Collectivity in small, srongly interacting systems

ended

In collisions of heavy nuclei at collider energies, for instance at the Large Hadron Collider (LHC) at CERN, the energy density is so high that an equilibrated Quark-Gluon Plasma (QGP), an exotic state of matter consisting of deconfined quarks and gluons, is formed. In proton-proton (p+p) collisions, on the other hand, the density of produced particles is low. The traditional view on such reactions is that final state particles are free and do not rescatter. This picture is challenged by recent LHC data, which found features in p+p collisions that are indicative of collective behaviour and/or the formation of a hot and dense system. These findings have been taken as signs of QGP formation in p+p reactions. Such an interpretation is complicated by the fact that jets, which are the manifestation of very energetic quarks and gluons, are quenched in heavy ion collisions, but appear to be unmodified in p+p reactions. This is puzzling because collectivity and jet quenching are caused by the same processes. So far there is no consensus about the interpretation of these results, which is also due to a lack of suitable tools. It is the objective of this proposal to address the question whether there are collective effects in p+p collisions. To this end two models capable of describing all relevant aspects of p+p and heavy ion collisions will be developed. They will be obtained by extending a successful description of p+p to heavy ion reactions and vice versa. The answer to these questions will either clarify the long-standing problem how collectivity emerges from fundamental interactions, or it will necessitate qualitative changes to our interpretation of collective phenomena in p+p and/or heavy ion collisions. The PI is in a unique position to accomplish this goal, as she has spent her entire career working on different aspects of p+p and heavy ion collisions. The group in Lund is the ideal host, as it is very active in developing alternative interpretations of the data.