HHeavy IonsExperimental Overview
Yvonne Pachmayer 𝑎, ∗ 𝑎 Physikalisches Institut der Universität Heidelberg,Im Neuenheimer Feld 226, D-69120 Heidelberg, Germany
E-mail: [email protected]
This article gives an overview of recent highlights from experimental measurements of heavy-ioncollisions at ultra-relativistic energies: Measurements of electroweak probes constrain both theinitial collision geometry and the nuclear parton distribution functions. Results from soft particleproduction show that the abundance of light-flavour hadrons from pions up to hypertriton and Hecan be described by a universal temperature and that these participate in the collective motion ofthe system. There are hints of these effects also in small systems, which will be further investigatedin future to understand the underlying mechanisms. Studies of hard probes, such as heavy quarksand jets show that parton energy loss plays an important role in heavy-ion collisions. Differentialmeasurements of J/ 𝜓 mesons elucidate their production mechanism, i.e. regeneration, and giveevidence for deconfinement in Pb–Pb collisions at LHC full energy. The large data samples atthe LHC enable studies of rare probes such as 𝜒 c1 (3872) and top–anti-top production. Further,measurements of antinuclei cross sections can provide input for dark matter searches. ∗ Speaker © 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 ] D ec eavy Ions, Experimental Overview Yvonne Pachmayer
1. Introduction
One of the main goals in modern heavy-ion physics is the study of QCD matter at extremeconditions of high temperature and/or high net-baryon number density ( 𝜖 ≈ ). Underthese conditions, which can be generated in ultra-relativistic heavy-ion collisions at laboratoriesusing accelerators such as the Large Hadron Collider (LHC) at CERN, a colour-deconfined stateof strongly-interacting matter, the quark–gluon plasma (QGP) [1] is created. At RHIC and LHCenergies, QCD matter is studied at nearly vanishing net-baryon density (baryochemical potential 𝜇 B ≈ 𝑇 C ≈ . ± . 𝑐 ; iii) expansion, collective motion, and cooling of the QGP;iv) hadronisation once the temperature drops below 𝑇 C ; v) after 5–10 fm/ 𝑐 [5] chemical freeze-out(inelastic collisions stop and hadron yields become fixed) and vi) kinetic-freeze out occur (elasticcollisions stop and momenta no longer change). The final-state particles or their decay particlescan then be measured in a detector to infer the characteristics of the QGP. It is not possible to detectthe QGP directly, because neither quarks nor gluons are colour-neutral objects. However, differentexperimental measurements are more sensitive to the various stages of the collision, and by com-paring the results with theoretical calculations, various properties of the QGP can be determined.In order to draw firm conclusions on effects induced by the hot medium (‘final-state effects’), thesame measurements are performed in pp collisions as a baseline and in proton–nucleus (pA) col-lisions to study initial-state effects in cold nuclear matter (CNM), such as the modifications of theparton distribution functions (PDF) in the nucleus with respect to that in the free proton. However,it has turned out that these measurements have provided in the last years far more interesting resultsthan being a pure reference (see below).The effects of the hot medium can be quantified using the nuclear modification factor 𝑅 AA , whichis defined as the ratio of the 𝑝 T distributions measured in AA collisions with respect to that in ppcollisions: 𝑅 AA = (cid:104) 𝑁 coll (cid:105) d 𝑁 AA / d 𝑝 T d 𝑁 pp / d 𝑝 T , where d 𝑁 AA / d 𝑝 T and d 𝑁 pp / d 𝑝 T are the 𝑝 T -differential yieldsof a given particle species in AA and pp collisions, respectively, and (cid:104) 𝑁 coll (cid:105) is the average numberof binary nucleon–nucleon collisions in the overlap region of the colliding nuclei. If in-mediumeffects are absent and cold nuclear matter effects are small, the ratio 𝑅 AA is unity in the regionwhere particle production from hard scattering processes dominates ( 𝑝 T (cid:38) 𝑐 ).
