Stars, planets, animals, plants, you and me – everything that can be directly observed in our Universe is ordinary matter. But the Universe didn’t always look this way. This ATLAS feature describes the quark-gluon plasma, a unique state of matter that existed shortly after the Big Bang. The nature and properties of this matter are being revealed at CERN’s ATLAS Experiment.

Most of the mass of ordinary matter comes from protons and neutrons, which build up the atomic nucleus. Protons and neutrons belong to a family of particles called hadrons, which are themselves composite subatomic particles made of two or more quarks held together by the strong force. They are analogous to molecules that are held together by the electromagnetic interaction. In the Standard Model of particle physics, the theory that best describes our current understanding of elementary particles and their interactions, gluons are the carriers of the strong force. Gluons compose the majority of particles inside of hadrons and, despite being massless themselves, are the major contributors to a hadron’s mass due to the interaction of the quarks with the gluon field.

A particle’s ability to interact via the strong force is described by its colour. Quarks and gluons, collectively known as partons, are thus colour-charged particles. Because of a phenomenon called colour confinement, they cannot be isolated from hadrons and therefore cannot be directly observed in normal conditions. Thus, the total colour charge of hadrons must be zero (they are colourless). Two families of hadrons can be identified: mesons, which consist of a quark of one colour and an antiquark of the corresponding anticolour, and baryons, which are composed of three quarks of different colours but still carrying net-zero colour charge. Today’s Universe is baryon dominated, with quarks bound into ordinary matter. In fact, antibaryons almost do not exist in nature and can only be produced in relatively high abundances in the laboratory.

But what if conditions can be established in which individual partons can be deconfined?

In the 1960s, German physicist Rolf Hagedorn from the Theory Division at CERN, while working on a statistical model for particle production, made an important prediction of particle yields at the highest accelerator energies available at the time (from the Proton Synchrotron at CERN). In his work, he introduced the concept of a “fireball”. In this approach, all the energy from a particle collision was regarded to be contained within a small space-time volume from which particles were radiated – like a burning fireball.

Hagedorn’s particle-production models turned out to be remarkably accurate at predicting yields for many different types of secondary particles produced from the primary high-energy collisions. He understood that while new particles are being produced, and more and more energy is poured into the system, the temperature does not increase. Instead it is the entropy that increases with the collision energy. If the number of particles of a given mass increases exponentially, the temperature becomes stuck at a limiting value called the Hagedorn temperature. It amounts to approximately 2 terakelvin (2 followed by 12 zeros), which is a million times higher than the temperature of the core of the Sun.

Later on, the Hagedorn temperature was interpreted as a limit where ordinary matter is no longer stable, and must either "evaporate" or be converted into quark matter; as such, it can be thought of as the "boiling point" of hadronic matter. In electronvolts, this temperature amounts to about 160 MeV, which is about 15% above the mass of the lightest hadron, the pion. Therefore, matter at Hagedorn temperature or above will spew out fireballs of new particles, and the ejected particles can then be detected by particle detectors. Thus, quarks and gluons cannot be separated from their parent hadron without producing new hadrons.

This brings us to a concept of the quark–gluon plasma (QGP), a state of matter in which the quarks and gluons that make up the hadrons of matter are freed of their strong attraction to one another under extremely high energy densities. In the QGP, quarks and gluons are deconfined. By producing the QGP in the laboratory, researchers can recreate and study the high energy density conditions that prevailed in the early Universe, shortly after the Big Bang, when matter was formed from free quarks and gluons. This corresponds to the time interval of 10⁻¹⁰–10⁻⁶ s after the Universe was born.

So far, the only way for physicists to produce the QGP is through the collision of two heavy atomic nuclei (called heavy ions as atoms are fully ionised). These nuclei are accelerated to energies of more than a hundred GeV, thus heating matter well above the Hagedorn temperature. Using the result of a head-on collision in a volume approximately equal to that of an atomic nucleus, it is possible to reproduce the conditions in the very first moments of the Universe.

