Studying the QGP with Jets at the LHC and RHIC

doi:10.1016/j.ppnp.2022.103940

Progress in Particle and Nuclear Physics

Volume 124, May 2022, 103940

https://doi.org/10.1016/j.ppnp.2022.103940 Get rights and content

Abstract

We review the current status of jet measurements in heavy-ion collisions at the Large Hadron Collider (LHC) and the Relativistic Heavy Ion Collider (RHIC). We discuss how the current measurements provide information about the quark-gluon plasma and discuss near future opportunities at both RHIC and the LHC.

Introduction

The main goal of the heavy-ion physics program at the Large Hadron Collider (LHC) and the Relativistic Heavy Ion Collider (RHIC) is to study quantum chromodynamics (QCD) at extremely high temperature. In order to do this, heavy nuclei are collided at ultrarelativistic energies. As the nuclei pass through each other, a region of extremely large energy density is created (greater than $12$ GeV/fm³ 1 fm after the collision [1]) and this results in the creation of matter known as the quark–gluon plasma (QGP) [2], [3], [4], [5], [6]. A major discovery of the RHIC and LHC experimental programs is that the QGP is well described as a nearly ideal fluid [7] with a maximal temperature of at least 300 MeV [8]. This strongly coupled fluid exhibits a viscosity to entropy ratio near the conjectured lower limit of $\frac{1}{4 π}$ [9], expected for quantum fluids [10] that can be described in picture of holographic duality between field theory and classical gravity [11].

A key aim is to understand how such a strongly correlated liquid arises from the underlying theory, QCD, and its degrees of freedom, the quarks and gluons. Jet measurements in heavy-ion collisions are of great interest to study the microscopic structure of the QGP liquid. Since jets are multi-scale objects, they probe the QGP at varying length scales. Jets have been identified as central to understanding the nature of the interactions which give rise to the fluid-like behavior of the QGP [12], [13].

Jets in hadronic collisions are formed by the point-like scattering of quarks and/or gluons. Jets are well-defined objects in QCD and are under good theoretical and experimental control in $p p$ collisions (see e.g Refs. [14], [15], [16], [17]). In heavy-ion collisions, the hard, elementary scatterings leading to jet production occur in the early stages of the collision. The evolution of the scattered quark or gluon towards hadronisation is embedded with and interacts with the evolving QGP medium, and is thus subject to modifications relative to $p p$ collisions.

The first description of the propagation of an energetic parton (quark or gluon) in the QGP appeared in Ref. [18]. Further studies identified the dominant mechanism of energy loss for high-energy partons to be gluon radiation induced by the QGP [19], [20], [21]. QGP-induced modifications to jet properties are generically called jet quenching because the most direct consequence of parton energy loss in the QGP is the reduced energy of jets, resulting in a reduced number of reconstructed hadrons and jets in heavy-ion collisions at a fixed momentum compared to expectations from $p p$ collisions.

Jet measurements in heavy ion collisions, as we will discuss in the next sections, attempt to capture the full dynamics of jet quenching across different jet radii, collision geometry and energy. They comprise survival rates constructed as ratios of jet (or hadrons from jets) cross sections relative to expectations based on $p p$ collisions as well as the jet radius, $R$ , dependence of jet cross sections, inter-jet correlations, jet azimuthal anisotropies and measurements of the jet shapes, fragmentation and substructure.

Determining QGP properties from the jet modifications is not trivial. First, the precise mechanisms of jet-medium interactions are currently under investigation and the predictive power of the different theoretical formalisms and approximations are still to be validated. Second, jet measurements are often affected by multiple confounding effects. Also, any measurement of jets is necessarily made after it has propagated through the entire time evolution of the QGP and effects preceeding and following the QGP existence can impact measured quantities. Finally, jet measurements in heavy-ion collisions are experimentally challenging due to the large and fluctuating background from the underlying event in a typical heavy-ion collision.

In this review, we will first discuss how jet measurements in heavy-ion collisions are performed. Then we will focus on three important questions related to the physics of jet quenching whose answers are not yet complete but will be within reach in the next few years due to new experimental data from RHIC and the LHC and theoretical advances:

•
How is jet energy transported within the QGP?
•
What are the effective degrees of freedom of the QGP?
•
Is there a critical size for QGP formation?

We will end with a brief conclusion and outlook.

