Jets are one of the key probes of the quark-gluon plasma (QGP) produced in high energy heavy ion collisions at the LHC and RHIC, allowing to build a tomographic picture of the intermediate QCD states. Although the QGP phase dominates most of the evolution of the medium in the aftermath of heavy ion collisions, it is preceded by a short-lived out-of-equilibrium state. This matter state, generated in parallel to the initial jet production, is initially often characterized by strong (classical) fields and undergoes multiple (initial) stages during its equilibration process towards the QGP phase. Understanding this initial stage is of critical importance to characterize the rich many-body properties of QCD. 

 The goal of this Theory Institute is to advance our understanding of the early stages of heavy ion collisions using high-energy probes. We aim to: understand which theoretical elements are missing from both sides of the problem and how they can be incorporated in a consistent way, discuss and propose novel jet observables that can be highly sensitive to features of the early stages, while not being much affected by the evolution in the QGP phase, and establish connections to related problems, including small collisional systems, pre-thermalization dynamics in gauge theories, and heavy flavor transport at high momenta. The key questions to be addressed during the Institute are thus summarized as: 

• How are jets modified during the early stages: virtuality cascade, medium-induced modifications, conceptual and technical challenges.

• What class of jet (high-pt) observables do we need to explore the early stages?

• Common aspects with other problems: small systems, thermalization dynamics, polarization effects, and heavy flavors.