Abstract
We consider the possibility that dark matter is stabilised by a discrete Z2 symmetry which arises from a subgroup of a U(1)′ gauge symmetry, spontaneously broken by integer charged scalars, and under which the chiral quarks and leptons do not carry any charges. A chiral fermion χ with half-integer charge is odd under the preserved Z2, and hence becomes a stable dark matter candidate, being produced through couplings to right-handed neutrinos with vector-like U(1)′ charges, as in the type Ib seesaw mechanism. We calculate the relic abundance in such a low energy effective seesaw model containing few parameters, then consider a high energy renormalisable model with a complete fourth family of vector-like fermions, where the chiral quark and lepton masses arise from a seesaw-like mechanism. With the inclusion of the fourth family, the lightest vector-like quark can contribute to the dark matter production, enlarging the allowed parameter space that we explore.
Highlights
A new version of the type I seesaw mechanism, named as the type Ib seesaw mechanism [56], that can be just as testable as the low scale seesaw models above has been proposed, with the light neutrino masses originating from a new type of Weinberg operator involving two Higgs doublets and a Dirac heavy neutrino
We consider the possibility that dark matter is stabilised by a discrete Z2 symmetry which arises from a subgroup of a U(1) gauge symmetry, spontaneously broken by integer charged scalars, and under which the chiral quarks and leptons do not carry any charges
We consider the possibility that dark matter is stabilised by a discrete Z2 symmetry which arises from a subgroup of a U(1) gauge symmetry, and under which the chiral quarks and leptons do not carry any charges
Summary
We introduce the U(1) extension of minimal type Ib seesaw model with a Majorana fermion singlet. The Yukawa interaction between charged fermions and Higgs doublets is forbidden by the U(1) symmetry. After φ gains a VEV φ = vφ/ 2, the Yukawa interactions generating fermion mass after spontaneous symmetry breaking (SSB) of Higgs doublets are. If the dark fermions share the same mass, either Dirac (mL,R = 0) or Majorana (mD = 0), the dark fermions are both stable and the predicted dark matter relic abundance is twice that in the case with hierarchical masses Between these two limits, the dark fermion masses could be quasidegenerate when mD mL,R = 0 or mL,R mD = 0 and the heavier dark fermion could have a long enough lifetime to play a role in dark matter production. Since the Higgs doublets are charged under the U(1) gauge symmetry, the mass of Z receives contributions from Φ1 and Φ2 after the electroweak (EW) symmetry breaking, which leads to mixing between the massive gauge bosons
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