Dark matter and Higgs boson physics

A vectorlike colorless fermion doublet and a singlet added to the standard model allow a consistent interpretation of dark matter in terms of the lightest neutral particle, as they may help in obtaining successful gauge coupling unification. We analyze in detail the mass range of the lightest neutral particle below the W mass, i.e. in a range of the parameters where the physics of the standard model Higgs boson may be substantially affected either directly or indirectly.