D-brane Disformal Coupling and Thermal Dark Matter

TL;DR

This paper investigates how D-brane-inspired conformal and disformal couplings influence the early universe's expansion rate, affecting dark matter relic abundance and annihilation, with implications for future experimental detection.

Contribution

It introduces a detailed analysis of D-brane-like couplings' effects on cosmological expansion and dark matter, highlighting the impact of disformal couplings on relic abundance predictions.

Findings

Disformal coupling always enhances the expansion rate before BBN.

Enhanced expansion rate leads to larger dark matter annihilation cross-sections.

The scale of enhancement depends on string coupling and string scale.

Abstract

Conformal and Disformal couplings between a scalar field and matter occur naturally in general scalar-tensor theories. In D-brane models of cosmology and particle physics, these couplings originate from the D-brane action describing the dynamics of its transverse (the scalar) and longitudinal (matter) fluctuations, which are thus coupled. During the post-inflationary regime and before the onset of big-bang nucleosynthesis (BBN), these couplings can modify the expansion rate felt by matter, changing the predictions for the thermal relic abundance of dark matter particles and thus the annihilation rate required to satisfy the dark matter content today. We study the D-brane-like conformal and disformal couplings effect on the expansion rate of the universe prior to BBN and its impact on the dark matter relic abundance and annihilation rate. For a purely disformal coupling, the expansion rate is always enhanced with respect to the standard one. This gives rise to larger cross-sections when compared to the standard thermal prediction for a range of dark matter masses, which will be probed by future experiments. In a D-brane-like scenario, the scale at which the expansion rate enhancement occurs depends on the string coupling and the string scale.