EEmergence of Quark–Gluon Plasma Phenomena
Jan Fiete Grosse-Oetringhaus ∗ CERN, 1211 Geneva 23, SwitzerlandE-mail: [email protected]
The discovery of QGP phenomena in small collision systems like pp and p–Pb collisions havechallenged the basic paradigms of heavy-ion and high-energy physics. These proceedings givea brief overview of the key findings and their implications, as well as today’s experimental andtheoretical situation. An outlook of future measurement is made.
European Physical Society Conference on High Energy Physics - EPS-HEP2019 -10-17 July, 2019Ghent, Belgium ∗ Speaker. c (cid:13) Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). https://pos.sissa.it/ a r X i v : . [ nu c l - e x ] J a n mergence of Quark–Gluon Plasma Phenomena Jan Fiete Grosse-Oetringhaus
1. Introduction
Heavy-ion collisions at ultrarelativistic energies allow to produce a Quark–Gluon Plasma(QGP) in the laboratory. The QGP is expected to have prevailed in the early universe and itsstudy allows to access the regime of deconfined quarks and gluons. With the measurements at theLHC at CERN and at RHIC at BNL, the last decade has advanced the field into the precision era,both, for the wealth of observables and the unprecedented quantification of the observed effects [1].In addition to the detailed study of Pb–Pb and Au–Au collisions, smaller systems (p–Pb and d–Aucollisions) as well as pp collisions are investigated, initially as a reference for the measurements inlarge systems. Surprising discoveries have been made in these smaller systems which have shakenthe basic paradigm of the field of heavy ions. This basic paradigm assumes that the phenomenaobserved in heavy-ion collisions requires the formation of a QGP. In turn the formation of a QGPrequires a large enough volume of hot and dense matter and therefore collisions of large objects.The experimental evidence discussed in this write-up questions this paradigm. The discoverieshave led to a tremendous experimental and theoretical activity in recent years: some of the relatedpublications rank among the highest cited publications of the ALICE, ATLAS and CMS collabo-rations [2, 3, 4]. A selection of results will be reviewed in the following, but it can be hardly givenjustice to the overall activity here due the space limitation. For a full review, the reader is invitedto consider Ref. [5, Chapter 9] and Ref. [6].In retrospect, the discovery of long-range correlations in two-particle correlations in very highmultiplicity pp collisions [4] marked the beginning of the study of emerging QGP phenomena.Correlations of particles with a transverse momentum of a few GeV/ c are dominated by a so-callednear-side peak structure and an away-side ridge structure. Both originate in the fragmentation of a2 → η ∆ -4 -2 0 2 4 φ ∆ ) φ ∆ , η ∆ R ( -202 >0.1GeV/c T (a) CMS MinBias, p η ∆ -4 -2 0 2 4 φ ∆ ) φ ∆ , η ∆ R ( -101 <3.0GeV/c T (b) CMS MinBias, 1.0GeV/c
0.1GeV/c T ≥ (c) CMS N η ∆ -4 -2 0 2 4 φ ∆ ) φ ∆ , η ∆ R ( -2-101 <3.0GeV/c T ≥ CMS N
Figure 1:
Two-particle correlations in high-multiplicity pp collisions (left panel, figure from Ref. [4]) andhigh-multiplicity p–Pb collisions after subtraction of the low-multiplicity counterpart (right panel, figurefrom Ref. [2]). mergence of Quark–Gluon Plasma Phenomena Jan Fiete Grosse-Oetringhaus and in pseudorapidity (1–2 units depending on the momenta of the involved particles), the awayside retains only the correlation back-to-back in azimuth, due to the fact that the center-of-masssystem of the scattering is not corresponding to the lab frame in hadronic collisions. In Ref. [4],an additional correlation is observed at large pseudorapidity differences on the near side for veryhigh multiplicity collisions which was named the ridge , see Fig. 1 (left). A similar structure wasobserved in p–Pb collisions [7] accompanied by a further ridge structure on the away side, seeFig. 1 (right), made visible employing a subtraction procedure [2, 3]. While the first ridge structurealready reminded of so-called elliptic flow observed in heavy-ion collisions [8], the observation oftwo ridge structures made this reminiscence indisputable. In heavy-ion collisions this phenomenonis directly attributed to the hydrodynamic expansion of the hot and dense matter [9].
