Unraveling the Physics of the Initial State of Heavy-Ion Collisions

One of the major uncertainties in the theoretical description of heavy ion collisions is the initial state, typically referring to how energy, momentum, and conserved charges are deposited in space immediately after the collision. Computing such initial states involves descriptions of the incoming nuclei at high energy, and a mechanism for energy, momentum and charge deposition. I will review the state of the art in modeling the initial state with focus on the color glass condensate effective theory, the use of Bayesian analyses to extract initial state properties, and how small systems can give additional insight into the details of the initial state. I will discuss how the increasing experimental precision in heavy ion collisions provides us with new access to nuclear structure at high energy, including detailed information on nuclear deformation and the size of fluctuating nuclear constituents. Finally, I will comment on the transition to hydrodynamics and what we know about the pre-equilibrium stage in heavy ion collisions.