Section snippets

A brief summary of the theoretical advances in jet quenching

A highly energetic parton that propagates through high-temperature and high-density QCD matter is expected to lose energy mainly via radiative processes [19], [20], [21]. These processes consist of gluon radiation induced by the scattering of the energetic parton with the medium constituents. A radiated single gluon spectrum was derived within the Baier–Dokshitzer–Mueller–Peigne–Schiff–Zakharov (BDMPS-Z) formalism in the 1990s [20], [21], [22] in the limit of infinite energy and medium length.

Jet reconstruction

The standard jet finding algorithm used in heavy ion collisions is the anti $- k_{T}$ algorithm [75] as implemented in the FastJet package [76] due to its wide adoption in the high-energy physics community, performance, and resilience to back-reaction [77]. Various constituents, underlying event subtraction and corrections for the jet energy resolution and detector effects have been used in heavy-ion measurements.

Jet measurements in heavy ion collisions have used different constituents for the jets.

What have we learned from jet measurements at the LHC and RHIC?

We have organized available heavy-ion jet data around three physics questions. First, what are the mechanisms responsible for the transport of energy from high-energy to low-energy modes within the QGP? Second, can we observe jets probing free quarks and gluons within the QGP? And finally, what is the critical size for the QGP formation? The first class of measurements constrains the mechanisms of jet-medium interactions since it comprises a vast set of observables that are differential in jet

Conclusions and outlook

This review covers the current status of measurements which use jets to study the properties of the quark–gluon plasma. The initial observations of jet quenching at RHIC and later at the LHC were only the beginning of this rich program.

Measurements covered here clearly show that the amount of energy loss a jet undergoes depends on the structure of the jet. While inclusive jets above 400 GeV do not have an $R$ -dependence to their suppression, other measurements do show significant dependence of

Acknowledgments

The authors thank Matthew Nguyen, Martin Rybar, Carlos Salgado, and Marco van Leeuwen for comments and suggestions to the draft. The authors also thank the ALICE, ATLAS, CMS, PHENIX and STAR Collaborations for the great experimental results. LCM is supported by the European Research Council project ERC-2020-COG-101002207 QCDHighDensityCMS. AMS acknowledges support from National Science Foundation, United States Award Number 2111046.

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sPHENIX is a next-generation detector experiment at the Relativistic Heavy Ion Collider, designed for a broad set of jet and heavy-flavor probes of the Quark-Gluon Plasma created in heavy ion collisions. In anticipation of the commissioning and first data-taking of the detector in 2023, a RIKEN-BNL Research Center (RBRC) workshop was organized to collect theoretical input and identify compelling aspects of the physics program. This paper compiles theoretical predictions from the workshop participants for jet quenching, heavy flavor and quarkonia, cold QCD, and bulk physics measurements at sPHENIX.
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ReviewStudying the QGP with Jets at the LHC and RHIC

Abstract

Introduction

Section snippets

A brief summary of the theoretical advances in jet quenching

Jet reconstruction

What have we learned from jet measurements at the LHC and RHIC?

Conclusions and outlook

Acknowledgments

Nuclear Phys. A

Nuclear Phys. A

Nuclear Phys. A

Nuclear Phys. A

Prog. Part. Nucl. Phys.

Phys. Lett. B

Nuclear Phys. B

Nuclear Phys. B

Nuclear Phys. B

Nuclear Phys. B

Nuclear Phys. A

Nuclear Phys. A

Phys. Lett. B

Phys. Lett. B

Nuclear Phys. A

Nucl. Instrum. Methods A

Phys. Lett. B

Nucl. Instrum. Methods A

Nucl. Instrum. Methods A

Ann. Rev. Nucl. Part. Sci.

Ann. Rev. Nucl. Part. Sci.

Phys. Lett. B

Phys. Rev. Lett.

New J. Phys.

Adv. Theor. Math. Phys.

J. High Energy Phys.

Eur. Phys. J. C

Rev. Mod. Phys.

Looking Inside Jets: An Introduction To Jet Substructure And Boosted-Object Phenomenology, Vol. 958

JETP Lett.

Phys. Rev. D

J. High Energy Phys.

J. High Energy Phys.

Phys. Rev. C

Phys. Rev. C

Phys. Rev. C

Phys. Rev. Lett.

Phys. Rev. Lett.

J. High Energy Phys.

J. High Energy Phys.

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J. High Energy Phys.

J. High Energy Phys.

J. High Energy Phys.

Erratum: J. High Energy Phys.

J. High Energy Phys.

J. High Energy Phys.

J. High Energy Phys.

Review
Studying the QGP with Jets at the LHC and RHIC