Figure 2:
Ratios of the strange particles K S , Λ , Ξ , and Ω to pions as a function of charged-particle multiplic-ity. Results from pp, p–Pb and Pb–Pb collisions areshown, overlaid by model comparisons. Figure fromRef. [10]. The second surprising observation isthat strange baryon production increasesfaster than multiplicity [10]. Increasedstrangeness production has been observed inlarge systems and is seen traditionally as asign of deconfinement as it is energeticallycheaper to produce a pair of strange quarksthan a pair of strange hadrons. Surprisingly,the increased strangeness production is al-ready present in pp collisions when studyinghigher multiplicity and connects smoothlyto p–Pb and then Pb–Pb collisions. Fig-ure 2 presents particle ratios for four strangeparticle species as function of multiplicity.One observes that traditional MC codes, likePYTHIA [11], completely fail to reproducethe trend which has been identified as a sig-nificant conceptual problem in such mod-els [12]. These discoveries triggered a largeexperimental programme as well as signif-icant theoretical modelling which is brieflyreviewed in the following section.
2. Experimental Situation Today
The ridge structures shown in Fig. 1 arequantified in detail by their Fourier coeffi-cients v n of the azimuthal distribution, de-fined as: dNd ϕ ∝ + ∑ n v n cos n ( ϕ − Ψ n ) , (2.1)where ϕ is the azimuthal angle of the particle and Ψ n the n th s order participant plane [13]. In A–Acollisions, the dominant component is the second-order coefficient v called elliptic flow , primarily2 mergence of Quark–Gluon Plasma Phenomena Jan Fiete Grosse-Oetringhaus offlinetrk N v = 13 TeVspp < 3.0 GeV/c T h |CMS |>2} hD {2, | sub2 v {4} v {6} v {8} v {LYZ} v offlinetrk N v = 2.76 TeV NN sPbPb < 3.0 GeV/c T h | offlinetrk N v = 5 TeV NN spPb < 3.0 GeV/c T h | Figure 3:
The coefficient v as a function of multiplicity in pp (left), p–Pb (center) and Pb–Pb collisions(right). A significant value is seen for all collision systems for measurements with up to 8 particles. Figurefrom Ref. [15]. driven by the elliptic shape of the overlap between the two colliding nuclei. However, also higher-order ( n >
2) components have a significant contribution which showed that the internal structureof the initial matter distribution in the colliding particles of the nuclei needs to be considered [14].The fluctuating positions of the nucleons in the nuclei lead to a different matter distribution event-by-event. These anisotropies of the initial matter distribution lead to asymmetries in the final-state momenta, when sufficient interactions between the constituents occur. This transition can bedescribed by hydrodynamic models which treat the QGP as a liquid with certain properties whichcan then be extracted by comparison of the measured v n coefficients to theoretical calculations.Given that the underlying symmetry planes are determined by the initial state of the collisions, theyare identical for all outgoing particles and therefore all particles are correlated with each other. Thishas to be separated from few-particle correlations stemming from jets or resonance decays.Given the much smaller overlap region in pp and p–Pb collisions, the measurements of signif-icant components in these collisions came as a surprise. Figure 3 shows v measurements in pp,p–Pb and Pb–Pb collisions as a function of multiplicity. In order to exclude that jet-like correla-tions have significant influence on the measured coefficients, multi-particle correlation techniquesare used showing that the observed effects involve at least 6 (8) particles in pp (p–Pb and Pb–Pb)collisions [15]. The overall magnitude is similar in pp and p–Pb collisions and somewhat smallerthan in Pb–Pb collisions. Generally, the similarities are striking.The large energy density of the hot and dense matter gives rise to a common velocity fieldwith which the constituents of the medium rapidly expand. A consequence of this so-called radialflow is that all particles have a similar β resulting in a mass-dependent influence on the particlemomenta. This well-known effect from Pb–Pb collisions has also been observed in pp and p–Pbcollisions, see the left panel of Fig. 4, providing further evidence for an expanding medium also inthese small collision systems. Further insight can be obtained from the study of heavier charm andbeauty quarks. Figure 4 (right) shows the measurement of heavy-flavour decay muons from charmand beauty decays [16]. This result and additional measurements involving heavy-flavour decayelectrons [17] as well as D and J / Ψ mesons [18] show that also the charm quark has a significant3 mergence of Quark–Gluon Plasma Phenomena Jan Fiete Grosse-Oetringhaus ) c (GeV/ T p | > . } hD { , | s ub2 v ALICE Preliminary = 5.02 TeV NN s p-Pb | < 0.8 h | 0-20% (V0A) – p – K )p p( recch N - v ATLAS -1 =13 TeV, 150 pbs pp <6 GeV T hD mfi c mfi b Figure 4:
