Heavy-ion collisions that take place at the world’s leading accelerator facilities, such as LHC at CERN and RHIC at BNL, offer an exclusive glimpse of a state of nuclear matter in extreme conditions that governed an early phase of the cosmological evolution of our Universe. A well of evidence point to the importance of the dynamics of elementary degrees of freedom, which are called quarks and gluons and constitute the building blocks of nuclei. This co-called quark-gluon plasma has strikingly simple emergent properties and behaves similarly to an ideal fluid. However, a complete microscopic understanding of the dynamical processes responsible for this behavior is still up for debate. Insights on nuclear matter in extreme conditions is also crucial for understanding supernovae and neutron stars.
The project mainly launched a two-pronged theoretical investigation of quark-gluon dynamics in the context of nuclear collisions. One the one hand, we studied the impact of introducing strong-coupling effects into the sophisticated framework of kinetic theory by incorporating a modified gluon component. This introduces a mass-scale, ultimately related to gluon confinement, and breaks the conformal invariance of the system. We found a profound impact on the bulk viscosity of such a plasma which is strongly enhanced at temperatures that are relevant for collisions at the LHC.
On the other hand, we focused on exploring the production of highly energetic sprays of particles, so-called jets, that serve as important probes of the plasma. We improved on the theoretical foundations of in-medium jet fragmentation by computing the process of two-gluon emission. This gives the foundations for developing a theoretically well-motivated computer simulation, a so-called Monte Carlo shower algorithm, that can be tested against experimental data.
During the project period, we proposed to use jet substructure observables as new tools for extracting properties of the medium and learning about in-medium jet fragmentation. This generated a lot of interest both from the experimental side and in the wider high-energy physics community. These observables will also be very important in the future heavy-ion program at RHIC and LHC. In addition to the theoretical advances, a two-week CERN Theory Institute was organized that gathered more than 50 participants from all over the world discussing these new ideas and setting up a plan for future improvements and measurements.
The project also involved work within cosmology initiated at CERN, where we studied a novel way of achieving a rapid acceleration of the Universe by coupling the Standard Model to new particles.
