Jet quenching for heavy-ion collisions at the LHC

Informations projet

JQ4LHC

N° de convention de subvention: 893021

DOI

10.3030/893021

Projet clôturé

Date de signature de la CE 4 Mars 2020

Date de début 1 Septembre 2021

Date de fin 31 Août 2023

Financé au titre de

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Coût total

€ 196 707,84

Contribution de l’UE

€ 196 707,84

Coordonné par

ECOLE POLYTECHNIQUE
France

Periodic Reporting for period 1 - JQ4LHC (Jet quenching for heavy-ion collisions at the LHC)

Période du rapport: 2021-09-01 au 2023-08-31

Résumé du contexte et des objectifs généraux du projet

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.

Travail effectué depuis le début du projet jusqu’à la fin de la période considérée dans le rapport et principaux résultats atteints jusqu’à présent

JQ4LHC builds upon a recent approach to the radiation process of jets induced by the QGP, which considers multiple scatterings between high-energy partons and the medium. The project has incorporated the longitudinal evolution of the QGP into this framework. To address the high computational demands inherent in evaluating these calculations within a dynamically evolving medium, JQ4LHC introduced a series of matching relations. These relations facilitate the pre-evaluation of calculations in specific simplified setups, significantly streamlining their practical implementation. Importantly, the accuracy of these matching relations was rigorously quantified. Moreover, JQ4LHC has upgraded this formalism through QCD analytical calculations to account for the transverse expansion of the QGP.

This formalism was also generalized to encompass the propagation of the hard parton in the initial stages, accounting from extra gluon emissions that occur before the formation of the QGP. Then, the impact of these additional emissions on the description of a pair of jet quenching observables was assessed. The results confirmed the crucial role of the treatment of jet quenching during the initial stages, not only for correctly describing these observables but also for determining the jet quenching parameter's extracted value.

To address the color coherence dynamics, JQ4LHC introduced the energy correlator approach to heavy-ion physics. This innovative strategy consists of examining the angular correlations between the energy deposited by the particles within the jets in the detectors, instead of reconstructing the full jet branching. By deriving and computing the two-point energy correlator within a heavy-ion jet in perturbative QCD, JQ4LHC has shown the potential of this observable to isolate the dynamics of color coherence.

These outcomes have been disseminated through scientific articles, including four published peer-reviewed papers, and two preprints currently undergoing review. Additionally, these findings were presented in various international conferences and workshops, contributing to the broader dissemination of this research across the scientific community.

Progrès au-delà de l’état des connaissances et impact potentiel prévu (y compris l’impact socio-économique et les conséquences sociétales plus larges du projet jusqu’à présent)

The perturbative QCD formalism developed in JQ4LHC stands as the forefront of the developments on energy loss calculations in jet quenching physics. It also lays the foundations for a full description of the propagation of the jets through the whole system evolution, including the initial stages. Furthermore, the introduced energy correlator approach represents a completely novel strategy to the study of the heavy-ion jets inner structure. This approach has already garnered substantial interest within the heavy-ion community and is expected to significantly impact the study of jet quenching physics in the years to come.

Specifically, JQ4LHC has achieved the following advancements beyond the sate of the art:

- The first complete evaluation of the BDMPS-Z framework with multiple scatterings without the use of the Harmonic Oscillator approximation in dynamically evolving media.
- The incorporation of the emitter's propagation and corresponding radiation in the stages before the formation of the QGP.
- The introduction and calculation of the two-point energy correlator of heavy-ion jets.

with the following impact:

- Significantly advancing our knowledge and understanding of the medium-induced radiation process, which is the main contribution to the energy loss experienced by jets when traversing a colored medium
- Introducing a completely novel tool, energy correlators, to study jet substructure in heavy-ion collisions.

Given the fundamental and theoretical nature of the project, its primary focus is on advancing knowledge. As such, no other immediate socio-economic implications are anticipated in the short term.

Heavy-ion jet