CERN Accelerating science

Heavy-ion collisions at Quark Matter 2018

by Panagiotis Charitos

CERN experiments presented their latest results from the study of lead-ion collisions at the annual Quark Matter conference, that was held this year in Venice, Italy. The programme includes a plethora of presentations on a variety of topics, spanning QCD at high temperature, QGP in small systems, initial state physics, collective dynamics, correlations and fluctuations, electroweak probes, jets, quarkonia and others.  All experiments report highly subtle measurements, bringing heavy-ion physics into a new era of high precision studies.

Heavy ion collisions at the Large Hadron Collider (LHC) energies result in a hot, dense medium called the quark-gluon plasma (QGP), in which the primary constituents are thought to be quarks and gluons. The study of these collisions is expected to shed more light on the strong interaction and how quarks and gluons interact to form stable particles. Of particular interest are new measurements of how ordinary particles emerge when the QGP cools down and reverts to more standard forms of matter.Moreover, the LHC experiments presented results from asymmetric collisions of lead ions with protons while for the first time Xe-Xe collisions produced at the LHC during a short test run in 2017. By comparing the result of pp, pPb and PbPb collisions physicists are able to study the role that the size of the colliding system plays in the final results.


In new results presented at Quark Matter 2018, ATLAS has studied “collective flow” and “jet quenching” in both xenon-xenon and lead-lead collisions. Comparisons of elliptic flow measurements in xenon and lead collisions provide a unique opportunity to study viscous effects in the hydrodynamic expansion of the QGP, as well as the effects of event-by-event fluctuations related to the geometry of the collision. These fluctuations are expected to be larger in xenon collisions compared to lead ions resulting in an enhancement of the observed flow. However, a counterbalancing effect comes from viscous effects that weakens the amplitude of the flow in peripheral collisions and even more in xenon collisions where there are larger spatial variations. These assumptions are confirmed by results presented in QM18. 

Left: charged particle v2 measured in xenon-xenon and lead-lead collisions as a function of centrality. Right: ratio of xenon to lead-ion v2 values compared to theoretical predictions. The error bars and bands indicate statistical and systematic uncertainties, respectively. (Image: ATLAS Collaboration/CERN).

Another interesting result from the comparison of the collisions of two different ions is the difference in jet quenching, one of the characteristic signature of QGP formation. Jet quenching is expected to weaken in xenon collisions due to the reduced density of the QGP and the smaller path lengths of the partons in the plasma compared to the larger QGP that is formed in lead-lead collisions. The reduction is quantified by the nuclear modification factor (RAA), the ratio of particle yields as a function of transverse momentum in nuclear collisions to that in proton-proton collisions, scaled by a factor that accounts for the flux of colliding quarks and gluons from each nucleus. The charged hadron RAA measured in xenon-xenon and lead-lead collisions are compared below as a function of the number of participating nucleons. It was shown that the xenon and lead results are similar, but slightly more suppression is observed in the xenon collisions, suggesting that the overlap geometry is an accurate parameter to determine the precise amount of jet quenching.

Charged hadron nuclear modification factor as a function of the number of nucleons (Npart) for xenon and lead-lead collisions in two different hadron pT intervals (left). Dijet xJ distributions in the 10% most central xenon-xenon and lead-lead collisions (right). (Image: ATLAS Collaboration/CERN). 

Another interesting result from ATLAS reported during QM18 comes from the study of the quark-gluon plasma using muon pairs produced during ultra-peripheral collisions by photon-photon interactions. To study dimuons in lead-lead collision data, ATLAS analysed two variables characterizing the kinematic properties of the two oppositely-charged muons, namely “acoplanarity” and “momentum asymmetry”. The “acoplanarity” variable measures how much the two muons are not back-to-back while the “momentum asymmetry” reflects the relative difference in the transverse momenta of the muons (or the momentum that is perpendicular to the beam line). Both quantities are large for muons decaying from heavy flavor mesons and nearly zero for the gamma-gamma process. For some years, it remained an open question whether dileptons from photon-photon interactions could be observed in more high-multiplicity interactions of heavy ions and, if so, whether they would be sensitive to the dramatically increased charge density inside the QGP.

To study dimuons in lead-lead collision data, ATLAS analysed two variables characterizing the kinematic properties of the two oppositely-charged muons, namely “acoplanarity” and “momentum asymmetry”. The “acoplanarity” variable measures how much the two muons are not back-to-back. The “momentum asymmetry” variable reflects the relative difference in the transverse momenta of the muons. Both quantities are large for muons decaying from heavy flavor mesons, while both are nearly zero for the gamma-gamma process. Results suggested that "acoplanarity" gets widder for more central collisions, an indication that can be linked with some sort of interaction between the muons and the QGP that is formed during these collisions. The result suggests that muon pairs from gamma-gamma interactions can be used to study the charge density of the quark-gluon plasma produced in central heavy ion collisions.

