Quantum chromodynamics (QCD) is one of the pillars of the Standard Model of particle physics. It describes the strong interaction – one of the four fundamental forces of nature. This force holds quarks and gluons – collectively known as partons – together in hadrons such as the proton, and protons and neutrons together in atomic nuclei. Two hallmarks of QCD are chiral symmetry breaking and asymptotic freedom. Chiral symmetry breaking explains how quarks generate the masses of hadrons and therefore the vast majority of visible mass in the universe. Asymptotic freedom states that the strong force between quarks and gluons decreases with increasing energy. The discovery of these two QCD effects garnered two Nobel prizes in physics, in 2008 and 2004, respectively.
High-energy collisions of lead nuclei at the Large Hadron Collider (LHC) explore QCD under the most extreme conditions on Earth. These heavy-ion collisions recreate the quark–gluon plasma (QGP): the hottest and densest fluid ever studied in the laboratory. In contrast to normal nuclear matter, the QGP is a state where quarks and gluons are not confined inside hadrons. It is speculated that the universe was in a QGP state around one millionth of a second after the Big Bang.
The ALICE experiment was designed to study the QGP at LHC energies. It was operated during LHC Runs 1 and 2, and has carried out a broad range of measurements to characterise the QGP and to study several other aspects of the strong interaction. In a recent review, highlights of which are described below, the ALICE collaboration takes stock of its first decade of QCD studies at the LHC. The results from these studies include a suite of observables that reveal a complex evolution of the near-perfect QGP liquid that emerges in high-temperature QCD. ALICE measurements also demonstrate that charm quarks equilibrate extremely quickly within this liquid, and are able to regenerate QGP-melted “charmonium” particle states. ALICE has extensively mapped the QGP opaqueness with high-energy probes, and has directly observed the QCD dead-cone effect in proton–proton collisions. Surprising QGP-like signatures have also been observed in rare proton–proton and proton–lead collisions. Finally, ALICE measurements of interactions of produced hadrons have also revealed novel features that have broad implications for nuclear physics and astrophysics.
Probing the QGP at various scales
The QGP can be inspected with various levels of spatial and energy resolution (scale) using particles produced in heavy-ion collisions. High-energy quarks and gluons rapidly cross the QGP and interact with it as they evolve to a spray, or “jet”, of partons that eventually form hadrons, or “hadronise”. The interaction with the QGP reduces the jet’s energy and modifies its structure. A jet with an energy of 20 gigaelectronvolts, for example, can probe distances of 0.01 femtometres (1 femtometre is 10-15 metres), well below the roughly 10-fm size of the QGP. The jet modification, known as jet quenching, results in several distinct effects that ALICE has seen, including significant energy loss for jets and a smaller energy loss for beauty quarks compared to charm quarks.
Lower-energy charm quarks also probe the QGP microscopically, and undergo Brownian motion – a random motion famously studied by Albert Einstein. ALICE has provided evidence that these lower-energy charm quarks participate in the thermalisation process by which the QGP reaches thermal equilibrium.
Bound states of a heavy quark and its antimatter counterpart, or “quarkonia”, such as the J/ψ (charmonium) and Υ(1S) (bottomonium), are spatially extended particles and have sizes of about 0.2 fm. They therefore probe the QGP at larger scales compared to high-energy partons. The QGP interferes with the quark–antiquark force and suppresses quarkonia production. For quarkonia made up of charm quarks, ALICE has shown that this suppression, which is stronger for more weakly bound states and thus “hierarchical”, is counterbalanced by charm quark–charm antiquark binding.
This recombination effect has been revealed for the first time at the LHC, where about one hundred charm quarks and antiquarks are produced in each head-on lead–lead collision. It constitutes a proof of quark deconfinement, as it implies that quarks can move freely over distances much larger than the hadron size. The hierarchical suppression can be explained assuming a QGP initial temperature roughly four times higher than the temperature at which the transition from ordinary hadronic matter to the QGP can occur (about two trillion degrees kelvin). An assessment of the QGP temperature was also obtained from the ALICE measurement of photons that are radiated by the plasma during its expansion, yielding an average temperature from the entire temporal evolution of the collision of about twice the QGP transition temperature.
Regarding the large-scale spatial evolution of the collision, ALICE has demonstrated that the QGP formed at LHC energies undergoes the most rapid expansion ever observed for a many-body system in the laboratory. The velocities of the particles that fly out of the QGP in a collective flow approach about 70% of the speed of light, and direction-dependent, or “anisotropic”, flow has been observed for almost all measured hadron species, including light nuclei made of two or three protons and neutrons. Small variations seen in some specific flow patterns of hadrons with opposite electric charge are influenced by the huge electromagnetic fields produced in non-head-on heavy-ion collisions.