2. Initial State
Photons with high transverse momentum as well as W and Z bosons (due to their large mass,they are produced before the QGP is created), are penetrating probes and provide information aboutthe initial state and the nuclear parton distribution functions. Due to the high collision energy at theLHC, these are copiously produced. The 𝑅 AA of isolated photons in AA collisions is consistent with2 eavy Ions, Experimental Overview Yvonne Pachmayer [GeV] g T E
20 40 60 80 100 120 140 160 180 200 AA R pp (5.02 TeV) -1 PbPb / 27.4 pb -1 b m CMS g h | uncertainty AA TLuminosity uncertaintyDataSystematic uncertaintyJETPHOX PbPb(EPPS16+CT14)/pp(CT14)JETPHOX PbPb(nCTEQ15)/pp(CT14)JETPHOX PbPb(CT14)/pp(CT14) mm cms y AA R NN s Pb, -
90% Pb - ALICE, 0 c > 20 GeV/ T m p Data CT14Uncorr. systematic CT14 + EPPS16NLO pQCD with CT14 as pp reference
Figure 1:
Nuclear modification factor of (left) isolated photons as a function of photon energy [6] and (right)Z bosons vs rapidity [8] in Pb–Pb collisions. unity [6] (see Fig. 1), since they do not carry colour charge and thus do not interact strongly. The dataare better described by calculations with EPPS16 and nCTEQ15 nuclear PDFs than by calculationswithout nuclear modifications. Measurements by the ATLAS collaboration in p–Pb collisions atforward, mid- and backward rapidity are described by calculations that include modestly modifiednPDFs [7]. The measurement of Z bosons by the ALICE collaboration in Pb–Pb collisions shownin Fig. 1 exhibits a clear deviation from the expectation using free-nucleon PDFs as a function ofrapidity, yielding a 3.4 𝜎 effect integrated over rapidity [8]. Measurements of Z boson production inp–Pb collisions at forward and backward rapidity by the LHCb collaboration agree with calculationsconsidering PDF sets with and without nuclear modification within current uncertainties [9]. Thesemeasurements can be used to better constrain initial state effects and nPDF global fits.
3. Soft Probes
The particle yields of light-flavour hadrons and nuclei from pions up to (anti-)hypertritonand (anti-) He, as measured by the ALICE collaboration in central Pb–Pb collisions at √ 𝑠 NN = 𝑇 chem = ± . 𝑣 n . The ‘elliptic flow’ coeffi-cient 𝑣 is shown in Fig. 2 for different particle species in central and semi-central collisions. Forall shown particle species large anisotropies due to collective effects are visible. For semi-centralcollisions the coefficients are larger due to the initial conditions of the collision. Moreover, at lowmomenta a mass ordering effect is visible characteristic for hydrodynamic collective expansion.Both deuterons and He are loosely bound composite objects and the data can also be used tounderstand the hadronisation mechanism. The He data are well described by a model that includescoalescence of nucleons, a hydrodynamic evolution of the fireball and a hadronic afterburner [11].3 eavy Ions, Experimental Overview
Yvonne PachmayerThe calculations also reveal that the produced matter has a small shear viscosity to entropy densityratio, i.e. the created fireball is an almost perfect liquid.
Figure 2:
Elliptic flow as a function of 𝑝 T for various particlespecies [12, 13]. The 𝑣 coefficient is shownin Fig. 3 as a function of chargedparticle multiplicity in pp, p–Pb,Xe–Xe and Pb–Pb collisions. Inthe large collision systems, atlarge multiplicities, due to theinitial geometry and strong inter-actions strong signs of collectiveeffects are seen, which are welldescribed by hydrodynamic cal-culations. In small collision sys-tems (pp, p–Pb, peripheral Pb–Pb), the sizeable coefficients, which are similar in the different systems at the same multiplicity,could be indicative of collective effects. However, the data are neither described by hydrodynamiccalculations nor by PYTHIA8 calculations, although the latter show a trend similar to data. Severaldetailed studies are ongoing to investigate the long-range correlations seen in the small systemsand the continuous evolution of strangeness enhancement across collision systems [14]. The AT-LAS collaboration e.g. measured in pp collisions the elliptic flow as a function of charged particlemultiplicity for inclusive and Z-tagged collisions (the impact parameter of the collision may beon average smaller) [15], where neither a significant multiplicity dependence nor an influence dueto the presence of the hard-scattering process is observed. In contrast to high-multiplicity (HM)pp collisions [16, 17], no indication of collective behaviour is observed in elementary HM ep ande + e − collisions as measured by the ZEUS [18], shown in Fig. 3, and BELLE collaborations [19].So far no signs of energy loss have been seen in small collision systems (see below). For furtherinvestigations, future measurements with large data samples and very HM pp collisions [20] andcollisions of light ions [21] are foreseen.
