NNuclear Physics A 00 (2020) 1–13
NuclearPhysics A / locate / procedia XXVIIIth International Conference on Ultrarelativistic Nucleus-Nucleus Collisions(Quark Matter 2019) “QM19 summary talk”:Outlook and future of heavy-ion collisions
ORNL, Oak Ridge, USA
A summary of the QM19 conference is given by highlighting a few selected results. These are discussed as examplesto illustrate the exciting future of heavy-ion collisions and the need for further instrumentation. (The arXiv version issigniﬁcantly longer than the printed proceedings, with more ﬁgures.)
The goal of heavy-ion physics  is to understand the phase diagram of Quantum Chromo Dynam-ics (QCD) as a function of the temperature ( T ) and baryon chemical potential ( µ B ), as shown in Fig. 1. Athigh temperature and / or high baryon chemical a transition occurs, from ordinary matter, where the hadronicdegrees of freedom are dominant, to a Quark Gluon Plasma (QGP), where the dominant degrees of freedomare quark and gluons, which in ordinary matter are conﬁned into hadrons. Lattice QCD calculations predict the presence of a Critical Endpoint (CE) somewhere in the region T < , µ B >
300 MeV with aﬁrst-order phase transition at higher µ B and a cross-over transition at T ≈
155 MeV and lower µ B . Immenseexperimental and theoretical e ﬀ ort is underway to characterize and understand the phase structure of QCD,and the emergence of collectivity and matter properties, and the underlying equation of state (EoS) fromﬁrst principles. Following the presentation at the conference , these proceedings are structured into thefollowing topics: • the high-density frontier (Sec. 2) to study the onset of deconﬁnement, to search for the CE and theﬁrst-order phase transition, and at very high µ B to provide constraints for the neutron star structureand its EoS; • the high-energy frontier (Sec. 3) to quantify the ﬂuid properties of QGP and to relate them to itsmicroscopic structure; • the cold nuclear matter or small- x frontier (Sec. 4) to characterize properties of cold nuclear matter,and understand the structure of protons and nuclei at small Bjorken- x ; a r X i v : . [ nu c l - e x ] J u l C.Loizides / Nuclear Physics A 00 (2020) 1–13
Fig. 1. The QCD phase diagram versus temperature and bayon chemical potential. Indicated are the nuclear matter, hadron gas andQGP phases, as well as the approximate region for the Critical Endpoint, and the ﬁrst-order and cross-over phase transitions. Currentand future facilities probing di ﬀ erent region in µ B are: LHC , FCC , RHIC , and the RHIC ﬁxed-target program , SPS [8, 9],NICA , FAIR  and J-PARC . Figure from . • the ultra-precision near and far future (Sec. 5) with the planned Electron Ion Collider (EIC) and theproposed new future experimental equipment at the Large Hadron Collider (LHC) and beyond.
