Speaker
Description
While collision centrality for large heavy ion systems is well defined
both theoretically and in the experiments, the same is not true when
very asymmetric systems (like p/d+A) collide. In light of some surprising
and in part contradictory results on the centrality dependence of certain
observables in p/d+A collisions that emerged since 2012 from RHIC and LHC,
the very concept of the geometrically inspired centrality gave way to
"event activity". Several phenomenological models try to explain the
counterintuitive results, but convincing experimental tests of the
diverging concepts are so far missing. After a critical review of the
relevant results from SPS, RHIC and LHC we will discuss whether and how
the measurement of the ``centrality'' dependence of high and low
transverse momentum direct photons (pQCD and "thermal") in p/d+A
collisions can help us break the impass. We will also explore whether
there are any lessons to be learned with respect to extreme event classes
in p+p and A+A collisions.