Calculations based on hydrodynamics, originally conceived to describe liquids at a few hundred degrees kelvin, describe all of the flow observables, and demonstrate that this theoretical framework is a good description of many-body QCD interactions at trillions of degrees kelvin. Such a description is achieved with the crucial inclusion of a small QGP viscosity, which is the smallest ever determined and thus establishes the QGP as the most perfect liquid.
Hadron formation at high temperatures
During the evolution of a heavy-ion collision, the QGP cools below the transition temperature and hadronises. After this hadronisation, the energy density may be large enough to allow for inelastic (hadron-creating) interactions, which change the medium’s “chemical” composition, in terms of particle species. Such interactions cease at the chemical freeze-out temperature, at which the particle composition is fixed. Elastic (non-hadron creating) interactions can still continue, and halt at the kinetic freeze-out temperature, at which the particle momenta are fixed.
ALICE measurements of hadron production over all momenta have provided an extensive mapping of this hadron chemistry, and they show that hadrons with low momentum form by recombination of quarks from the QGP. Theoretical models, in which a hadron “gas” is in chemical equilibrium after the QGP phase, describe the relative abundances of hadron species using only two properties: the chemical freeze-out temperature, which is very close to the transition temperature predicted by QCD, and a “baryochemical potential” of zero within uncertainties, which demonstrates the matter–antimatter symmetry of the QGP produced at the LHC.
In addition, ALICE investigations into the hadron-gas phase indicate that this phase is prolonged, and that the decoupling of particles from the expanding hadron gas is likely to be a continuous process.
What are the limits of QGP formation?
Studying how observables such as the particle production yields and multi-particle correlations change with multiplicity – the total number of particles produced – for proton–proton and proton–lead collisions provides a means to explore the thresholds required to form a QGP. A suite of ALICE measurements of high-multiplicity proton–proton and proton–lead collisions exhibit features similar to those observed in lead–lead collisions, where these are associated with QGP formation. The effects include the enhancement of yields of particles with strange quarks, the anisotropic flow determined from particle correlations, and the reduction of the yield of the feebly bound charmonium state ψ(2S) in proton–lead collisions. These observations were among the most surprising and unexpected from the first ten years of LHC running.
The ability of the hydrodynamic framework and of theoretical models of a strongly interacting system to describe many of the observed features, even at low multiplicities, suggests that there is no apparent spatial limit to QGP formation. However, alternative models that do not require the presence of a QGP can also explain a limited number of these features. These models challenge the idea of QGP formation, and this might be supported by the fact that jet quenching has not been observed to date in the small proton–lead colliding system. However, such absence could also be caused by the small spatial extent of a possible QGP droplet, which would decrease the jet quenching. Therefore, the quest for the smallest collision system that leads to QGP formation remains open.
Exploring few-body interactions
ALICE investigations of few-body QCD interactions, such as those that take place in proton–proton collisions or in heavy-ion collisions in which the colliding nuclei only graze past each other, have provided a wide range of measurements. Examples include precise measurements showing that in these collisions the formation of hadrons from charm quarks differs from expectations based on electron-collider measurements, and the first direct observation of the dead-cone effect, which consists of a suppression of the gluons radiated by a massive quark in a forward cone around its direction of flight.
Grazing collisions, known as ultra-peripheral collisions, provide a means of exploring the internal structure of nucleons (protons or neutrons) via the emission of a photon from one nucleus that interacts with the other nucleus. ALICE studies of these collisions show clear evidence that the internal structure of nucleons bound in a nucleus is different from that of free protons.
The large data samples of proton–proton and proton–lead collisions recorded by ALICE have allowed studies of the strong interaction between protons and hyperons – unstable particles that contain strange quarks and may be present in the core of neutron stars. ALICE has shown that the interactions between a proton and Lambda, Xi or Omega hyperon are attractive. These interactions may play a part in the stability of the observed large-mass neutron stars. In addition, ALICE measurements of the lifetime and binding energy of hypertriton – an unstable nucleus composed of a proton, a neutron and a Lambda – are the most accurate to date and shed light on the strong interaction that binds hypernuclei together.
The present and future of ALICE
After a major upgrade, the ALICE experiment started to record Run 3 proton–proton collisions in July 2022. The next full-scale data-taking of lead–lead collisions is planned for 2023, with a proposed pilot run expected in late 2022. The upgraded detector will reconstruct particle trajectories much more precisely and record lead–lead collisions at a higher rate. With the resulting, much larger Run 3 and then Run 4 data sets, rare probes of the QGP that were already used in the past decade, such as heavy quarks and jets, will become high-precision tools to study the QGP. ALICE will also continue to use the small colliding systems to investigate, among other things, the smallest QGP droplet that can be formed and the proton’s inner structure.
Besides further smaller-scale but highly innovative upgrades for the next LHC long shutdown, the ALICE collaboration has prepared a proposal for a completely new detector to be operated in the 2030s. The new detector will open up even more new avenues of exploration, including the study of correlations between charm particles, of chiral-symmetry restoration in the QGP, and of the time-evolution of the QGP temperature.