Left: v coefficient for π , K, and p as a function of p T in p–Pb collisions. A characteristic splittingand crossing of the v of the different particle species is observed. Right: v coefficient as a function ofmultiplicity for b and c heavy-flavour decay muons. A positive value is observed for charm quarks, whilethe result for b is consistent with 0. It should be noted that the measurement is for 4 < p T < / c andthus does not include the low-momentum region. Figure from Ref. [16]. v component. For the b to date, no signal has been seen at large p T . While this could indicatethat the beauty quark is too heavy to participate in the system evolution, the low-momentum regionremains to be studied before a final answer can be given.In order to investigate if the observed ridge structures could be related to a fundamental processand thus not need any final-state interactions, archived e + e − collisions recorded by ALEPH havebeen analyzed. No signal has been observed and Fig. 5 (left) compares the obtained limit withresults from pp, p–Pb and Pb–Pb collisions. At multiplicities below 30, the limit on the associatedyield is about 10 − , while the uncertainties in hadronic collision systems are of the order of 10 − .At larger multiplicities, the signal observed in hadronic systems is finite but compatible with the(poorer) limit in e + e − collisions. While multiplicities between the systems may not be directlycomparable, the call is still out if there is a significant difference between e + e − and hadroniccollisions.The versality of RHIC allowed a detailed comparison of collisions with different shapes of theinitial overlap region. This has been achieved by colliding p, d, and He on Au. While p–Au israther round, d–Au and He–Au have a large elliptic component. In addition, He–Au has on aver-age about twice the triangularity than the other two systems. The performed measurements in thesecollisions, show that the shape of the initial state determines the strength of the measured v and v coefficients [20]. This has important consequences: the transition from the initial-state shape to thefinal-state momenta requires interactions of the constituents. Furthermore, hydrodynamic modelsimplementing those correctly predicted the measured values [21, 22] while models involving onlyinitial-state momentum correlations cannot reproduce the effects [23]. Figure 5 (right) presentsthese coefficients in He–Au collisions compared to these model calculations.As discussed, the presented results support the idea of final-state interactions in small colli-4 mergence of Quark–Gluon Plasma Phenomena
Jan Fiete Grosse-Oetringhaus 〉 corrtrk N 〈 − − − − − −
10 110 A ss o c i a t ed y i e l d ALEPH Archived Data =91 GeVs hadrons, → − e + e Thrust coordinatesLab coordinates (shifted right)Scaled CMS Resultpp 7 TeVpPb 5.02 TeVPbPb 2.76 TeV (GeV/c) T p0.5 1 1.5 2 2.5 3 n v = 200 GeV 0-5% NN s He+Au iEBE-VISHNU n v SONIC n v MSTV, PRL Erratum n v Data v Data v PHENIX
Figure 5:
Left: limits on near-side ridge yields in e + e − collisions recorded by ALEPH compared to resultsfrom the LHC. For a detailed discussion of the comparison see text. Figure from Ref. [19]. Right: v and v as a function of p T in He–Au collisions compared to hydrodynamic calculations (iEBE-VISHNU andSONIC) which predicted the values well. The calculation involving only initial-state momentum correlations(MSTV) cannot reproduce the measurement. Figure adapted from Ref. [20]. sion systems. If such interactions are indeed present, the outgoing partons should also lose energyby this mechanism. This phenomenon is well known from collisions of large systems where high p T hadrons and jets lose a significant fraction of their energy [24]. However, in small systems asignal of