GUT-based NMSSM with Singlino Dark Matter: Describing the ATLAS/CMS Excesses

One of the main goals of the ongoing LHC program is the search for BSM physics, with EW SUSY partners still allowed with masses as low as a few hundred GeV. Over the last years, searches for the ``golden channel'', $pp \to \tilde\chi^0_2 \tilde\chi^\pm_1 \to \tilde\chi^0_1 Z^{(*)} \, \tilde\chi^0_1 W^{\pm (*)}$ show consistent excesses between CMS and ATLAS in the 2 soft-lepton and 3 soft-lepton plus missing $E_T$ searches, favoring mass scales of $m_{\tilde\chi^0_2} \approx m_{\tilde\chi^{\pm}_1} \ge 200$ GeV and $\Delta m_{21} := m_{\tilde\chi^0_2} - m_{\tilde\chi^0_1} \approx 20$ GeV. We analyze these excesses in the framework of the Next-to-Minimal Supersymmetric Standard Model (NMSSM). We assume a singlino dominated lighest neutralino, $\tilde\chi^0_1$, as our Dark Matter (DM) candidate. The second and third lightest neutralinos, $\tilde\chi^0_{2,3}$ are higgsino like, with the higgsino mixing parameter $\mu$ being smaller than the soft SUSY-breaking bino and wino masses, $M_1$ and $M_2$. In particular, we assume the approximate GUT relations $M_1 \sim M_2/2 \sim M_3/6$, yielding a gluino mass $m_{\tilde g} \sim M_3 \ge 2$ TeV, in agreement with the LHC search limits. The scalar masses are assumed to heavy and do not play a role in our analysis. We demonstrate that this scenario is in agreement with all relevant experimental constraints, comprising the DM direct detection limits and the upper limit on the DM relic density, the LHC searches for SUSY particles and additional Higgs bosons, as well as the LHC Higgs-boson rate measurements. This constitutes the first explanation of the soft-lepton excesses in a model with GUT relations among the soft SUSY-breaking parameters.