Heavy ion physics

Primordial matter, existing until ~10 microseconds after the Big Bang, was a Quark-Gluon Plasma (QGP), an extremely hot and dense phase of strongly interacting matter consisting of deconfined quarks and gluons. The QGP is recreated in ultra-relativistic heavy-ion collisions at the LHC.  The main physics objective of the ALICE experiment at the LHC is the detailed exploration of QGP properties, using as prime probes charmonia and other hadrons containing heavy quarks.

Charmonia, mesons built from a charm-anti-charm quark pair, are among the main probes of QGP properties, and display a complex and interesting behaviour in the nuclear medium. The data has shown strong indications of a novel QGP signature, charmonium production through a recombination mechanism, permitted by the high density of charm-anti-charm quark pairs in this unprecedentedly hot plasma and competing with thermal dissociation of the bound state. A particular challenge is the disentanglement of final-state (QGP) and initial-state (cold nuclear matter, CNM) medium effects. This can be done by comparison of data from different systems, where Pb-Pb, p-Pb and p-p collisions represent the hot QGP and the CNM / vacuum baseline, respectively.

The ALICE-Oslo group has extensive experience with charmonium physics and analysis. The charmonium state measured is the J/Psi meson reconstructed in its di-electron decay channel using ALICE charged particle tracking detectors.