parton energy loss has not been observed to date, neither for inclusive hadrons [24, 25](see Fig. 6, left panel), nor jets [26, 27], nor D mesons [28, 29], nor B and J / Ψ from B [30, 31].Also h–jet coincidence measurements can only provide an upper limit on parton energy loss in p–Pb collisions [32]. This creates an apparent inconsistency as the well-established observable R AA showed a difference from unity (the expectation if A–A collisions were an incoherent superposi-tion of nucleon–nucleon collisions) in peripheral collisions. The latter are at similar multiplicitieswhere unity was measured in p–Pb collisions. This inconsistency was recently understood by ameasurement in very peripheral Pb–Pb collisions (80–100%) where an unphysical reduction of R AA was observed [33] and explained by a simple superposition model [34], see Fig. 6 (right).This model includes the variation of the impact parameter of the single nucleon–nucleon collisionsand its effect on the event classification. In consequence, signals of parton energy loss seem tobe absent in peripheral Pb–Pb collisions and p–Pb collisions, although the presence of final-stateinteractions should give rise to them to some extent.In addition, to this puzzling absence of energy loss in small systems, on open question is themagnitude of v coefficients at low multiplicity in pp collisions. Their extraction in low-multiplicitycollisions is very challenging due to dominating jet-like correlations and resonance decays. De-pending on the utilized subtraction method a finite [35] or close to zero [15] v is extracted inlow-multiplicity pp collisions. The fact that the result is procedure-dependent, means that the col-lective nature in dilute systems is not understood, yet.5 mergence of Quark–Gluon Plasma Phenomena Jan Fiete Grosse-Oetringhaus ) c (GeV/ T p p P b R , P b P b R TeV, charged particles = 5.02 NN s p-PbALICECMS Pb-Pb,0-5%ALICECMS Norm. < | h < cms h < < | h ALICE |
Centrality (%)0 20 40 60 80 100 AA R | < 0.8 η = 5.02 TeV, charged particles, | NN sPb Pb, c < 20 GeV/ T p ALICE data, 8 < HG PYTHIA, PLB 773 (2017) 408
Figure 6:
Left: R AA and R pA as a function of p T from ALICE and CMS. Figure from Ref. [24]. Right: R AA for 8 < p T <
20 GeV / c as a function of centrality compared to a simple superposition model not involvingparton energy loss. Figure from Ref. [33].
3. Explanations & Modelling
The observations of QGP phenomena in small systems have received wide attention. Theirtheoretical explanation and description attempts can be grouped into three areas: • Extending the hydrodynamic description valid in large collision systems involving manyconstituents to small systems. This approach assumes many scatterings between the con-stituents. • An approach showing that few scatterings can already create anisotropies called escapemechanism . • Considering the effect of momentum correlations in the initial state of the colliding objects.In this approach, no final-state interactions are considered, although it can be combined withthe other approaches.These three areas span the entire field between fluid dynamics (many scatterings) and the free-streaming limit (no scatterings). Figure 7 illustrates the two modelling directions which follow.The first approach starts from a valid description in large systems (for example hydrodynamics orstatistical models), and extends it in the direction of smaller systems. The second approach beginswith a valid description in vacuum (for e + e − ) possibly amended by multiple parton interactions,color reconnection and ropes (for pp) and extends it to larger systems. In both approaches thedegree of complexity increases when moving towards intermediate systems like p–A collisions.In practice today, hydrodynamics is rather successful in describing the observed phenomenain p–A collisions. While such calculations require in principle local thermal equilibrium, the cal-culations are quantitatively successful even if the calculations are far from equilibrium (see also6 mergence of Quark–Gluon Plasma Phenomena Jan Fiete Grosse-Oetringhaus
Figure 7:
Illustration of the landscape of modelling from pp collisions in the single-process limit to Pb–Pbcollisions in the thermal limit. Models are extended along both directions, where the degree of complexityincreases going away from one of the limits.
