Ultralight Fermionic Dark Matter

TL;DR

This paper proposes a scenario where numerous fermionic dark matter species evade traditional mass bounds, leading to relaxed constraints on dark matter mass and implications for particle physics and cosmology.

Contribution

It introduces a model with many fermionic species that avoids the Tremaine-Gunn bound, analyzing gravitational and collider constraints on their properties.

Findings

LHC constrains the number of light species to N ≲ 10^62.

Degenerate fermionic dark matter can be heavier than ~10^{-14} eV.

Constraints from cosmic rays and black hole observations limit particle masses.

Abstract

Conventional lore from Tremaine and Gunn excludes fermionic dark matter lighter than a few hundred eV, based on the Pauli exclusion principle. We highlight a simple way of evading this bound with a large number of species that leads to numerous non-trivial consequences. In this scenario there are many distinct species of fermions with quasi-degenerate masses and no couplings to the standard model. Nonetheless, gravitational interactions lead to constraints from measurements at the LHC, of cosmic rays, of supernovae, and of black hole spins and lifetimes. We find that the LHC constrains the number of distinct species, bosons or fermions lighter than $\sim 500$ GeV, to be $N \lesssim 10^{62}$. This, in particular, implies that roughly degenerate fermionic dark matter must be heavier than $\sim 10^{-14}$ eV, which thus relaxes the Tremaine-Gunn bound by $\sim 16$ orders of magnitude. Slightly weaker constraints applying to masses up to $\sim100$ TeV exist from cosmic ray measurements while various constraints on masses $\lesssim10^{-10}$ eV apply from black hole observations. We consider a variety of phenomenological bounds on the number of species of particles. Finally, we note that there exist theoretical considerations regarding quantum gravity which could impose more severe constraints that may limit the number of physical states to $N\lesssim 10^{32}$.