4. Hard Probes (parton energy loss, jets, quarkonia)
In the large collision systems, such as in central Pb–Pb collisions, the yield of charged hadrons(see Fig. 4) and of jets with large transverse momenta up to 𝑝 T =
900 GeV/ 𝑐 , as measured by theATLAS collaboration [23], is strongly suppressed compared to pp collisions scaled by the numberof binary collisions. The comparison with results from p–Pb collisions, where this effect is absentas seen in Fig. 4, and theoretical calculations show that the suppression in Pb–Pb collisions is dueto a final state effect, where the parton traversing the medium looses energy due to radiative andcollisional energy loss. The large data samples at the LHC allow for many differential studies. Forexample, the CMS collaboration measured for the first time Z-tagged charged particle yields inPb–Pb collisions, which are, as shown in Fig. 4, compared to the expected yields in pp collisionsstrongly suppressed at high 𝑝 T , but increase towards low 𝑝 T [22]. The jet substructure was studiedby the ALICE collaboration [24] as a function of the jet resolution parameter in Pb–Pb and ppcollision using the grooming technique, where the large angle soft gluon radiation is removed to4 eavy Ions, Experimental Overview Yvonne Pachmayer
Figure 3: (left) Elliptic flow coefficient 𝑣 as a function of charged particle multiplicity in pp, p–Pb, Xe–Xeand Pb–Pb collisions [17]. (right) Two-particle correlation 𝐶 ( Δ 𝜂, Δ 𝜑 ) for HM ep collisions [18]. ) c (GeV/ T p p A R π ALICE, = 5.02 TeV NN s , p-Pb, ± h (preliminary) = 8.16 TeV NN s p-Pb, (EPJC 78 (2018) 624) = 5.02 TeV NN s p-Pb, (preliminary) = 5.02 TeV NN s (JHEP 1811 (2018) 013) ALICE (JHEP 1704 (2017) 039)
CMS
ALI−PREL−349201
Figure 4: (left) Nuclear modification factor of 𝜋 mesons and charged hadrons in p–Pb and Pb–Pb collisions.(right) Ratio of Z boson-tagged charged hadron spectra in Pb–Pb and pp collisions [22]. identify a hard splitting within the jet. The fully corrected data show that the jet substructure ismodified by the hot medium, the selected hard splitting being narrower, which is described by e.g.models that include incoherent energy loss [24].Heavy quarks (charm and beauty) are created in initial hard scattering processes and thusexperience the whole spatial and temporal evolution of a heavy-ion collision, thus providing essentialinformation on the interactions of partons with the hot medium and its properties. Gluons are arguedto loose more energy than quarks due to their stronger colour coupling to the medium. In addition,several mass-dependent effects (dead-cone effect and mass dependent spatial diffusion coefficient)are expected to influence the amount of energy loss of the heavy quarks. Thus a hierarchy of theparton energy loss is predicted, namely Δ 𝐸 gluon > Δ 𝐸 charm > Δ 𝐸 beauty , resulting in an expectedordering: 𝑅 𝜋 AA ≤ 𝑅 DAA ≤ 𝑅 BAA for pions (mostly originating from gluons), D, and B mesons.Measurements (e.g. those by the CMS collaboration in Fig. 5) show that 𝑅 hadronAA ≈ 𝑅 DAA for 𝑝 T ≥ 𝑐 . The 𝑅 AA is not only influenced by the energy loss, but also by the parton 𝑝 T spectrum andthe fragmentation into hadrons. Theoretical calculations including these effects described the data5 eavy Ions, Experimental Overview Yvonne Pachmayer ) c (GeV/ T p - v = 5.02 TeV NN s Pb - Pb , |y|<0.5, 30-50% p ALICE ALICE prompt D, |y|<0.8, 30-50% e, |y|<0.8, 30-50% fi ALICE b |<2, 30-40% h e, | fi ATLAS c |<2, 30-40% h e, | fi ATLAS b
Figure 5:
Nuclear modification factor 𝑅 AA of charged hadrons (mainly pions), prompt D mesons, B ± ,non-prompt J/ 𝜓 as a function of 𝑝 T in Pb–Pb collisions at √ 𝑠 NN = .
02 TeV [25]. Elliptic flow 𝑣 of pions,prompt D mesons and electrons from charm- and beauty-hadron decays in the same collision system [26–28]. well [29]. However, comparing with J/ 𝜓 mesons from B-hadron decays and new measurementsof prompt and non-prompt D mesons by the ALICE collaboration a quark mass depend energyloss at intermediate 𝑝 T is indeed observed. At high transverse momentum ( 𝑝 T (cid:38)
20 GeV/ 𝑐 ),all hadrons including prompt J/ 𝜓 [30] show within uncertainties surprisingly the same values of 𝑅 AA . Differential new complimentary measurements by the ALICE and ATLAS collaborations (seeFig. 5) show that heavy quarks participate in the collective motion [26–28]. Also here, at small 𝑝 T , amass-ordering effect becomes visible with electrons from beauty-hadron decays showing a positive 𝑣 with 3.8 𝜎 significance indicating a partial thermalisation of beauty quarks in comparison to alarge degree or complete thermalisation of charm quarks. At intermediate 𝑝 T , the coefficients aresimilar for pions and D mesons, maybe due to a coalesence effect of a light and a charm quark. Athigh 𝑝 T , the distributions merge suggesting that the path-length dependent energy loss dominates.
