What Quark-Gluon Plasma in small systems might tell us about nucleons

1. WhatQuark-Gluon Plasma in small systems mighttellusaboutnucleons Peter ChristiansenLund University

2. Go outside of the comfort zone • Try to give a microscopic picture for • Perfect liquid degrees of freedom • Why and how it expands • Phase transition condition • Hadronization • Why QGP forms in small systems • Will be qualititative • Hope someone can make it quantitative Goals

3. Motivation Essential ingredients in small system modelling Reversibility in the QGP What have we learned about nucleons from QGP studies What could we learn about nucleons from QGP in small systems Outline

4. MOTIVATION

5. Not expected from: • Asymptotic freedom • Original understanding of LQCD calculations • Bag model Against all odds:we found the most perfect liquid

6. How ideal is it? (1/2) Ideal = reversible → linear response to initial state: → fluctuations should be purely geometrical

7. How ideal is it? (2/2) Left: v2 response is linear at low and high pT (=global property) Right: v2 and v3 correlations are given by pure Glauber model! (solid curves)

8. The perfect liquid is IMO the most beautiful QCD result in 20+ years • But • No breakthrough in our understandings of QCD • No change in Big Bang model • Small systems is IMO the battle that we cannot afford to loose • QGP → QCD • Is there a link between the perfect liquid and the Xover phase transition? – both are reversible! • What about nucleons? The perfect liquid disaster

9. Could reversibility be the main feature of the QCD physics (realized in nature) in the QGP near Tc? • Reversibility → perfect liquid • Reversibility → Xover phase transition • Reversibility → Time reversal (start from back)? • Reversibility → Duality between QGP and nucleons? Idea here: focus on reversibility

10. ESSENTIAL INGREDIENTS IN SMALL COLLISION SYSTEM MODELS Is it a QGP we see in small system?

11. Now ridges and c2{4} in most models • Hydro • AMPT • CGC • Ropes + shoving • Interference? • Even not clear that c2{4}<0 means collective or vice versa • What else can discriminate? • dN/dη? I think not… How discriminative are ridges and c2{4}?

12. In the end the quest for origin of QGP-like effects in small systems is about validation and falsification of models • Fundamental caveat: models are incomplete • Missing physics, approximations, LO and tuning • Idea: to falsify models we need to separate out the essential ingredients and focus on the principle differences in models rather than their quantitative predictions Other challenge and how to make progress

13. Long range correlations for 100 ≤ Nch < 120 NB! this is after Short Range Correlations have been subtracted LRC figures from ATLAS-CONF-2015-051 Long range correlations (e.g. UA5 results) was one of the first indications of collectivity in small systems, because it requires that longitudinal color fields, long ranged in rapidity, are formed in the initial collisions.

14. Origin of the saddle point (?) Slide from: http://indico.cern.ch/event/223909/contribution/11/attachments/367751/511867/MGyulassy-MIT051713v2.pdf http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.39.1120 I want to construct simple models based on this idea that particle production factorizes into a sum of “triangles”!

15. Particle production from a single nucleon (at mid-rapidity) Inspired by BGK triangle Start: , Stop: flat in rapidity: Each string produces on average particles (random in y) – Nch is taken from Poisson distribution Particles are randomly distributed in rapidity dN/dy “A Lund-like string” 1 yBeam yBeam+Δy

16. Simulation of long range correlations for 100 ≤ Nch < 120 LRC figures from ATLAS-CONF-2015-051 The simple string simulation reproduces the saddle point shape and the relative magnitude Stringsimulation

17. Saddle point means that longitudinal color fields (strings) enters from left and right • There is essentially a factorization (p-Pb model) • Similar to the one that is now in PYTHIA • Magnitude of saddle point gives the dN/dyof each string because particle production is only correlated inside a string What is the physics?

18. ATLAS studies of pseudorapidity correlations ”SPAM” ”HAM”

19. All initial states have to be generated in a similar way: pp / p-Pb / Pb-Pband are essentially just more of the same → no new initial state physics or non-linearity • Lund strings: OK (but needs tuning) • CGC “strings”: all strings have to be the same! • Qs varies a lot () →Qs cannot affect dN/dy for the strings • QGP • Need long range correlations • Have to be produced early → in quark-gluon state • But that could suggest that you need recombination! Why is this so exciting?

