On dark atoms, massive dark photons and millicharged sub-components

Felix Kahlhoefer,Einar Urdshals

doi:10.1016/j.physletb.2020.135601

Abstract

We present a simple model of two dark matter species with opposite millicharge that can form electrically neutral bound states via the exchange of a massive dark photon. If bound state formation is suppressed at low temperatures, a sub-dominant fraction of millicharged particles remains at late times, which can give rise to interesting features in the 21 cm absorption profile at cosmic dawn. The dominant neutral component, on the other hand, can have dipole interactions with ordinary matter, leading to non-standard signals in direct detection experiments. We identify the parameter regions predicting a percent-level ionisation fraction and study constraints from laboratory searches for dark matter scattering and dark photon decays.

Highlights

The defining property of dark matter (DM) is that it does not participate in electromagnetic interactions in the same way as visible matter
The cosmology of this so-called atomic DM has been studied in detail in the literature [12,13,14]
We focus on the process e + p → H + A where A is an on-shell dark photon

Summary

Introduction

The defining property of dark matter (DM) is that it does not participate in electromagnetic interactions in the same way as visible matter. The idea of a sub-component of millicharged DM has recently received substantial attention because such a species would have interactions with baryons that become stronger as the Universe cools down This makes it possible to satisfy constraints on DMbaryon interactions from the Cosmic Microwave Background (CMB), while allowing for observable effects at later times. We assume that dark protons and dark electrons interact with each other through the exchange of a dark photon A with fine structure constant αD, such that the bound-state formation cross section is proportional to αD2 The cosmology of this so-called atomic DM has been studied in detail in the literature [12,13,14].

Model set-up

Bound state formation

Direct detection

Discussion and outlook