10 20 ) c (GeV/ T p / D + c L Pb,|y|<0.5,5.02TeV -
10% Pb - ALICE preliminary 0Pb, |y|<1, 5.02 TeV - - CMS 0 Au, |y|<1, 0.2 TeV -
80% Au - STAR 0ALICE pp, |y|<0.5, 5.02 TeVCMS pp, |y|<1, 5.02 TeV
Figure 6: Λ c /D ratio as a function of 𝑝 T in pp andheavy-ion collisions [31–33]. To better understand the aforementionedmechanisms, yields of hadrons containing heavyquarks are also studied. The ALICE and theSTAR collaboration measured the ratio of strangeto non-strange D mesons in Pb–Pb collisionsat √ 𝑠 NN = .
02 TeV and Au–Au collisions at √ 𝑠 NN = . s /B + ). The data hint at an enhanced produc-tion of strange hadrons due to the strangeness-richQGP. Thanks to the large available data samples,6 eavy Ions, Experimental Overview Yvonne Pachmayerbaryon-to-meson ratios can also be studied in the charm sector, which might provide insight intothe hadronisation mechanism of charm baryons. The ratio of Λ c /D in pp collisions shown in Fig. 6is much larger compared to the one in e + e − collisions ( Λ c /D ≈ .
11) and shows an even largervalue for heavy-ion collisions as measured by the ALICE, CMS and STAR collaborations [31–33].The measurements are qualitatively described by the statistical hadronisation model [10] and theCatania model with hadronisation via a combination of fragmentation and coalescence [34].Quarkonia were proposed as a signature of the deconfinement in the QGP. It was predicted thatquarkonium production would be suppressed due to a screening of the heavy-quark potential in thecolour-deconfined medium [35]. With increasing temperature of the hot medium, the quarkoniumstates with decreasing radius were predicted to melt subsequently, thus providing a sort of ‘ther-mometer’ of the QGP. Another competing mechanism is the production via (re)generation duringthe QGP phase or at hadronisation which depends e.g. for the J/ 𝜓 on the 𝑐𝑐 production cross section( 𝑁 J / 𝜓 ∝ 𝑁 𝑐 ) [36, 37]. The LHC data allow the production mechanism of J/ 𝜓 to be elucidated.While the 𝑅 AA of J/ 𝜓 decreases with increasing charged-particle density in Au–Au collisions at √ 𝑠 NN = . 𝑐𝑐 crosssection at the LHC and is a clear signature of deconfinement. The differential studies vs 𝑝 T show anincrease of the 𝑅 AA with decreasing 𝑝 T , which is well described by theoretical transport models andthe statistical hadronisation model that include the regeneration mechanism that dominates at low 𝑝 T . The observed strong sign of collective effects of J/ 𝜓 mesons [38], including also the observedpositive 𝑣 coefficient with a significance of 2.5 𝜎 at midrapidity, confirms the regeneration scenario.On the other hand, the suppression mechanism dominates for the bottomonium family as observedby an increasing suppression of Υ (1S), Υ (2S), and Υ (3S) in measurements by the ALICE, ATLAS,CMS and STAR collaborations [39–42], see e.g. Fig. 7. The data are described by theoreticalmodels that include the suppression mechanism and feedown from higher lying resonances. ALI-PREL-358983 æ part N Æ AA R Preliminary
ATLAS -1 = 5.02 TeV, L = 0.26 fb s , pp -1 = 5.02 TeV, L = 1.38 nb NN s Pb+Pb, ATLAS Krouppa, Strickland <30 GeV, |y|<1.5 T p (1S) ¡ (2S) ¡ <40 GeV, |y|<2.4 T p /s=1 hp (1S) 4 ¡ /s=2 hp (1S) 4 ¡ /s=3 hp (1S) 4 ¡ /s=1 hp (2S) 4 ¡ /s=2 hp (2S) 4 ¡ /s=3 hp (2S) 4 ¡ Figure 7: 𝑅 AA of J/ 𝜓 (left) and Υ states (right) [39] as a function of charged-particle density and averagenumber of participants, respectively.