20. QGP picture: initial state (rapidity correlations) → hydro → freeze-out • Rope picture: initial state strings/ropes → shoving →final state fragmentation (rapidity correlations) • Perfect liquid picture (reversibility) suggest that either way is the same because correlations in the initial state (q and g) will be preserved in the hadronization(else entropy would grow) • Main difference is time scale: • QGP: hadrons at Tc • Ropes: hadrons at tfragmentation (supposedly independent of system size) • Else rope model is in some sense a QGP model (and vice versa) • Point of view: once you start to add final state interactions to pp non-QGP models they become difficult to separate from QGP models (in small systems) QGP and hydro or ropes and shoving?

21. Select Npart from Glauber INEL calculation • Npart p = 1, Npart Pb = Npart – 1 • (per Npart) is fixed by dN/dη (ALICE) • Fluctuations of Nstrings are modelled via NBD matched to pp data • Each string is boosted from CM to LAB frame • The goal was to restrict parameters to avoid tuning • Essentially only one choice: proton is assigned Nstrings as largest Pb participant (the proton can get more “wounded”) Extend my simple string model to p-Pb collisions

22. Comparison with dN/dηin p-Pb collisions Multestiator: |η|<1.4 0-5%, 5-10%, 10-20%, 20-40%40-60%, 60-80%, 80-100% Data from ALICE: Phys. Rev C 91 (2015) 064905 • The string model only works well at mid-rapidity so I use that multiplicity estimator • It describes the data within ~10% suggesting that non-linear effects for dN/dη are small • For more details, see:https://indico.cern.ch/event/487649/timetable/#18-ideas-for-a-data-driven-mod

23. CGC early predictions for p-Pbor Can we actually measure Qs? Phys. Lett. B 728, 662 (2014) ~exp(-y) Spectacular prediction: The proton scatters of the Pb CGC while for hydro <pT> follows the “energy density”. See also K.Deja, K.Kutak, PRD 95 (2017), 114027

24. Alt. CGC predictions for p-PbPhys. Rev. C 94, 024917 (2016) Partons Hadrons From paper abstract: “We update previous predictions for the pTspectra using the hybrid formalism of the CGC approach and two phenomenological models for the dipole-target scattering …. and demonstrate that the ratio <pT(y)>/<pT(y=0)>decreases with the rapidity and has a behaviorsimilar to that predicted by hydrodynamical calculations.”

25. Can we measure Qs (opposite dependence of hydro) or not? • If we can measure Qs (first prediction) : strong prediction that can be tested • If not (second prediction) : “leading order” CGC picture is qualitatively wrong • Dangerous because you need a full calculation to relevant (unknown?) orders • In this case we need a full (ideally open source) generator implementation so one can understand that all approximation makes sense at the same time What is correct?

26. REVERSIBILITY of the QGP

27. The good thing • From studies of Pb-Pb collisions we have developed a fantastic understanding and model description of the phases of a collisions • That I will take over as a baseline for the dilute pp collision (assuming that a QGP is produced in the UE of each pp collision)

28. No apparent onset! QGP in very small systems?

29. The problem • Physics of initial collisions are not well understood • Proposed solution: start from the back! ?

30. Final state particles(at kinetic freezeout) The very forward going particles have most of the energy and are very important for the formation of the QGP, but only for creating it. After that they decouple.

31. Now we reverse time(to the moment of Hadronization) Even in heavy-ion collisions the time between hadronization and chemical and kinetic freeze-out is supposedly short. For these very dilute systems (less than MB pp collisions) the main non-reversible effects is supposedly strong decays. But even that must be a small effect.

32. Now we reverse time(into the QGP phase) The phase transition is a Xover so that is reversible!

33. Now we reverse time(in the QGP phase) The QGP behaves as a nearly ideal liquid, so it is as reversible as it can be. This suggests that we create a QGP every time in collisions of even very few low energy hadrons!?

