Strongly-Interacting Ultralight Millicharged Particles

TL;DR

This paper explores a novel model of ultra-light, strongly-interacting dark matter composed of dark baryons with baryon number, which can form dense cores in halos and address the core-cusp problem, differing from bosonic dark matter.

Contribution

It introduces a dark QCD-based model of ultra-light fermionic dark matter with strong self-interactions, providing astrophysical solutions to small-scale structure issues.

Findings

Dark matter halos can have dense, neutron star-like cores.

Strong self-interactions violate traditional fermionic mass bounds.

Distinct detection signatures from ultra-light bosonic dark matter.

Abstract

We consider the implications of an ultra-light fermionic dark matter candidate that carries baryon number. This naturally arises if dark matter has a small charge under standard model baryon number whilst having an asymmetry equal and opposite to that in the visible universe. A prototypical model is a theory of dark baryons of a non-Abelian gauge group, i.e., a dark Quantum Chromo-Dynamics (QCD). For sub-eV dark baryon masses, the inner region of dark matter halos is naturally at 'nuclear density', allowing for the formation of exotic states of matter, akin to neutron stars. The Tremaine-Gunn lower bound on the mass of fermionic dark matter, i.e., the dark baryons, is violated by the strong short-range self-interactions, cooling via emission of light dark pions, and the Cooper pairing of dark quarks that occurs at densities that are high relative to the (ultra-low) dark QCD scale. We develop the astrophysics of these STrongly-interacting Ultra-light Millicharged Particles (STUMPs) utilizing the equation of state of dense quark matter, and find halo cores consistent with observations of dwarf galaxies. These cores are prevented from core-collapse by pressure of the 'neutron star', which suggests ultra-light dark QCD as a resolution to core-cusp problem of collisionless cold dark matter. The model is distinguished from ultra-light bosonic dark matter through direct detection and collider signatures, as well as by phenomena associated with superconductivity, such as Andreev reflection and superconducting vortices.