Abstract
We study the superheavy dark matter (DM) scenario in an extended B−L model, where one generation of right-handed neutrino νR is the DM candidate. If there is a new lighter sterile neutrino that co-annihilate with the DM candidate, then the annihilation rate is exponentially enhanced, allowing a DM mass much heavier than the Griest-Kamionkowski bound (∼105 GeV). We demonstrate that a DM mass MνR ≳ 1013 GeV can be achieved. Although beyond the scale of any traditional DM searching strategy, this scenario is testable via gravitational waves (GWs) emitted by the cosmic strings from the U(1)B−L breaking. Quantitative calculations show that the DM mass mathcal{O} (109−1013 GeV) can be probed by future GW detectors.
Highlights
As proposed in refs. [33, 34], dark matter (DM) mass beyond the GK bound is possible within the thermal freeze-out framework, as long as there is a lighter unstable species that co-annihilates with the DM candidate and exponentially enhances the interaction rate
We study the superheavy dark matter (DM) scenario in an extended B−L model, where one generation of right-handed neutrino νR is the DM candidate
We propose a zombie annihilation mechanism that is associated with the breaking of a U(1) symmetry, which leads to the formation of cosmic strings that can be detected via the gravitational wave (GW) signals at current or future gravitational waves (GWs) detectors
Summary
Assume the reheating temperature after inflation is higher than MνR and both νR and ψ are originally in thermal equilibrium with each other and the SM particles, and the number densities are described by neαq = 2. When the interaction rate is lower than the Universe expansion rate, νR deviates from the chemical equilibrium and eventually freeze-out to be the DM candidate. The freeze-out process can be characterized by a set of Boltzmann equations, which are discussed in detail below. We define the particle abundance as Yα = nα/s, i.e. the ratio of number density to the entropy density, and the abundances of the equilibrium distributions are. We assume Mφ, MZ MνR so that the B − L scalar φ and gauge boson Z do not participate in the Boltzmann equations explicitly, but Z contribute to γνRνR→ffand γψψ→ff (where f denotes the SM fermions in equilibrium) via the off-shell s-channel diagrams. The late time ψ decay after νR freeze-out plays an important role, as discussed in the subsection
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