Dark matter from CP symmetry of order 4: evolution in the asymmetric regime

Abstract

Multi-Higgs models equipped with global symmetries produce scalar dark matter (DM) candidates stabilized by the unbroken symmetry. It is remarkable that a conserved CP symmetry can also stabilize DM candidates, provided it is a CP symmetry of order higher than two. CP4 3HDM, the three-Higgs-doublet model with CP symmetry of order 4, is the simplest example of this kind. It contains two mass-degenerate scalar DM candidates φ and φ̄, each of them being a CP4 eigenstate and, therefore, its own antiparticle. A novel phenomenological feature of this model is the presence of φφ → φ̄φ̄ conversion process, which conserves CP. It offers a rare example of DM models in which self-interaction in the dark sector can significantly affect cosmological and astrophysical observables. Here, we explore the thermal evolution of these DM species in the asymmetric regime. We assume that a mechanism external to CP4 3HDM produces an initial imbalance of the densities of φ and φ̄. As the Universe cools down, we track the evolution of the asymmetry through different stages, and determine how the final asymmetry depends on the interplay between the conversion and annihilation φφ̄ → SM and on the initial conditions. We begin with the analytic treatment of Boltzmann equations, present a detailed qualitative description of the process, and then corroborate it with numerical results obtained using a dedicated computer code. Finally, we check if the model can produce an observable indirect detection signal.