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Probing the Strongly-Coupled Quark-Gluon Plasma with Jets

Periodic Reporting for period 1 - ResolvedJetsHIC (Probing the Strongly-Coupled Quark-Gluon Plasma with Jets)

Reporting period: 2015-11-01 to 2017-10-31

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.
The work performed in course of the project involves theoretical calculations for a wide range of topics. We have studied material properties (transport coefficients) of a plasma consisting of gluons that are sensitive to confinement effects. We have developed calculations of multi-particle emissions in the plasma and were centrally involved in the start of studying QCD jet substructure observables in heavy-ion collisions. In particular, this involved developing a new formalism for dealing with energy loss of a set of correlated particles moving simultaneously through the medium. The principal investigator (PI) and collaborators has proposed new observables that influence experimental searches. Finally, we have also proposed a new mechanism for an accelerated epoch of the cosmological evolution called Inflation through a novel coupling to new microscopic degrees of freedom. This has resulted in a total of 13 publications (including 5 conference proceedings).

In addition to the theoretical work, the PI has also actively engaged in disseminating the new results in the wider (high-energy) physics community through participation in international workshops and conferences (11 talks in total, including 3 plenary), seminars and colloquia (8 in total) and by giving lectures at international PhD schools (3 in total). Most importantly, he was the main organizer of a two-week CERN Theory Institute (combined with the 5th Heavy Ion Jet Workshop) in August 2017. The meeting is the first of its kind, where theorists and experimentalists work together on a final report outlining a novel strategy for future jet observables (to appear shortly). A follow-up workshop is planned for 2018 at CERN.

Finally, the PI has been active in the CERN outreach activities, delivering approximately 20 outreach talks for high-school students and teachers from Norway, Finland, Singapore and Thailand. He has also initiated a collaboration with published comic artist Håvard S. Johansen on a (web-)comic related to CERN activities in Norwegian (see enclosed example illustration).
The work at CERN facilitated by the grant has allowed to establish a new theoretical formalism for treating jets in heavy-ion collisions. This lead to seminal contributions on substructure observables that will be increasingly important for future theoretical and experimental studies at RHIC and LHC and opens the way to establish more precise computer simulations of jet processes. Finally, this can also impact our understanding of how collective features of the quark-gluon plasma arises from microscopic dynamics.
Example page from outreach program.