  Black holes have a unique sensitivity to the presence of ultralight matter
fields or modifications of the underlying theory of gravity. In the present
paper we combine both features by studying an ultralight, dynamical scalar
field that is nonminimally coupled to the gravitational Chern-Simons term. In
particular, we numerically simulate the evolution of such a scalar field around
a rotating black hole in the decoupling approximation and find a new kind of
massive scalar hair anchored around the black hole. In the proximity of the
black hole, the scalar exhibits the typical dipolar structure of hairy
solutions in (massless) dynamical Chern-Simons gravity. At larger distances,
the field transitions to an oscillating scalar cloud that is induced by the
mass term. Finally, we complement the time-domain results with a spectral
analysis of the scalar field characteristic frequencies.

General Relativity and Quantum Cosmology;High Energy Physics - Phenomenology