Hunting Quasi-Degenerate Higgsinos

Abstract

We present a new strategy to uncover light, quasi-degenerate Higgsinos, a likely ingredient in a natural supersymmetric model. Our strategy focuses on Higgsinos with inter-state splittings of O(5-50) GeV that are produced in association with a hard, initial state jet and decay via off-shell gauge bosons to two or more leptons and missing energy, $pp \to j + \text{MET}\, + 2^+\, \ell$. The additional jet is used for triggering, allowing us to significantly loosen the lepton requirements and gain sensitivity to small inter-Higgsino splittings. Focusing on the two-lepton signal, we find the seemingly large backgrounds from diboson plus jet, $\bar tt$ and $Z/\gamma^* + j$ can be reduced with careful cuts, and that fake backgrounds appear minor. For Higgsino masses $m_{\chi}$ just above the current LEP II bound ($\mu \simeq 110\,$) GeV we find the significance can be as high as 3 sigma at the LHC using the existing 20 fb$^{-1}$ of 8 TeV data. Extrapolating to LHC at 14 TeV with 100 fb$^{-1}$ data, and as one example $M_1 = M_2 = 500$ GeV, we find 5 sigma evidence for $m_{\chi} \lesssim\, 140\,$ GeV and 2 sigma evidence for $m_{\chi} \lesssim\, 200\,$ GeV . We also present a reinterpretation of ATLAS/CMS monojet bounds in terms of degenerate Higgsino ($\delta m_{\chi} \ll 5\,$) GeV plus jet production. We find the current monojet bounds on $m_{\chi}$ are no better than the chargino bounds from LEP II.