2. The high-density frontier
Predictions for the existence and exact location of the CE at ﬁnite µ B have been continuously improvedover the past years. New lattice QCD results from the WB collaboration  using the imaginary µ B method are consistent with previous calculations by the HotQCD collaboration, but have much smalleruncertainty, and hence add further evidence that the location of CP is disfavoured for µ B <
300 MeV. Wherepossible, lattice QCD predictions were already confronted with experimental data . The experimentalapproach  is to probe di ﬀ erent regions of the QCD matter phase diagram by extracting the freeze-out parameters ( T , µ B ) from statistical model ﬁts to hadron yields measured at LHC (few TeV) down toSIS18 (few GeV) collision energies. In this way, the vicinity of the phase boundary will be explored from T ≈
155 and µ B ≈ T =
80 and µ B =
900 MeVas shown in Fig. 1.The CE is characterized as a divergence in the correlation length of the underlying system, and hencemanifests as a divergence of the associated susceptibilities. Experimentally, ratios of susceptibilities areaccessible through event-by-event ﬂuctuations in conserved quantities, such as electric charge or baryonnumber. In particular, higher-order moments of net-proton distributions, which are a proxy for baryonnumber, are expected to be sensitive to the CE. First results of C / C of net-proton distributions werereported , which for central AuAu collisions at √ s NN =
200 GeV are negative, as expected from latticeQCD, while they were found to be positive for central AuAu collisions at √ s NN = . σ -e ﬀ ect at present, the result may indicate the expected O (4) criticality in the cross-over region .Furthermore, a crucial study was performed  by checking that the freeze-out parameters deduced frommean hadron yields agree with those from grand-canonical ﬂuctuations of conserved quantities for centralcollisions. This demonstrates for the ﬁrst time, using ﬂuctuation observables, that a femto-scale system .Loizides / Nuclear Physics A 00 (2020) 1–13 attains thermalization. A low √ s NN ( < . ﬀ erent relaxation times for di ﬀ erent moments.In view of the search for the CE this should be followed-up on.A ﬁrst-order phase transition is characterized by an unstable co-existence (spinodal) region correspond-ing to a softest point in the EoS. Direct ﬂow ( v ) which is sensitive to the compressibility of the createdmatter, is hence a key observable in the beam energy scan (BES) program. The slope of the rapidity-oddcomponent at mid-rapidity was reported as a function of beam energy for various identiﬁed mesons andbaryons . Mesons and anti-baryons exhibit negative slope over the whole range of beam energies, whilebaryons (p, Λ ) exhibit a change of slope around 14 . v , and hence was proposed as a signature of a ﬁrst-order phase transition . How-ever, to consistently treat e ﬀ ects from spectator matter and baryon stopping, calculations in the frame ofthe BEST collaboration  would be needed using more realistic 3D hydrodynamic calculations at ﬁnite µ B , with an EoS with and without CE . First data from the STAR ﬁxed-target (FT) program, the v of the φ -meson at √ s NN = .
3. The high-energy frontier
The high-energy experiments at RHIC and LHC continue to produce data with ever increasing precisionin measuring bulk properties. At the conference, for example, new results of longitudinal ﬂow decorrelationsat LHC  and their energy dependence at RHIC , linear and non-linear ﬂow modes  and higherorder cumulant elliptic ﬂow and ﬂuctuations  of identiﬁed particles in PbPb collisions at √ s NN = . ﬀ erent descriptions of the various stages of the collision. Furthermore, we need toinsist that contributing authors release their pieces of code as open source promptly.The precision reached in the J /ψ nuclear modiﬁcation factor at mid-rapidity from ALICE  is nowgood enough to clearly see predicted behavior of the R AA from statistical hadronization or regeneration [32–34] counteracting Debye screening: A clear minimum is observed in peripheral collisions ( N part ∼ R AA rises for more central collisions almost reaching unity in central collisions. Conse-quently, one also observes large ﬂow as well as triangular ﬂow due to “approximately thermalized” charmat low p T . The total charm cross section needs to be measured, since it is the natural normalization forJ / ψ measurements. Knowing it precisely allows one to reduce model uncertainties by constraining shadow-ing e ﬀ ects, and set precise limits on the expected R AA . Unity is not apriori a limiting value; the J / ψ R AA can be larger than one  With a “brute-force” combinatorial extraction of D meson production down to0.5 GeV / c , ﬁrst constraints of the total charm cross section in PbPb were placed . Better performanceand more statistics will be expected in Run-3 after the LS2 ALICE upgrades.The situation for the Y is di ﬀ erent as due to its much heavier mass one does not expect a signiﬁcantregeneration component. Broadening of its spectral widths on the lattice for ﬁnite temperature, presented atthe conference , is compatible with the sequential dissociation picture. Sequential dissociation is clearlyestablished for the Y family, and observed to be signiﬁcantly stronger in PbPb than pPb collisions . InpPb collisions, the suppression usually is attributed to comoving nuclear matter, but could in principle alsobe partially from the onset of screening. Elliptic ﬂow was found to be consistent with zero [39, 40], howeverfrom extrapolating Blast Wave ﬁts of existing data, one expects signiﬁcant v only for p T >
10 GeV .Understanding and characterizing parton energy loss continues to evolve from rather qualitative to more-and-more sophisticated and precise level. A milestone was reached with the ﬁrst measurement of the jetfragmentation of jets recoiling of an isolated photon . The data clearly exhibit the expected “text book”result that soft (low- z ) particles are enhanced, while hard (high- z ) are suppressed, as predicted since more C.Loizides / Nuclear Physics A 00 (2020) 1–13 than 20 years (e.g.see  and references therein). Detailed measurements using isolated photon–jet cor-relations will play a key role to precisely quantify jet modiﬁcation in Run-3 / and in thesPHENIX era at RHIC. Measurements of inclusive jet R AA were performed from about 50 to 500 GeV / c for R = . √ s NN = .
