How low can SUSY go? Matching, monojets and compressed spectra

If supersymmetry (SUSY) has a compressed spectrum then the current mass limits from the LHC can be drastically reduced. We consider a possible 'worst case' scenario where the gluino and/or squarks are degenerate with the lightest SUSY particle (LSP). The most sensitive searches for these compressed spectra are via the nal state LSPs recoiling against initial state radiation (ISR). Therefore it is vital that the ISR is understood and possible uncertainties in the predictions are evaluated. We use both MLM (with Pythia 6) and CKKW- L (with Pythia 8) matching and vary matching scales and parton shower properties to accurately determine the theoretical uncertainties in the kinematic distributions. All current LHC SUSY and monojet analyses are employed and we nd the most constraining limits come from the CMS Razor and CMS monojet searches. For a scenario of squarks degenerate with the LSP and decoupled gluinos we nd M~ q > 340 GeV. For gluinos degenerate with the LSP and decoupled squarks, M~ > 500 GeV. For equal mass squarks and gluinos degenerate with the LSP, M~ q;~ > 650 GeV.