Dynamics of Dark Matter Misalignment Through the Higgs Portal

TL;DR

This paper investigates the cosmological production mechanisms of ultra-light scalar dark matter via the Higgs portal, analyzing how thermal and vacuum misalignment processes influence relic abundance across different mass ranges.

Contribution

It introduces a detailed analysis of scalar dark matter production through thermal and VEV misalignment, revealing novel resonance effects and mass-dependent relic abundance predictions.

Findings

Thermal misalignment dominates for masses above 10^{-3} eV, leading to robust relic predictions.

VEV misalignment influences low-mass scalars below 10^{-5} eV, sensitive to initial conditions.

Intermediate masses exhibit resonance effects causing oscillation amplitude variations.

Abstract

A light singlet scalar field feebly coupled through the super-renormalizable Higgs portal provides a minimal and well-motivated realization of ultra-light bosonic dark matter. We study the cosmological production of dark matter in this model by elucidating the dynamics of two sources of scalar field misalignment generated during the radiation era. For large scalar masses (above order $10^{-3}\,{\rm eV}$), dark matter is produced through thermal misalignment, by which the scalar field is driven towards large field values as a result of the finite-temperature effective potential. The dominance of thermal misalignment in this mass range leads to a sharp relic abundance prediction which is, to a significant extent, insensitive to the initial conditions of the scalar field. On the other hand, for low mass scalars (below order $10^{-5}\,{\rm eV}$), dark matter is produced via VEV misalignment, which is caused by the induced scalar field vacuum expectation value triggered by the electroweak phase transition. We show that the relic abundance in this low mass range is sensitive to the scalar field initial conditions. In the intermediate mass range, the relic abundance is a consequence of a competition between thermal misalignment and VEV misalignment, leading to novel forced resonance effects which cause a recurring enhancement and suppression in the late time oscillation amplitude as a function of the scalar mass. We compare our relic abundance predictions with constraints and projections from equivalence principle and inverse square law tests, stellar cooling, resonant molecular absorption, and observations of extra-galactic background light and diffuse X-ray backgrounds. New experimental ideas are needed to probe most of the cosmologically motivated regions of parameter space.