Our understanding of QGP behaviour was overthrown in 2010 when the LHC started. Until then, the QGP was studied by comparing particle properties in heavy systems and in lighter systems where the QGP is a priori not produced. Proton-proton (pp) and proton-lead (p-Pb) collisions were then considered as references. Surprisingly, at LHC energies, in a small fraction of these collisions the number of produced particles is similar to that in nucleus-nucleus collisions at lower energies where the QGP is observed. Even more surprisingly, emblematic signatures of QGP formation were observed in lighter systems. The burning question is how the QGP can be formed in small systems? And the corollary, what mechanisms are involved in the initial state of the collision that creates a high enough temperature for the phase transition of nuclear matter?
One of the best initial state candidates is the multi-parton interaction mechanism where several individual interactions between quarks and gluons in proton-proton collisions can happen in parallel. This mechanism is able to explain the observed particle multiplicity in the final state which can eventually be related to the initial energy density of the collision but is still to be investigated theoretically and experimentally, especially the connection with Quarkonia. Quarkonia are particles made of one heavy quark and one heavy anti-quark with the same flavour, charm or beauty, like the J/Psi or the Upsilons. Studying correlations between quarkonium production and charged particle multiplicity is a key to understand the impact of multi-parton interactions in the initial stage of hadronic collisions.
This seminar will give an overview on these open questions and recent measurements and will try to address the potential impact on studies carried out at the LHC.