, because these higgsinos are nearly mass degenerate, they release very little visible energy in their decays, and (even putting aside triggering issues) signals from electroweak higgsino pair production typically remain buried under Standard Model backgrounds. Prospects for detecting higgsino pair production via events with monojets or mono-photons from initial state radiation are also bleak because of signal-to-background rates typically at the 1% level. Here, we consider the possibility of reducing backgrounds by requiring the presence of soft daughter leptons from higgsino decays in monojet events. We find that LHC14 experiments with an integrated luminosity of 300 fb^{-1} should be sensitive to light higgsinos at the 5\sigma level for \mu < 170 GeV with a S/B ~ 8.5%. For an integrated luminosity of 1000 fb^{-1} (which should be possible at a high luminosity LHC) the corresponding sensitivity to \mu extends to over 200 GeV though the systematic uncertainty would have to be controlled to considerably better than 5%. The corresponding reach from measurements of the rate asymmetry between monojet events with same flavour vs. opposite flavour dileptons is ~10-15 GeV smaller, but does not suffer the systematic uncertainty from the normalization of the background.