  We study spherical collapse of a scalar cloud in scalar-Gauss-Bonnet gravity
- a theory in which black holes can develop scalar hair if they are in a
certain mass range. We show that an additional quadratic coupling of the scalar
field to the Ricci scalar can mitigate loss of hyperbolicity problems that have
plagued previous numerical collapse studies and instead lead to well-posed
evolution. This suggests that including specific additional interactions can be
a successful strategy for tackling well-posedness problems in effective field
theories of gravity with nonminimally coupled scalars. Our simulations also
show that spherical collapse leads to black holes with scalar hair when their
mass is below a mass threshold and above a minimum mass bound and that above
the mass threshold the collapse leads to black holes without hair, in line with
results in the static case and perturbative analyses. For masses below the
minimum mass bound we find that the scalar cloud smoothly dissipates, leaving
behind flat space.
