Long-term LHC Discovery Reach for Compressed Higgsino-like Models using VBF Processes

Abstract: The identity of Dark Matter (DM) is one of the most active topics in particle physics today. Supersymmetry (SUSY) is an extension of the standard model (SM) that could describe the particle nature of DM in the form of the lightest neutralino in R-parity conserving models. We focus on SUSY models that solve the hierarchy problem with small fine tuning, and where the lightest SUSY particles ($\tilde{\chi}{1}^{0}$, $\tilde{\chi}{1}^{\pm}$, $\tilde{\chi}{2}^{0}$) are a triplet of higgsino-like states, such that the mass difference $\Delta m(\tilde{\chi}^{0}{2},\tilde{\chi}^{0}_{1})$ is 2-50 GeV. We perform a feasibility study to assess the long-term discovery potential for these compressed SUSY models with higgsino-like states, using vector boson fusion (VBF) processes in the context of proton-proton collisions at $\sqrt{s} = 13$ TeV, at the CERN Large Hadron Collider. Assuming an integrated luminosity of 3000 fb$^{-1}$, we find that stringent VBF requirements, combined with large missing momentum and one or two low-$p_{T}$ leptons, is effective at reducing the major SM backgrounds, leading to a 5$\sigma$ (3$\sigma$) discovery reach for $m(\tilde{\chi}^{0}_{2}) < 180$ $(260)$ GeV, and a projected 95\% confidence level exclusion region that covers $m(\tilde{\chi}^{0}_{2})$ up to 385 GeV, parameter space that is currently unconstrained by other experiments.