02 TeV, and rather good agreement was found between dataand models [45, 46]. It was hence surprising that the predictions  for larger radii and to higher p T varyconsiderably indicating that the limited available measurements in the past year attracted the calculations. Inorder to avoid tuning of models to speciﬁc results, we could deﬁne a set of reference measurements (accord)that every jet model has to be able to describe before one trusts its prediction for a di ﬀ erent observable.The CMS collaboration released new results  of R AA for high energy jets up to R = .
0, which yield R AA ≈ . / c for central PbPb collisions at √ s NN = .
02 TeV. Peripheral collisions are con-sistent with unity, in particular when taking into account also the presence of the centrality bias . Thejet suppression for large radii in central collisions is similar to that of charged particles at ∼
200 GeV / c ,indicating the sought-after consistency between inclusive charged particle and jet R AA at high p T and largeenough radius. Still, with new data from Run-3 /
4, it is important to measure both to even higher p T , toreduce uncertainties, and to investigate if R AA ∼ p T and large radii ( ≥ .
4) can be made by using Machine Learning methods trained on propertiesof the jet constituents in pp collisions . First results have already been presented, and conﬁrm thepreviously established picture of strong quenching at low jet p T . However, in particular for large jet radii of R = . / wide angle energy loss picture have been presented atthe conference. Symmetric splittings for wide-angles were reported to be suppressed relative to vacuum and single sub-jets less exhibit less suppression than jets with multiple subjets .One of the frontiers of the high-energy program is to study the evolution and onset of QGP phenom-ena in smaller collision systems like pA and pp collisions . The yields of strange and multi-strangeparticles (relative to pions) increase smoothly with multiplicity, rather independent of collision species andenergy  reaching a value predicted by statistical models in central PbPb collisions. The increase withmultiplicity in smaller systems can be regarded as lifting of canonical suppression . The rapid rise atlow multiplicity plus the presence of many QGP signatures in pp and pPb collisions  lead to the de-velopment of “core / corona” models. In these models, the core “hydrodynamizes”, while the corona doesnot and instead is treated as a superposition of independent nucleon–nucleon collision. In this way, gooddescription of data across all systems is achieved with a rather universal approach (similar to EPOS) .The observed enhancement in the strange particle yields versus multiplicity result from the fact thatthere is a pedestal e ﬀ ect: The strange particle yields depend approximately linearly on multiplicity but onlyabove some minimum multiplicity (threshold), while the pion yields do not exhibit any apparent threshold.Hence, normalizing the strange particle yields with the pion yields creates an enhancement ∝ (1 − M / M ),which leads to a rise with M that is most apparent at low multiplicities. In microscopic models one can inter-pret the observed threshold in several ways: i) The yield increases with the number of overlapping strings,i.e. as a multi-string e ﬀ ect related to the string density driving the increase of strangeness production thathowever quickly saturates at higher multiplicity. ii) There is minimum associated multiplicity necessaryfor multi-strange particle production and / or di ﬀ erent scaling of soft and (semi-)hard processes, i.e. a singlestring e ﬀ ect leading to suppression at low multiplicity that is reduced at higher multiplicity. To further probeproduction mechanism of strangeness can be achieved by measuring two-particle angular correlations (as-sociated production) between same and opposite sign strange and non-strange particles . These studiesin particular when they involve also the Ω baryon will greatly beneﬁt from the large increase in statistics ofthe 200 / pb pp program planned for Run-3 / Λ + c / D ratio versus multiplicity at mid-rapidity measured in several p T -ranges for pp, pPb and PbPbcollisions at √ s NN = .
