Probing the stau-neutralino coannihilation region at the LHC with a soft tau lepton and a jet from initial state radiation

We present a feasibility study, to search for dark matter at the LHC, in events with one soft hadronically decaying tau lepton and missing transverse energy recoiling against a hard $p_{T}$ jet from initial state radiation. This methodology allows the search for Supersymmetry in compressed mass spectra regions, where the mass difference between the lightest neutralino, $\tilde\chi_1^0$, and the stau (the tau superpartner), $\tilde{\tau}$, is small. Several theoretical models predict a direct connection between thermal Bino dark matter and staus within this scenario. We show that compressed regions, not excluded by ATLAS nor CMS experiments, are opened up with the increase in experimental sensitivity reached with the proposed methodology. The requirement of a hard jet from initial state radiation combined with a soft tau lepton is effective in reducing Standard Model backgrounds, providing expected significances greater than 3$\sigma$ for $\tilde{\chi}_{1}^{\pm}$ masses up to 300 GeV and $\tilde{\tau}$-$\tilde{\chi}_{1}^{0}$ mass gaps below 25 GeV with only 30 fb$^{-1}$ of 13 TeV data from the LHC.