Abstract
We consider extensions of the standard model with fourth generation fermions (SM4) in which extra symmetries are introduced such that the transitions between the fourth generation fermions and the ones in the first three generations are forbidden. In these models, the stringent lower bounds on the masses of fourth generation quarks from direct searches can be relaxed, and the lightest fourth neutrino is allowed to be stable and light enough to trigger the Higgs boson invisible decay. In addition, the fourth Majorana neutrino can be a subdominant but highly detectable dark matter component. We perform a global analysis of the current Large Hadron Collider (LHC) data on the Higgs production and decay in this type of SM4. The results show that the mass of the lightest fourth Majorana neutrino is confined in the range ~41–59 GeV. Within the allowed parameter space, the predicted effective cross-section for spin-independent DM–nucleon scattering is ~3×10-48–6×10-46 cm 2, which is close to the current XENON100 upper limit and is within the reach of the XENON1T experiment in the near future. The predicted spin-dependent cross sections can also reach ~8×10-40 cm 2.
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