The root-mean-square of the distribution of additional transverse momentum broadening, relative to the muon direction. Results are shown as a function of the number of participating nucleons, with peripheral events on the left and central events on the right. (Image: ATLAS Collaboration/CERN). 

During QM18, ATLAS also presented final results on quarkonium suppression in PbPb collisions, with particular attention to contributions from prompt quarkonia, produced directly in the plasma, and particles that result from the decay of B mesons, typically outside the medium. A new technique of correlating harmonic amplitudes and transverse momentum in PbPb collisions is presented. 


ALICE, the LHC's dedicated experiment for the study of heavy-ion collisions presnted a wealth of new measurements sheding light on the mechanisms of QCD and the physics of strong interactions. More than 70 new preliminary results and 16 new papers were published on time to be presented in QM18.

New results on heavy flavour production in p-Pb collisions show that the charmed baryon production rate is much larger than was expected from electron-positron collisions, and the baryon-to-meson ratio is characterized by a maximum at intermediate pT, which is also seen for light flavour baryon-to-meson ratios. This suggests that there is a common production mechanism for light flavour baryons such as protons and Λ baryon and for the charmed Λc baryon. A first result of Λc baryon production in Pb-Pb collisions was presented, which also shows a large baryon/meson ratio. Improving the precision of these measurements is one of the goals of the ALICE detector upgrade programme, which was discussed in another session.

Heavy quark baryon/meson ratio similar to Λ/K​.

Increase of strange baryon production observed in pp, pPb and PbPb collisions​.

Moreover, the collaboration presented the latest results on the production of particles ranging from photons to nuclei and hypernuclei. New studies were also presented on the dependence of particle production on the size of the colliding system, including new measurements on the data from a short run in 2017 in which nuclei of xenon, instead of protons or lead nuclei, were accelerated in the LHC. The relative abundances of light flavour hadrons in the new Xe-Xe data confirm the previously-established picture that particle chemistry depends mostly on final state particle multiplicity at LHC energies.

Finally, for high-momentum particle production, a similar nuclear modification factor in Xe and Pb was measured when comparing collisions with the same multiplicity. This is qualitatively in line with expectations, since parton energy loss depends on the density and the volume of the system, but more detailed model comparisons are being pursued.

Multiplicity/Npart ‘scales’ (approximately) between xenon and lead-ion collisions. Results suggest a sharper increase for central collisions that needs to be further investigated. 

Another intriguing result comes from a dedicated study of the nuclear modification factor (RAA) of peripheral Pb-Pb collisions shows that, while the suppression of high-momentum particle production that is associated with parton energy loss initially decreases when the collisions become less central, it increases again for very peripheral collisions. This non-monotonic behaviour suggests that there is a different mechanism that suppresses high-momentum particle production in very peripheral collisions; one possible explanation is that the individual nucleon-nucleon collisions in the nuclear collision have larger impact parameters and that this reduces the number of parton scatterings, and thus the particle production at high transverse momentum. It is also relevant for the interpretation of collisions of small systems, where the observed azimuthal anisotropy suggests that final state interactions are important, but no suppression of final state particle production is found.

Nuclear modification factor for peripheral events showing evidence of increasing in very peripheral collisions. 

ALICE also presented a first attempt to measure azimuthal anisotropy of direct photons at the LHC, which probes the time evolution of the temperature and pressure in the Quark Gluon Plasma. The measured signal is large, suggesting the importance of late emission of photons. However, the uncertainties are still sizeable and further improvements are needed to firmly establish this conclusion.

Finally, ALICE presented new results on the nuclear modification factor for jets, as well as studies of the substructure of jets, which aim to be directly sensitive to the radiation of gluons by fast partons that travel through the QGP. The observed suppression of large-angle symmetric splittings, suggests that partons in a parton shower interact independently with the QGP for large enough angles. 


The CMS collaboration presented fourteen new results and five other results recently submitted for publication. The majority of them exploit the high luminosity 8.16 TeV pPb, and 5.02 TeV pp and PbPb data delivered by the LHC in 2016 and 2015, respectively. Three of the analyses use the xenon-xenon collisions data at 5.44 TeV delivered during a one day run in October 2017.

Data from xenon collisions indicate that the charged-particle production depends on collision geometry, not the system size. CMS continues to investigate collective effects in small systems, searching for the onset of these effects in events with a small multiplicity as well as for similar effects in events which produce particles with relatively large mass or momentum. Significant v2 (a measure of azimuthal anisotropy) is observed in high-multiplicity asymmetric proton-lead collisions for particles with either a single charm quark (D0) or a charm-anticharm pair (J/ψ). CMS reports also the first v3 measurement using 4-particle cumulants in proton-lead collisions, providing further evidence that v2 and v3 in asymmetric collisions are caused by initial state fluctuations.

