Constraints on the cosmic string loop collapse fraction from Primordial
  Black Holes
  A small fraction, $f$, of cosmic string loops can collapse to form Primordial
Black Holes (PBHs). Constraints on the abundance of PBHs can therefore be used
to constrain $f$. We update these calculations, taking into account the PBH
extended mass function, and find $f < 10^{-31} (G \mu/ c^2)^{-3/2}$. This is
roughly two orders of magnitude tighter than previous constraints. The
improvement from the tighter constraints on the abundance of PBHs is partly
off-set by refinements to the theoretical calculation of the cosmic string loop
formation rate.