02 TeV  shown in Fig. 2exhibits surprising features: The ratios, in particularfor lower p T , smoothly increase with multiplicity from low multiplicity pp to central PbPb collisions. At At LHC also Z–jet correlations are becoming available  .Loizides / Nuclear Physics A 00 (2020) 1–13 Λ + c / D ratio versus multiplicity at mid-rapidity measured in several p T -ranges for pp, pPb and PbPb collisions at √ s NN = .
02 TeV compared to e + e − collisions . the lowest measured multiplicity interval (below the pp average) the ratio at 2 < p T < / c is alreadysigniﬁcant (4 times) larger than that measured in e + e − collisions, while for the highest multiplicity intervalit approaches the value seen in central PbPb collisions. The result is in qualitative agreement with the“recombination” hypothesis saturating already few ( ∼
6) times mean the average pp multiplicity, whichrequires the presence of a local space-time density. Alternative mechanisms like implemented in PYTHIA(“color reconnection”) can describe the data (but are not universal). The new data open up research onhadronization in pp collisions and future ep (eA) colliders.In pp collisions at 13 TeV, heavier particles up to D-mesons exhibit ﬁnite v >
0, while v ≈ v in Z-tagged pp events (which lead to more “central” events), andin photo-nuclear reactions (which can be regarded as a ρ –nucleus collision), reveal ﬁnite v > v ≈ v ≈
0, itwould be good to follow up with other measurements like identiﬁed particle p T spectra, mean p T , or yieldratios versus p T to check for absence of collectivity in observables related to v . New calculations for smallsystems, where initial and ﬁnal state e ﬀ ects were consistently treated, suggest the dominance of the ﬁnalstate e ﬀ ects beyond d N / d η >
10 . Together with the large set of observables, which in PbPb collisionsundoubtedly would be interpreted as QGP signals, this makes it plausible that we indeed observe the sQGPeven down to multiplicities just above that of minbias pp collisions at LHC energies. While in 2015, therewas a large debate whether it is possible , we should now focus on understanding how it is realized inQCD and what it implies.Although the presence of ﬂow without observable jet quenching does not rule out having a “mini” QGP,one of the key questions is the apparent absence of parton energy loss in small systems. Above a fewGeV / c , nuclear modiﬁcation factors measured in minimum bias pA collisions at mid-rapidity are consistentwith pQCD calculations including nuclear PDFs (see references in ). Observing a suppression directlyin light-particle ﬂavor spectra is complicated due to the selection biased introduced by the “event-activitycategorization” . At high p T >
10 GeV / c , v is understood to reﬂect the path-length dependence ofparton energy loss in PbPb collisions . Therefore, the new data from ATLAS , which demonstratedsigniﬁcant non-zero v values up to high p T ∼
50 GeV / c in pPb collisions measured using the templateﬁt method, are puzzling. They can not be consistently interpreted as due to parton energy loss, since thelatter would imply R pPb to be signiﬁcantly below unity, not seen in data. Furthermore, the similarity of the p T shape of v between the pPb and PbPb systems provokes the question, whether there is a yet unknown sourceof v in both systems? In the intermediate region, the anisotropies were found to be larger in minimum-biasthan in jet-selected events, and the observed v may come from changing the admixture of particles from C.Loizides / Nuclear Physics A 00 (2020) 1–13 - - - - - -
10 1 x Q ( G e V ) EM and DIS measurementscentral LHC M FT LHCbFOCALLHCb
