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, $\bar{t}t$ and $Z/γ^* + j$ can be reduced with careful cuts, and that fake backgrounds appear minor. For Higgsino masses $m_{\ddot{I}}‡$ just above the current LEP II bound ($\hat{I}¼\simeq 110$) 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 $m_{\ddot{I}}‡\lesssim 140$ GeV and 2 sigma evidence for $m_{\ddot{I}}‡\lesssim 200$ GeV . We also present a reinterpretation of ATLAS/CMS monojet bounds in terms of degenerate Higgsino ($\hat{I}´m_{\ddot{I}}‡\ll 5$) GeV plus jet production. We find the current monojet bounds on $m_{\ddot{I}}‡$ are no better than the chargino bounds from LEP II.

25 pages, 12 figures, 3 tables