  A cosmological pseudoscalar field coupled to hypercharge topological number
density can exponentially amplify hyperelectric and hypermagnetic fields in the
symmetric phase of the electroweak plasma while coherently rolling or
oscillating, leading to the formation of a condensate of topological number
density. The topological number can be converted, under certain conditions,
into baryon number in sufficient amount to explain the observed baryon
asymmetry of the universe. We focus on a singlet elementary hypercharge axion
(HCA) whose only coupling to standard model fields is to hypercharge
topological number density. This model has two parameters: the mass $m$ and
decay constant $M$ of the HCA. We describe this new mechanism for baryogenesis
and outline the region of the parameter space in which the mechanism is
efficient. We show that present colliders can already put interesting
constraints on both parameters, and that future colliders will improve the
detection capabilities very significatively.