NMC/EMCEIC f RH I C ( P b ) s Q ( p ) s Q Fig. 3. Approximate ( x , Q ) coverage of various experiments for regions probed by DIS measurements including at the planned EIC, aswell as possible future direct photon and Drell-Yan measurements at RHIC and LHC, The estimated saturation scales for proton andPb are also indicated. The horizontal dashed line and the dashed curve indicate the kinematic cuts above which data were included inthe nNNPDF ﬁts. To calculate x and Q the approximate relations x = p T / √ s NN exp( − y ) and Q = p T was used with √ s NN = . √ s NN = . hard scattering and the underlying event. At p T >
10 GeV / c , the ATLAS data essentially report long-rangecorrelation between hard and soft particles on the level of 2%, which is remarkably similar to that in PbPbcollisions, maybe reﬂecting some source of residual correlation from the likely presence of a di-jet. It isclear that this question will need to be followed up, maybe by measurements in peripheral PbPb collisions,or by studies using event generators trying to disentangle the various contributions.Peripheral PbPb collisions, most of the beam energy systems from STAR, as well as small systems (pAand ppcollisions) exhibit v but no measurable e ﬀ ect from parton energy loss on the p T spectra [73–75].Hence, one of the next key questions is to study the onset of parton energy loss at RHIC and LHC withlighter ions, like for example in OO or ArAr collisions . Unlike in pp or pA collisions, multiplicitydistributions in AA collisions exhibit a pronounced plateau that simpliﬁes the centrality determination, andreduces e ﬀ ects from biases induced by the event selection. Indeed, it will be very beneﬁcial to perform thesame measurements also at √ s NN = . ﬀ er greatly.Furthermore, it may be useful to describe the R AA data from the RHIC beam energy scan , as well asthe SPS data  with models including state-of-the-art initial and ﬁnal state e ﬀ ects.
4. The cold-nuclear matter or small- x frontier At small longitudinal momentum fraction x and momentum transfer Q , parton dynamics is expectedto be a ﬀ ected by non-linear QCD evolution, where the rate of gluon–gluon fusion is in competition withthat of gluon splitting. In this kinematic regime, the extremely high gluon density may even saturate, pos-sibly leading to the existence of another pre-collision state of matter – the so-called colour glass conden-sate (CGC) . The saturation scale, where for a given x the competing processes are in balance, isenhanced in nuclei by a factor A / compared to protons, and hence comparisons between measurements inpp and pA collisions are of particular interest.Figure 3 gives an overview of the approximate ( x , Q ) coverage of various curent and future experimentsfor EM or DIS measurements.At RHIC and EIC a region down to about x ∼ − will be probed with .Loizides / Nuclear Physics A 00 (2020) 1–13 the future direct photon and Drell-Yan measurements enabled by the RHIC cold nuclear program , forwhich STAR and sPHENIX plan forward upgrades at 2 . < η < x of the proton or nucleus, down to x ∼ − , over a large range in Q . The LHCbexperiment  is a single-arm spectrometer equipped with tracking and particle-identiﬁcation detectors aswell as calorimeters with a forward angular coverage of about 2 < η <
5. The FoCal is high-granularitySi + W electromagnetic calorimeter and metal + scintillator hadron calorimeter at 3 . < η < . x than any ofthe other experiments. The FoCal will access the smallest x ever measurable until the possible advent of theLHeC  or FCC . Compared to RHIC, the LHC will give access to a signiﬁcantly larger region ofphase space that is potentially a ﬀ ected by parton saturation. In particular, the region of gluon saturation willextend to p T values high enough that perturbative QCD should be applicable. ] c [GeV/ T p p P b R NN s LHCbForward LHCbEPS09LOEPS09NLO nCTEQ15CGC
Fig. 4. Nuclear modiﬁcation factor R pPb as a function of p T for prompt D integrated over 2 . < | y ∗ | < . p T < / c and2 . < | y ∗ | < . < p T <