The idea that particle production could help achieve a high enough particle density to allow deconfinement was first recognised around 1978. Physicists realised that matter made of hadrons would melt into a boiling QGP phase. In the following decade, two experimental facilities began to search for this new phase of matter: lead nuclei were collided at the Super Proton Synchrotron (SPS) at CERN, and gold nuclei were collided the Relativistic Heavy Ion Collider (RHIC) at the Brookhaven National Laboratory (BNL) near New York City. The nuclei accelerated by these facilities would travel almost to the speed of light before being directed towards each other – creating a "fireball" in the rare event of a collision.

But how can physicists recognise whether the QGP medium is created in a collision? Several probes have been proposed to search for signatures of deconfined matter. This feature article will be highlighting strangeness production and elliptic flow, with the phenomena of jet quenching reviewed in detail.

Strangeness – that is, the production of strange quarks in relativistic heavy-ion collisions – is a signature and a diagnostic tool of the QGP formation. In 1982, strange quarks (and their antiquarks) were first proposed as signatures of the QGP. This relies on the fact that strange quarks and antiquarks are not present in ordinary matter since, when produced, they promptly undergo a "weak" interaction decay, akin to naturally radioactive isotopes. Moreover, the mass of strange quarks (and antiquarks) is below and close to the temperature at which protons, neutrons and other hadrons dissolve into quarks. Strange quarks are thus sensitive to the conditions, structure and dynamics of the deconfined matter phase and, if their number is large, it can be assumed that deconfinement conditions were reached. However, in the hot QGP environment, strange quarks can be produced abundantly and, due to their natural radioactivity, are relatively easy to observe. The ensuing production of matter particles comprising strangeness (especially multi-strange antibaryons) is the “gold standard” for the formation of the QGP.

In 2006, the NA57 Experiment at CERN found that hadrons made entirely from newly created quarks were produced up to 15-20 times more abundantly in heavy-ion reactions when compared to expected values from the reference proton-proton system. The pattern of enhancement follows the QGP prediction. There is no known explanation for these experimental results other than QGP formation.

Perhaps the most striking feature of heavy-ion collisions is that the geometry of the initial overlap between the two nuclei is reflected in the anisotropy of the final particle distributions. In order to understand this, it is first useful to classify collisions according to their impact parameter. Collisions are categorised into “centrality classes” depending on the magnitude of the impact parameter: central collision events with the largest overlap correspond to small impact parameter values, and peripheral collision events with the smallest overlap correspond to large impact parameter values. This is known as elliptic flow and it is illustrated in Figure 1.

Elliptic flow is a fundamental observable. It reflects the initial almond-shaped, azimuthally asymmetric region in the transverse plane of the nuclear overlap region, directly translated into the observed momentum distribution of particles. In very central collisions, elliptic flow is small, because the overlap of the two nuclei is nearly circular. As the overlap becomes more eccentric, the elliptic flow becomes larger. The reaction plane defines the minor axis of this eccentric shape. The high energy density in the overlap region creates pressure gradients which are largest in the reaction plane and particles are preferentially pushed out in that direction, similar to a ball rolling faster down a steep hill than a gentle slope. Elliptic flow is especially sensitive to the early stages of system evolution, when the spatial anisotropy is at its largest. In particular, the magnitude of the anisotropy is sensitive to the viscosity of the expanding fireball medium. The elliptic flow is measured to be nearly as strong as it could be, given the eccentricity of the overlap region. This means that the viscosity is nearly as small as it could be (see here). Elliptic flow is interpreted as strong evidence for the existence of the QGP.

Jet quenching is another unique phenomenon that can occur in relativistic heavy-ion collisions. Jets are produced in both proton-proton and heavy-ion collisions at the LHC. In both collision systems they are produced by two partons, one from each incoming projectile, colliding and exchanging a very large amount of momentum. These partons scatter at a large angle. As they leave the collision region, the scattered parton develops a shower. This is the process by which a colour-charged quark or gluon becomes an observable collection of colour-neutral particles. In proton-proton collisions, this showering process happens in the vacuum, but in nucleus-nucleus collisions, the shower develops inside the QGP (see Figure 2). Thus the showering process actually probes the short distance scale properties of the QGP itself. This makes jets one of the most powerful probes of QGP properties.

