  Ultrarelativistic collisions between heavy nuclei briefly generate the
quark-gluon plasma (QGP), a new state of matter characterized by deconfined
partons last seen microseconds after the Big Bang. The properties of the QGP
are of intense interest, and a large community has developed over several
decades, to produce, measure and understand this primordial plasma. The plasma
is now recognized to be a strongly-coupled fluid with remarkable properties,
and hydrodynamics is commonly used to quantify and model the system. An
important feature of any fluid is its vorticity, related to the local angular
momentum density; however, this degree of freedom has received relatively
little attention because no experimental signals of vorticity had been
detected. Thanks to recent high-statistics datasets from experiments with
precision tracking and complete kinemetic coverage at collider energies,
hyperon spin polarization measurements have begun to uncover the vorticity of
the QGP created at the Relativistic Heavy Ion Collider. The injection of this
new degree of freedom into a relatively mature field of research represents an
enormous opportunity to generate new insights into the physics of the QGP. The
community has responded with enthusiasm, and this book (to be published as a
volume of Lecture Notes in Physics series by Springer) represents some of the
diverse lines of inquiry into aspects of strongly interacting matter under
rotation.