10 GeV / c for pPb collisions at √ s NN = .
02 TeV as measured by LHCb compared to theoreticalpredictions of di ﬀ erent pQCD calculations using nuclear PDFs and a recent CGC calculation. The ﬁgure is adapted from . Precise information at forward rapidity probing small x at the LHC is provided by the measurement ofprompt D-meson production at 2 . < y < . c ¯ c production is gluon fusion gg → c ¯ c . Themeasured nuclear modiﬁcation factor R pPb as a function of p T at forward rapidities (Fig. 4)exhibits thatthe forward production of prompt D-mesons is suppressed compared to pp collisions, with R pPb ∼ . p T and increasing mildly with p T . The measured suppression is consistent with expectations based onthe various calculations using nuclear PDFs or the CGC framework. The suppression of charm productionin the calculations with nuclear PDFs is a direct result of the reduced gluon density at x (cid:46) − , whichis commonly referred to as gluon shadowing . The calculated values range from R pPb about 0 . x gluon density. This directlyconﬁrms that shadowing at small x is large, and that the data place constraints on nuclear PDFs, as discussedat the conference .However, a quantitative determination of the amount of gluon shadowing based on hadron productionmeasurements is complicated by the fact that hadronic ﬁnal state e ﬀ ects (rescattering) may also play a role inthe observations. Recent forward measurements from LHCb at √ s NN = .
16 TeV include the observations
C.Loizides / Nuclear Physics A 00 (2020) 1–13 c (GeV/ T p p P b R g Isolated EPPS16+CT14nNNPDF1.0
ALICE projectionFoCal upgrade = 8.8 TeV NN sp-Pb < 4.75 cms h -1 = 50 nb L - - - - - -
10 1 x g R nNNPDF 1.0EIC fitFOCAL fitPb reweighting =10 GeV Q90% CL
Fig. 5. Left panel: Expected uncertainty for the nuclear modiﬁcation factor of isolated photons at √ s NN = . ﬃ ciency and energy scale, as well as thedecay photon background determination. The current EPPS16 and nNNPDF1.0 uncertainties are indicated by the black line and theshaded band, respectively. Right panel: The nuclear modiﬁcation of the gluon distribution for Pb versus x at Q =
10 GeV / c for x > − compared between nNNPDF1.0 parameterization and ﬁts to the FoCal pseudo-data (red band) and “high energy” EICpseudo-data (green band). 90% conﬁdence-level uncertainty bands are drawn, and the nuclear PDFs are normalized by the protonNNPDF3.1. Figures from . of a stronger suppression of the cross section ratio of Y (1 S ) to J / ψ from b in pPb compared to pp collisions,as well as stronger nuclear modiﬁcation of Y (2 S ) compared to Y (1 S ) . Both require the presence ofadditional (ﬁnal-state) e ﬀ ects to describe the data. Also, the forward / backward ratio of D-meson productionat high p T in pPb collisions at √ s NN = .
16 TeV  di ﬀ ers from expectation of models and data at 5 TeV.A clean probe providing direct access to partons are direct photons, since they couple to quarks, andare not a ﬀ ected by ﬁnal-state nor hadronization e ﬀ ects. At leading-order more than 70% are produced byCompton ( qg → γ q ) process, hence directly sensitive to the gluon density. An essential design goal ofproposed FoCal  is ability to reconstruct π → γγ decays at forward rapidity up to large transversemomenta p T ∼
20 GeV / c with high e ﬃ ciency. This will enable precise discrimination between directphotons and decay photons, hence enabling to measure direct photons from low transverse momentum up to ∼
20 GeV / c at large rapidity. The unique performance of a future isolated photon measurement with FoCalis demonstrated in Fig. 5. An accuracy of about 20% is expected at 4GeV / c , improving to about 5% at 10GeV / c and above, which will strongly constrain nuclear PDFs below x ∼ .
