Superradiant pion clouds around primordial black holes
  We show that highly spinning primordial black holes of mass $M\sim 10^{12}$
kg, potentially born in a matter-dominated era after inflation, can produce
clouds of pions in their vicinity via the superradiant instability, with
densities up to that of nuclear matter. We discuss the electromagnetic
signatures of this process, via neutral pion decay and charged pion
annihilation into photons, computing in particular their contribution to the
isotropic gamma-ray background. This allows us to place upper bounds on the
abundance of such primordial black holes that are comparable to the ones
obtained from Hawking evaporation. We also discuss the possibility of directly
observing such clouds in high-redshift superclusters.
