Development of Event Generators for Heavy Ion Collisions

It is generally agreed that in high energy collisions between heavyions (HI), it is possible to create and study the phase of matterknown as the Quark-Gluon Plasma (QGP), which we assume was the state ofthe Universe around a microsecond after the Big Bang.Since the observation of the near-side ridge in proton-proton(pp) collisions by the CMS experiment in 2010, it has become clearthat some of the collective effects used as evidence for a QGP in HIcollisions, are also present in high-multiplicity pp collisions.

Thisrequires us to ask ourselves the question if a thermalised QGP is alsoformed in pp, or if there could be another explanation for theobserved phenomena in both kinds of collisions.In this project we want to follow the latter avenue, by exploringother models for describing the measurements, based on an extension ofthe Lund string fragmentation that includes interactions betweenstrings in a dense environment.

So far we have shown how the so-calledshoving and rope models implemented in the Pythia generator candescribe some of the key measurements in pp.

Now we want to developthese models so that they can be used together with our recentlyimplemented framework for generating HI collisions in Pythia, calledAngantyr.

If we in this way also can describe observables ofcollectivity in HI collisions, it would cast serious doubts on thecurrent consensus in the HI community, that a thermalised QGP can bestudied in such collisions.