5. The ultra-precision near and far future
An overview of the timeline of planned and proposed near- and far-future instrumentation for high-energy nuclear physics, grouped into di ﬀ erent categories (high density, high energy, small- x and ultra-precision future) is given in Fig. 6. Besides the ongoing experimental program, in particular at RHIC andSPS, new dedicated future experiments and facilities are being built over the next 5–10 years to characterizethe phase structure of strongly-interacting matter at high µ B . In addition, at the LHC, data can be takenin ﬁxed-target mode, for example by LHCb with the SMOG2 system , allowing to probe the freeze-outcurve up to µ B ≈
400 MeV (see summary in ).In 2021 the ALICE LS2 upgrades [96, 97] to improve the capabilities for rare probes at low p T will havebe completed, which in particular include a new inner tracking system based on MAPS , the GEM-basedTPC readout , and the forward muon tracker . LHCb, will complete its LS2 upgrades , with .Loizides / Nuclear Physics A 00 (2020) 1–13 among more minor improvements, a new pixel vertex locator  and a new high-granularity silicon micro-strip planes upstream and scintillating-ﬁbre downstream tracker  usable in up to 30–100% central PbPbcollisions. In 2023, sPHENIX  is expected to start operating speciﬁcally designed for measurementsof hard probes at RHIC. The major upgrades for CMS  and ATLAS  are prepared for data takingin 2027 after LS3. For CMS, they mainly are a new high-granularity pixel and Si-strip tracking systemup to η < . η < ﬃ ciency and momentum resolution at low p T . Fig. 6. Timeline of planned and proposed near- and far-future instrumentation for high-energy nuclear physics, grouped into: thehigh-density frontier (green) with BESII , [email protected] , MPD , LHCb ﬁxed-target , NA62 , CEE , CBM ,NA60 + , J-PARK-HI ; the high-energy frontier (red) with ALICE LS2 upgrades [96, 97], sPHENIX , CMS LS3 up-grades  and ATLAS LS3 upgrades , ALICE ITS3 ; the small- x frontier (blue) with LHCb phase-1 upgrade ,forward sPHENIX , forward STAR , FoCal , EIC ; the ultra-precision future (black): LHCb phase-2 upgrade ,NGHI , FCC , SppS . Besides the LHCb after its phase-1 upgrade , the proposed forward upgrades [80, 81] at RHICand FoCal  at the LHC, there is the planned EIC  expected to begin data-taking around 2030. TheEIC will be ep and eA polarized (up to small nuclei) collider dedicated to the study of proton and nucleistructures. The collider will enable DIS o ﬀ a proton or a nucleus with a range of 20 < √ s <
140 GeV and aluminosity of 10 cm − s − . It will allow us to systematically explore correlations inside protons and nuclei,as well as to study saturation and hadronization with a controllable initial state. For an overview of the EICphysics program and the connection to heavy-ion physics, given at the conference, see .For data-taking at similar timescale as the EIC, the successor of ALICE, the Next Generation HeavyIon (NGHI) has been proposed . It is designed as a fast, ultra-thin detector with precise tracking andtiming, and will provide the ultimate performance for measurements related to (multi-)heavy-ﬂavor, softhadrons and thermal radiation over large range in rapidity η ≤
4. Eventually one can add far-forward track-ing and particle identiﬁcation capabilities to access the high net-baryons and / or a far forward calorimeterfor ultra-soft- x photons. In 2030, LHCb will also have concluded its phase-2 upgrade , after which itstracking detectors will be able to cope with even higher track densities of the HL-LHC. This will also allowthe reconstruction of central PbPb data, and open up the full suite of particle identiﬁcation provided by theLHCb spectrometer at forward rapidity inPbPb collisions. Together with ATLAS and CMS, these experi-ments will provide an incredibly rich, precision high-density QCD and heavy-ion program at the LHC, inparallel to the precision cold nuclear matter program the EIC. As indicated also in Fig. 6, the far-future per-spective is equally bright, given the activities related to the FCC  including preparations for a heavy-ionprogram  and the CEPC-SppC .
Instead of a “summary of the summary”, let me just point out that —without any doubt— we are expe-riencing the golden age of high-density QCD and heavy-ion physics, with a numerous interesting problemsto solve and an extremely bright future in instrumentation ahead. C.Loizides / Nuclear Physics A 00 (2020) 1–13
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