5. Rare probes
The large data samples recorded at the LHC, allow for new measurements of very rare probes.Studies of the 𝜒 c1 (3872) in heavy-ion collisions could provide additional insight into the production7 eavy Ions, Experimental Overview Yvonne Pachmayermechanism and the nature of this exotic particle. Figure 8 presents the ratio of the yield of 𝜒 c1 (3872)to 𝜓 (2S) in pp and Pb–Pb collisions measured by the ATLAS and CMS collaborations. The ratiois larger in the heavy-ion collision, also because the CMS collaboration reported a significantsuppression of 𝜓 (2S) in Pb–Pb collisions. In future, it will be important to extend the measurementto low 𝑝 T , where the regeneration mechanism dominates as seen for the J/ 𝜓 meson (see above).Evidence of top quark production in Pb–Pb collisions was for the first time reported by the CMScollaboration [43]. The measurement is performed via the semileptonic decay channel of Wbosons with and without the presence of b-quark jets. The observed cross section 𝜎 tt as shown inFig. 8 is compatible, but somewhat lower than pQCD calculations and the result from pp collisionscorrespondingly scaled. The measurement of top quark production in heavy-ion collisions mighthave the potential to probe the time structure of the QGP [44] and thus will be an important studyfor the future, maybe for the LHC Run 5.
10 20 30 40 50 60 70 T p - -
10 110 R InclusivePromptNonpromptpp (7 TeV, CMS)|y| < 1.2pp (8 TeV, ATLAS)|y| < 0.75PromptPbPb (5.02 TeV, CMS)|y| < 1.6, Cent. 0-90%
Preliminary
CMS (2018 PbPb 5.02 TeV) -1 - b] m [ s CMS
NNLO+NNLL TOP++NNPDF30 NNLONNLO+NNLL TOP++CT14 NNLO = 5.02 TeV)s, ( -1 pp, 27.4 pb ) (scaled by A b-tag +jets/l+N OS JHEP 03 (2018) 115
NNLO+NNLL TOP++CT14 NLOEPPS16 NLOCT14 NNLO x = 5.02 TeV) NN s, ( -1 PbPb, 1.7 nb OS b-tag +N OS syst ¯ Exp unc: stat, stat scale ¯ Th unc: PDF, PDF
Figure 8: (left) Ratio of the fully-corrected yields of 𝜒 c1 (3872) and 𝜓 (2S) in pp and Pb–Pb collisions [45]and (right) tt cross sections in pp and Pb–Pb collisions [43].
6. Impact beyond the physics of heavy-ion collisions
Figure 9:
Inelastic antideuteron cross section vs. mo-mentum at which the interaction occurs [46].
Results from high-energy heavy-ion colli-sions also have impact on other fields of physics.Measurements of the cross sections of anti-nuclei provide important input for dark mattersearches. The precise 𝑝 T -differential antipro-ton cross section measurements in p–He colli-sions [47], a measurement in fixed-target modeperformed by the LHCb collaboration (He isinjected into the beam pipe close to the interac-tion point), help to constrain theoretical mod-els. The ALICE collaboration has measured forthe first time the inelastic antideuteron–nucleuscross section at small momenta (see Fig. 9) us-ing the detector as an absorber [46]. These kindof measurements will be extended to He and He in LHC Run 3 and 4.8 eavy Ions, Experimental Overview
Yvonne Pachmayer
7. Future of heavy-ion physics
The many new important results from the LHC and RHIC lead to an improved understandingof the interaction of partons with the hot medium, its properties and initial-state effects. They alsoconstrain the nPDF global fits and provide input for other fields of physics.With the ongoing upgrades (ALICE, ATLAS, CMS, LHCb) for LHC Run 3 and 4 an unprecedentedlevel of precision will be reached and the search for rare probes significantly extended. Further, theEuropean strategy for particle physics encourages the heavy-ion programme at CERN in the HL-LHC era. There is a plan for a next-generation LHC heavy-ion experiment: ALICE 3 constructedalmost entirely from silicon [48] for LHC Run 5 and beyond. In addition, projects such as sPHENIXand STAR at RHIC (incl. Beam Energy Scan), the fixed-target programme at CERN, facilities inthe high net-baryon number density frontier (NICA, FAIR, ...) and the electron-ion collisions atEIC will provide further insight into different kinematic regimes of the QCD phase diagram.
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