CMS continues to perform detailed studies of the jet-quenching phenomenon, which gives rise to the striking dijet pT asymmetries observed in lead-lead collisions (2011). Tracing the energy lost in hard-scattered partons in the dense QGP remains a fascinating challenge for the field.The first measurement was reported of the detailed shapes of jets in events with a back-to-back photon+jet pair. The results show that some of the energy in the core of the jet is redistributed to large distances from the jet axis. At the same time, the analysis of jet substructure shows that the overall distribution of particles in the core itself is hardly affected by the medium.

For heavy quark studies, the first measurement of the radial profile of D mesons in jets in heavy ion collisions is reported. CMS continues to enrich its programme of studying b (beauty) quarks, by adding the first measurement of Bs mesons and D mesons from the decay of beauty hadrons (called non-prompt D). For particles with charm-anticharm pairs, the measurement of Psi(2S) and J/ψ mesons in PbPb, pPb, and pp collisions at 5.02 TeV reveals that the production of the Psi(2S) is suppressed with respect to J/ψ in both pPb and PbPb. 

The distribution of jet-correlated charged-particle tracks with |Δϕ|<|Δϕ|< 1.0 as a function of ΔηΔη in pp (top left) and PbPb (middle row) collisions. The PbPb results are shown for different centrality regions. The bottom row shows the difference between the PbPb and pp data.

CMS reported also on the scattering of a photon from one Pb nucleus off of a photon from the other (so called light-by-light scattering) in ultra-peripheral PbPb collisions.

 The fragmentation functions of jets associated with isolated photons are measured for the first time in pp and PbPb data, using CMS samples collected at centre-of-mass energy of 5.02 TeV as function of the ξjet parameter. When compared to the results found using pp data, the ξjet   and ξTγ distributions in central PbPb collisions show an excess of low- particles and a depletion of high- particles inside the jet.  

This measurement shows for the first time the in-medium parton shower modifications for events with well-defined initial parton kinematics, and constitutes a clear reference for testing theoretical models of the parton?s passage through the QGP.  


The LHCb collaboration presented a wide range of results on charm production in various types of collisions. These include the production of the Λc baryon in pPb collisions and of D0 and J/ψ mesons in the fixed-target collisions with He and Ar. The production of heavy quarks in nucleus-nucleus interactions is well suited to the study of the transition between ordinary hadronic matter and the hot and dense Quark-Gluon Plasma (QGP). The production of J/ψ mesons in nucleus-nucleus interactions, its possible suppression in the quark-gluon medium and/or later charm-anti-charm quark recombination are all studied in order to shed light into the mechanisms governing such a phase transition. The LHCb pPb and fixed-target results utilising proton interactions with different nuclei at different energies provide precious reference results in conditions in which the formation of the QGP is not expected. The images below show a signal mass peak of Λc baryons decaying into a proton, a K and a π (above left) as well as of D0 mesons decaying into a K and a π (above right).

In ultra-relativistic heavy nuclei PbPb collisions, two-photon and photonuclear interactions are enhanced in ultra-peripheral collisions (UPC). The collisions are either coherent, where the photon couples coherently to all nucleons, or incoherent, where the photon couples to a single nucleon. In the case of coherent J/ψ production in UPC, the photon-lead interaction can be modelled by the exchange of a colourless propagator, identified as a single object called a Pomeron, that interacts with the photon. The LHCb collaboration reported the cross-section measurement of coherent J/ψ production in PbPb collisions at 5 TeV and compared this to predictions from different phenomenological models.

The image shows that the coherent J/ψ production (blue line) can be clearly separated from the other contributions in the natural logarithm of the J/ψ transverse momentum squared distribution.

High-energy collisions involving ions have the best chance to produce gluon condensates, where the gluon wave functions start to overlap producing a collective behaviour. Saturated gluons are expected to be observed only at small angles relative to the beam axes, where the number and the size of the gluons are the largest. LHCb has the unique capability of measuring photons coming from these high density gluon regions and the announcement of initial measurements of these photons caused a lot of excitement. It is the first indication that gluons can be probed in this region, never achieved by any experiment so far. 

The image shows the angular distribution between isolated photons and other particles taken during the 2016 pPb run. he peak at the angle π indicates the presence of photons from gluons. The blue band is the background from other processes.​

In summary, Quark Matter 2018 meeting was the latest in a long line of this venerable conference series, which has played a central role in defining the field of relativistic heavy ion physics. The foreseen detector upgrades of the LHC experiments will increase the experiment's capabilities to search for new phenomena and shed light in the understanding of the QCD mechanisms in the new era of HL-LHC. 

This embarrassment of experimental riches will be well-served by a new level of quantitative theoretical understanding made available by increased computational resources, new algorithms, and developing sophistication in techniques and modeling. These are promising sign of a bright future for the heavy-ion community that keeps growing and making the most of CERN's accelerator complex. 


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