In the first few microseconds of its existence the Universe was filled with an extremely hot medium called “quark-gluon plasma” (QGP). As the QGP cooled down, it eventually converted into bound states called hadrons, such as protons and neutrons, which later formed atomic nuclei, the building blocks of the visible matter in today’s Universe. Modern-day experiments where atomic nuclei are accelerated and collided at high energies, such as the Relativistic Heavy-Ion Collider (RHIC) at Brookhaven Lab in the US and the Large Hadron Collider (LHC) at CERN in Europe, provide the fascinating opportunity to recreate the QGP in the laboratory, 14 billion years after it last existed, while future experiments at the Electron Ion Collider (EIC) in the US will study fundamental aspects of cold nuclear matter. The QGP created in the laboratory is very short-lived, and its properties have to be inferred from the thousands of particles that are produced and subsequently measured in large detectors. Hydrodynamic simulations of the expansion dynamics of the QGP fireballs have found that it behaves as a “nearly perfect liquid”, with a record low value for the ratio of viscosity over entropy density, close to a conjectured lower bound set by quantum mechanics. While the fundamental theory of the strong nuclear forces, Quantum Chromodynamics (QCD), is well established, the emergence of the properties of QCD matter from its fundamental interactions remains a great challenge in modern research.
The heavy charm and bottom quarks (referred to as ”heavy flavor”), which are produced upon initial impact of the colliding nuclei, are premier probes of the QGP: their large masses cause a diffusive motion that can serve as a “Brownian” marker of the interactions in the QGP, including the fundamental process of the quark binding into hadrons. However, a reliable analysis of heavy-flavor measurements requires a broad knowledge of the various processes that heavy quarks undergo throughout a nuclear collision until their final observation in experiment. The objective of this collaboration is to combine the capabilities of leading US researchers in the field to develop a rigorous and comprehensive theoretical framework to describe heavy flavors in QCD matter, from the initial production of heavy quarks when the nuclei first collide, their subsequent diffusion through the QGP and their hadronization into heavy-flavor particles. This framework will be embedded into realistic numerical simulations that enable quantitative comparisons to experimental data.
The methods employed include first-principles lattice-QCD computations of novel heavy-quark operators in hot QCD matter, analyzed using heavy-quark effective field theories and quantum many-body theory to derive transport coefficients, quantum transport approaches combined with rigorous statistical data analysis, and advanced perturbative calculations for initial heavy-quark production. The resulting framework will enable accurate assessments of QGP transport properties and unravel the underlying microscopic processes driving them, thereby providing unprecedented insights into the inner workings of the QGP based on QCD. Phenomenological analyses of experimental data will take full advantage of the upcoming precision-measurement campaigns by the sPHENIX collaboration at RHIC and dedicated upgrade programs at the LHC, and develop connections to heavy-flavor probes at the future EIC. The RHIC and LHC programs place great emphasis on bottom-quark observables as its large mass allows for a greater theoretical precision than in the charm sector. Junior researchers will be hired to achieve these scientific goals, synergizing the expertise of the senior personnel. A diverse working environment of mutual respect and individual responsibility will be created, enabling the junior researchers to grow into future leaders of the field.