In these interactions, the energy of the partons is reduced through collisional energy loss and medium-induced gluon radiation, the latter being the dominant mechanism in the QGP. The effect of jet quenching in the QGP provides a main motivation for studying jets, as well as high-momentum particles and their correlations, in heavy-ion collisions.

The heavy-ion programme at ATLAS was launched spectacularly with a paper on the discovery of jet quenching. In contrast to the back-to-back jets observed in proton-proton collisions, in lead-lead collisions, one jet was observed with the opposing jet either entirely missing or having a much smaller energy. Figure 3 shows an event display from one lead-lead collision with a single prominent jet and no clear corresponding opposing jet. Overall, pairs of jets (dijets) in lead-lead collisions were found to be much less balanced than in proton-proton collisions.

This observation was based on the first LHC lead-lead data from 2010. However, with the increased integrated luminosity of lead-lead collisions accumulated over the last several years at the LHC, more interesting questions can be asked and answered. The R_AA of jets as a function of their momentum in the transverse direction (perpendicular to the beam) was measured in 2019. R_AA is the ratio of the number of jets measured in lead-lead collisions compared to measurements from proton-proton collisions scaled by a factor to account for the thickness of the lead nuclei compared to protons; R_AA being less than one indicates jets are losing energy. R_AA of inclusive jets in the most head-on lead-lead collisions was found to be between 0.4 and 0.6 (depending on the momentum of the jet). This indicates substantial energy loss.

LHC physicists are now focused on measurements addressing how this jet suppression actually happens. Two questions of current interest are:

1. How does the suppression depend on the type of jet?
2. How does the energy loss depend on the path length the jet travelled through the QGP?

How does the suppression depend on the type of jet?

Jets can be classified in a number of ways. One common way is to classify them by the type of parton that gave rise to the jet – for example quarks and gluons. At the LHC, most jets are gluon jets. Separating quark jets from gluon jets is a difficult task on a jet-by-jet basis, but creating a sample of jets with an enhanced quark fraction can be done by looking for jets which are opposite to a photon rather than another jet. Those jets are dominantly produced in the process quark+gluon → photon+quark. The R_AA of these photon-associated jets is also shown in Figure 4. These jets have a larger R_AA and are thus less suppressed than inclusive jets. This is the expected effect, as gluons jets couple to the QGP more strongly because the colour charge of gluons is larger than that of quarks. It is also possible to sort quark jets by the different types of quarks. Bottom quarks are of particular interest due to their very large mass. The R_AA of jets arising from bottom quarks is found to be larger than that of inclusive jets. This is expected, at least in part, from a suppression of gluon radiation inside the QGP due to the quark mass.

Another way to classify jets is by their structure. The developing parton shower is what interacts with the QGP, therefore jets with different structures interact differently with the QGP. ATLAS researchers have classified jets according to the angular distance between two prongs of the parton shower (r_g). These prongs are constructed via the Soft-Drop procedure and the R_AA is calculated as a function of both transverse momentum (p_T) and r_g. Very collimated (small r_g) jets are shown to be suppressed less than inclusive jets, while wide (large r_g) jets are suppressed more than inclusive jets.

As the structure and partonic origin of the jet are expected to be related to each other, these two ways of classifying jets are not independent. What they both show is that the amount any jet is quenched by depends on the details of the jet itself – this is exactly what is needed to understand the QGP from its interactions with jets.

How does the energy loss depend on the path length the jet travelled through the QGP?

In order to investigate the path length dependence of energy loss, the angular distributions of jets with respect to the event planes are measured. In this way, one can directly change the average amount of QGP that the jet traverses through the QGP. The variation in the yield with respect to those planes provides information on the sensitivity to the path length. Like the flow measurements discussed earlier, these measurements are directly sensitive to the shape of the QGP. Figure 5 shows a pair of jets created in the QGP and shows the differing path lengths depending on the orientation of the jets with respect to the QGP.

