Do primordial quark pellets solve the dark matter puzzle?

Abstract

We show that primordial quark pellets (PQP), ultra dense quark matter mini-stars formed at around 1 TeV, naturally arise in a radiation dominated universe if rare baryon overdensities are produced by collapsing Peccei-Quinn domain walls or similar superhorizon structures. Solving the Tolman-Oppenheimer-Volkoff equation with a hot quark equation of state, we find stable solutions with a maximum mass of $10^{-2}$ solar masses and radii of approximately 100 meters, although the formation mechanism favours much smaller objects. Once formed, PQPs cool and evolve intro mini neutron stars or stable strange matter nuggets, depending on the QCD ground state. Formation probabilities of $10^{-9}$ to $10^{-4}$ per horizon volume could suffice to reproduce the present dark matter density without altering Big Bang Nucleosynthesis or requiring entropy dilution. PQPs completely evade microlensing constraints if their mass is below $10^{-7}$ solar masses, a range that could easily accommodate most of the dark matter. PQPs thus potentially constitute a conservative, Standard-Model based, and observationally testable solution to the dark matter puzzle.