Fig. 5, right panel) and for large differences between longitudinal and transverse pressure. WithinMC models [36] and kinetic theory [37] it has been shown that few interactions are sufficient tocreate an anisotropy measurable in the final state. If the system is small enough, the single-hit limitis even close to the full transport [38] while there are large deviations for larger systems, see Fig. 8(left).The inability of traditional MC codes like PYTHIA to describe the strange baryon production,see Fig. 2, and the fact that this cannot be resolved by tuning [12], have prompted work to extendthe baryon production mechanisms considered. Mechanisms that connect the colour flow fromdifferent parton–parton interactions (as used in PYTHIA and DIPSY [39]) or an explicit collectiveexpansion (as used for example in EPOS [40]) bring the models closer to the data, but are also notyet satisfactory [41]. Recently, a promising attempt is to extend PYTHIA into A–A collisions witha model called Angantyr [42] whose evolution is worth to be closely followed.
4. Future
There are very interesting opportunities ahead to study these open questions further, and un-derstand the underlying QCD processes which give rise to the discussed observations.At the LHC, the next run (2021–2024) will allow to study extremely rare high-multiplicitypp events, illustrated in Fig. 8 (right) for an integrated luminosity of 200 pb − . About 25 000events with a multiplicity of 14–16 times the average multiplicity are expected, which is higherthan the multiplicity of 65% central Pb–Pb collisions where significant effects associated withQGP formation are observed. Detailed studies of higher-order flow cumulants, the increase ofstrangeness production and the search for energy-loss signals will become possible. A detaileddiscussion of the opportunities with this data sample can be found in Ref. [5, Chapter 9].Furthermore, the study of O–O collisions may give important insight into understanding thepuzzling absence of parton energy loss. This symmetric collision system allows for a good selection7 mergence of Quark–Gluon Plasma Phenomena Jan Fiete Grosse-Oetringhaus γ ^ E lli p iti c f l o w : v / ∈ τ s =0.15 R τ s =0.55 R τ s =0.75 R τ s =0.95 R Kinetic theory pre-hydrodynamic stage:Single hit Ideal hydroFull transport h (| ch N - - - - - - - - - - - - -
10 1 ) c h P ( N Pb-Pb 5.5 TeVp-Pb 5.5 TeVpp 14 TeV| < 1.5 h | > c h - < N > c h - < N > c h - < N > c h - < N > c h - < N > c h - < N Figure 8:
Left: Anisotropy as a function of system size in a single-hit scenario compared to the full transportcalculation. Figure from Ref. [38]. Right: Extrapolated multiplicity distribution in pp collisions at 14 TeVcompared to p–Pb and Pb–Pb collisions. Figure from Ref. [5]. of the collision geometry and has a similar size than p–Pb collisions, and therefore is still largeenough to exhibit parton energy loss [5]. This is currently under discussion at LHC and RHIC.
5. Summary
The discovery of QGP phenomena in small collision systems have challenged two paradigms:they have challenged the descriptions explaining phenomena in large heavy-ion collisions.
Whatis the smallest system where they remain valid?
At the same time, the intriguing effects observedin high-multiplicity pp and p–Pb collisions which are not described by state-of-the-art models,have challenged the standard descriptions used in pp collisions.
Can these remain standard?
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