JQ4LHC · Jet quenching for heavy-ion collisions at the LHC

Jet quenching for heavy-ion collisions at the LHC

Quantum Chromodynamics (QCD) is the universally recognized theory that describes the strong interaction in terms of quarks and gluons, collectively known as partons. Under normal conditions, partons remain confined within protons and neutrons. However, when subjected to extraordinarily high temperatures and densities, partons are freed from their confinement, giving rise to an exotic state of matter known as the quark-gluon plasma (QGP). The QGP filled the early Universe during the first microseconds after the Big Bang and is nowadays recreated in the laboratory by colliding heavy atomic nuclei at ultrarelativistic velocities. The detailed characterization and comprehensive understanding of the QGP are the primary objectives of the ongoing heavy-ion programs at the Relativistic Heavy-Ion Collider (RHIC) at BNL (USA) and at the Large Hadron Collider (LHC) at CERN. JQ4LHC was led by Carlota Andres, under the mentorship of Cyrille Marquet, at the Centre de Physique Theorique (CPHT) at Ecole Polytechnique (France) and aims at improving the general understanding of several phenomena being measured in such collisions. This type of research in fundamental physics not only expands our knowledge of the universe, but often fuels innovation, fostering the development of technologies that can have important transformative effects on society. Due to the extremely short lifetime of the QGP produced in heavy-ion colliders, it is not possible to use external probes to probe its dynamics. Fortunately, as in proton-proton collisions, high transverse momentum scatterings can occur in heavy-ion collisions, resulting in highly-energy parton pairs moving in opposite directions. In a p-p collision, each parton emits gluons at small angles ultimately fragmenting into a collimated spray of hadrons, which we refer to as a jet. In heavy-ion collisions, the partons within the jets traverse the QGP, interacting with it through the strong force. Such interactions induce modifications in heavy-ion jets compared to p-p jets.This phenomenon, known as jet quenching, is an excellent tool for probing and characterizing the properties of the QGP. Describing heavy-ion jets and their interactions with the QGP poses significant technical challenges. Consequently, current jet quenching Monte Carlo approaches include a fair amount on modeling, making it difficult to drawing definitive conclusions about the QGP dynamics. JQ4LHC implements a bottom-up semi-analytical framework for jet quenching based on perturbative calculations in QCD, tailored for phenomenological applications. This framework aims to offer an alternative perspective to Monte Carlo approaches, serving as a complementary method in the study of jet quenching phenomena. The theory of jet quenching usually neglects the propagation of the jets during the stages of the system prior to the formation of the QGP, known as initial stages. JQ4LHC extended this theoretical framework to account for the propagation of the initial high-energy parton through these initial stages. Furthermore, JQ4LHC has delved into the dynamics of color coherence, a phenomenon anticipated to significantly impact the fragmentation and evolution of heavy-ion jets within the QGP. A new set of jet observables, known as energy correlators, were proposed, showcasing their potential to isolate the long-sough-after color coherence dynamics.

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