To quantify this, ATLAS physicists have measured the yield of jets with respect to the angle of the impact parameter (Ψ). This has been quantified by measuring the distribution of jets with respect to Ψ. More jets are observed at Ψ = 0 and Ψ = π than at Ψ = π/2; this distribution is fit to a sinusoidal distribution and the amplitude of that variation is v₂. Figure 6 shows v₂ of jets as a function of centrality. In the most central collisions (smallest impact parameter), v₂ is very small; this is understood to be due to the fact that the path length differences are very short when the QGP is nearly circular. However, in less central collisions v₂ grows as the eccentricity of the QGP increases. This clearly shows that the number of jets varies with the path length through the QGP.

Summary

Recent ATLAS results have shown that jet quenching is sensitive to both the structure of jets as they pass through the QGP and the path length of the jet travelling through. This demonstrates the usefulness of jet measurements for understanding the structure of the QGP. These experimental results are being incorporated into theoretical models which contain the microscopic quark and gluon interactions between the jets and the QGP. These models will have to describe these (and other) data to be possible interpretations of these interactions.

The ATLAS measurements presented here are part of a rich scientific programme seeking to understand the nature and properties of the QGP, and are but a small fraction of the exciting results to come out of ATLAS Run-2 data. Additional data collected in Runs 3 and 4 of the LHC will allow for more differential studies to further constrain the parton type, jet structure and path length dependence of jet quenching.

About the authors

Anne M. Sickles is an Associate Professor of Physics at the University of Illinois at Urbana-Champaign. She is a member of the ATLAS and sPHENIX collaborations and has worked on many experimental studies of the quark-gluon plasma created in ultra-relativistic collisions of nuclei. Her present research deals primarily with measurements of jets and their properties in heavy-ion collisions. Iwona Grabowska-Bold is a Full Professor of Physics at the AGH University of Science and Technology (Kraków, Poland) and member of the ATLAS Collaboration. She has been working on many topics including trigger preparations, data quality for heavy-ion data, as well as data analysis with a focus on weak bosons, heavy quarks and ultra-peripheral collisions (UPC). Her present research activity focuses on understanding strong and electroweak interactions using inelastic and UPC heavy-ion data.

Further Reading

• Mass matters – but it isn't the only thing!, ATLAS Physics Briefing, April 2022
• ATLAS gives new insight into dijet suppression in heavy-ion collisions, ATLAS Physics Briefing, April 2022

Scientific articles

• ATLAS Collaboration: Observation of a Centrality-Dependent Dijet Asymmetry in Lead-Lead Collisions at 2.76 TeV with the ATLAS Detector at the LHC (PRL 105 (2010) 252303)
• ATLAS Collaboration: Measurement of the nuclear modification factor for inclusive jets in Pb+Pb collisions at 5.02 TeV with the ATLAS detector (PLB 790 (2019) 108)
• ATLAS Collaboration: Comparison of inclusive and photon-tagged jet suppression in 5.02 TeV Pb+Pb collisions with ATLAS (ATLAS-CONF-2022-019)
• ATLAS Collaboration: Measurement of the nuclear modification factor of b-jets in 5.02 TeV Pb+Pb collisions with the ATLAS detector (arXiv: 2204.13530)
• ATLAS Collaboration: Measurement of substructure-dependent jet suppression in Pb+Pb collisions at 5.02 TeV with the ATLAS detector (ATLAS-CONF-2022-026)
• ATLAS Collaboration: Measurements of azimuthal anisotropies of jet production in Pb+Pb collisions 5.02 TeV with the ATLAS detector (arXiv: 2111.06606)
• E. V. Shuryak: Quantum chromodynamics and the theory of superdense matter (Phys. Rept. 61 (1980) 71-158)
• J. Rafelski, B. Müller: Strangeness Production in the Quark-Gluon Plasma (Phys. Rev. Lett. 48 (1982) 1066)
• NA57 Collaboration: Enhancement of hyperon production at central rapidity in 158 A GeV/c Pb–Pb collisions (J. Phys. G 32, 427 (2006))
• J. E. Bernhard, et al.: Bayesian estimation of the specific shear and bulk viscosity of quark–gluon plasma (Nat. Phys 15 (2019) 11)
• Y. L. Dokshitzera, D.E. Kharzeev: Heavy-quark colorimetry of QCD matter (PLB 519 (2001) 199)
• A. J. Larkoski, et al.: Soft drop (JHEP 05 (2014) 146)