A very heavy sneutrino as viable thermal dark matter candidate in U(1)′ extensions of the MSSM

We study the Standard Model singlet ("right-handed") sneutrino $\tilde \nu_R$ dark matter in a class of $U(1)'$ extensions of the MSSM that originate from the breaking of the $E_6$ gauge group. These models, which are referred to as UMSSM, contain three right--handed neutrino superfields plus an extra gauge boson $Z'$ and an additional SM singlet Higgs with mass $\simeq M_{Z'}$, together with their superpartners. In the UMSSM the right sneutrino is charged under the extra $U(1)'$ gauge symmetry, it can therefore annihilate via gauge interactions. In particular, for $M_{\tilde \nu_R} \simeq M_{Z'}/2$ the sneutrinos can annihilate by the exchange of (nearly) on--shell gauge or Higgs bosons. We focus on this region of parameter space. For some charge assignment we find viable thermal $\tilde \nu_R$ dark matter for mass up to $\sim 43$ TeV. This is the highest mass of a good thermal dark matter candidate in standard cosmology that has so far been found in an explicit model. Our result can also be applied to other models of spin$-0$ dark matter candidates annihilating through the resonant exchange of a scalar particle.