Mixed sneutrinos, dark matter, and the CERN LHC

We study the phenomenology of supersymmetric models in which gauge-singlet scalars mix with the minimal supersymmetric standard model (MSSM) sneutrinos through weak-scale $A$ terms. After reviewing the constraints on mixed-sneutrino dark matter from measurements of ${\ensuremath{\Omega}}_{\mathrm{CDM}}$ and from direct-detection experiments, we explore mixed-sneutrino signatures relevant to the LHC. For a mixed-sneutrino lightest supersymmetric particle (LSP) and a right-handed slepton next-to-lightest supersymmetric particle (NLSP), decays of the lightest neturalino can produce opposite-sign, same-flavor (OSSF) dileptons with an invariant-mass distribution shifted away from the kinematic end point. This signature is possible for parameters that lead to a cosmologically viable mixed-sneutrino LSP. We also consider signatures that require larger mixing angles than preferred for mixed-sneutrino dark matter, but which are possible regardless of whether a mixed-sneutrino is the LSP. In some parameter regions, the charginos and neutralinos produced in cascades all decay dominantly to the lighter sneutrinos, leading to a kinematic edge in the jet-lepton invariant-mass distribution from the decay chain $\stackrel{\texttildelow{}}{q}\ensuremath{\rightarrow}{\ensuremath{\chi}}^{\ensuremath{-}}q\ensuremath{\rightarrow}{\stackrel{\texttildelow{}}{\ensuremath{\nu}}}^{*}lq$, without an OSSF dilepton signature. We explore the possibility of using mass-estimation methods to distinguish this mixed-sneutrino jet-lepton signature from an MSSM one. Finally, we consider signatures associated with Higgs-lepton or $Z$-lepton production in cascades involving the heavier sneutrinos.