Unfolding the Time Structure of Quantum Chromodynamics in Heavy-Ion Collisions

Ultra-relativistic heavy-ion collisions recreate, for fleeting instants, the hottest and densest conditions known in the Universe — a phase where quarks and gluons exist in a strongly-coupled state known as the Quark–Gluon Plasma (QGP). These environments, produced at facilities like the Large Hadron Collider (CERN) and the Relativistic Heavy Ion Collider (Brookhaven National Laboratory), allow us to reveal fundamental aspects of Quantum Chromodynamics (QCD), the theory of the strong interaction. Jets, collimated sprays of particles initiated by high-energy quarks or gluons, are among the most powerful probes, capable of traversing the evolving QGP and interacting with its constituents.


Traditionally studied in energy-momentum space and regarded as mathematical tools rather than physical entities, jets were long considered to be virtual constructs. Yet recent theoretical and phenomenological efforts have revealed that, in the presence of a medium, jets acquire real space-time features, a new physical layer that leave measurable imprints on experimental observables. This interaction framework renders previously unobservable quantities — such as the time at which a parton splits — physically meaningful. Rather than treating the QGP as a static modifier of jet properties, this perspective redefines jet evolution as a process intrinsically shaped by the medium's own time evolution.


In this talk, I will present the key developments that have led to this new perspective, including how formation-time-sensitive observables can be used to map the unfolding of the parton shower in time. I will also discuss how this approach enables us to revisit jet quenching from a dynamical, space-time viewpoint, and even test the onset of QGP formation in small systems. This shift, from virtual to time-resolved jets, introduces a novel axis for QCD studies, one that is only now beginning to be explored.