  We argue that, because of the peculiar properties of the $\eta'$ meson, it is
a promising probe of "chiral" dynamics. In particular, we show that a rotating
gluon-dominated plasma might lead to an enhanced production of $\eta'$ w.r.t.
statistical model expectations. The presence of a strong topological
susceptibility might give a similar effect. In both cases, unlike the
statistical model,we expect a non-trivial dependence on event geometry, such as
initial volume and impact parameter.
  Hence, an observation of $\eta'/\pi^0$ ratio depending strongly on impact
parameter might be a good indication of chiral effects, either from vorticity
or topological phases of QCD
