Ultralight Dilatonic Dark Matter

TL;DR

This paper investigates ultralight dilatonic dark matter, exploring its theoretical protection via supersymmetry, and finds that experimental detection is extremely challenging due to tiny couplings.

Contribution

It introduces a novel dilaton stabilization mechanism and analyzes the implications of supersymmetry for ultralight dilaton dark matter.

Findings

Dilaton mass can range from 10^{-11} to 1 eV via misalignment.

Supersymmetry can protect the hierarchy but leads to negligible dilaton couplings.

Experimental detection of such dilatons is beyond current and future capabilities.

Abstract

The dilaton, a pseudo-Nambu-Goldstone boson (pNGB) of broken scale invariance, is an appealing ultralight dark matter (DM) candidate. Its mass is protected by conformal invariance and it can be searched for in tabletop experiments. However, contrary to standard pNGBs of internal symmetries, the dilaton generically has a large non-derivative self-coupling, leading to radiative contributions to its mass of the order of its decay constant. Hence typical ultralight dilatons should also have sub-eV decay constants, which would incur significant deviations from standard DM behavior at structure formation times, in severe tension with observations. Therefore, a fine-tuning is required to generate a hierarchy between the mass and the decay constant. In this work, we consider whether supersymmetry (SUSY) can be used to protect this hierarchy from quantum corrections. To ensure an ultralight dilaton mass robust against realistic SUSY-breaking contributions, we must consider a novel dilaton stabilization mechanism. The observed DM abundance can be produced by the misalignment mechanism for dilaton masses ranging from $10^{-11}$ to $1$ eV. Unfortunately, irreducible SUSY-breaking corrections due to gravity restrict the couplings between the dilaton and the Standard Model to be extremely small, beyond the reach of any current or proposed experiments. Our work demonstrates that constructing a consistent model of ultralight dilaton DM is quite involved.