34. If produced in all pp collisions then we can select the softest with the least particlesTime reversal → no energy threshold • Not trivial to understand what Quantum Mechanics means for reversibility • Scattering theory typically assumes that we start from –infinity and ends up at +infinity • Also in low energy systems, ridge must be limited by rapidity gap • Difficult to detect • But the QGP should also be there! Is QGP produced in all hadronic collisions?

35. If QGP is produced in the underlying event of all pp collisions then we can select pp collisions that are particularly simple • Few particles at mid-rapidity • No heavy resonances • The near reversibility of the perfect liquid and the Xover phase transition close to the chemical freeze-out then implies that one always produce QGP in the underlying event of hadronic collisions (no onset in Xover regime) Summary of this key point

36. INTERLUDE: What have we learned about nucleons from qgp studies

37. The BAG model gives a weakly coupled QGP that is falsified by the perfect liquid→ unnatural that bag model should describe nucleon Bag model idea:weakly coupled picture Nucleon: vacuum pressure stabilized by uncertainty principle QGP: vacuum pressure stabilized by temperatrure

38. No evidence for baryon production via baryon junctions in pp collisions→ Surprising if Baryon Junction would play a big role other places Baryon Junctions and CR C. Bierlich, J. R. Christiansen, Phys.Rev. D92 (2015) no.9, 094010 Rise not seen indata

39. A DUALITY between qgp and hadrons? For more details have a look at arXiv:1709.03415

40. If QGP is produced in all small systems and time reversal is a good symmetry then it suggests to me that there must be a relation between the QGP and hadrons • I propose that there is a duality between them • Would explain how QGP can form in small systems • What does duality mean? A duality between QGP and hadrons?

41. My idea: degrees of freedom are similar in QGP and hadrons • If we zoom in on QGP and hadrons we see similar structure • Strongly interacting QGP→ strongly interacting protons • CGC is an example of interacting quarks and gluons in the dense (non-linear) proton regime→ can we generalize this to QGP Let’s try to do that What does such a duality mean

42. Idea: we have the same type of dense fields (CGC like) in the QGP and hadrons • Characterized by scale Qd (d for domain) • In hadrons Qd = Qh • Proton and neutron made up of 3 dense fields of size/scale Qh • In QGP Qd > Qh • Fields expands hydrodynamically (a la shoving) and hadronizes vis recombination at Qh Dense fields

43. Evolve CGC to proton • Several problems • CGC: 2D → Proton: 3D • CGC weakly coupled →proton strongly coupled • But let’s try to do it • Assume that for proton (mass shared by 3 fields) → • Number of fields in proton ~ 2.99 • Proton consists of such fields • So the bulk proton is always dense • No emergence of a Qs scale, it is always there Proton as 3 dense fields?

44. Dense fields → No diffusion • From long range correlations I have argued that we recombination is a possible solution • The dense fields naturally recombines when Qd = Qh • Entropy is conserved • Number of dense fields is conserved: Qdexpands to Qh • Dense fields are constituent-quark-like degrees of freedom Why is it nice with dense fields in the QGP

45. Qualitative dense field picture of QGP evolution Qd > Qh (~EKRT) Qd → Qh Qd > Qh Picture is qualitatively similar to the expanding cold gas of strongly interacting atoms

46. Assumptions • Asymptotic freedom (QCD relation) means that a color charge in vacuum physically grows into a dense field • Energy of dense field scales as 1/size (Heisenberg, similar to Bag model): • To restrict size I assume that • Simplest case which has a minimum , for • Close to constituent quark mass • Expansion driven by energy minimization • Phase transition condition (minimum energy of dense field) What is the origin of the dense fields?

47. My idea is that it must be similar to what causes showing in string models • The dense field works as a superconductor to repel external fields • Similar ideas can be found in CGC related literature A. H. Mueller, NPB 643, 501 (2002)E. Iancu, K. Itakura and L. McLerran, NPA 724, 181 (2003): • In both works they found that the state is color neutral for gluon wavelengths larger than 1/Qs due to active shielding from other gluons (while color appears randomly distributed for wavelengths less than 1/Qs). • If true there must also be some relation to the Debye screening which scales with T (Qd). Why do the dense fields repel?

48. Density of dense fields (from size): • Energy density of dense fields:as the running of s is small near Qh, so • Proportionality constant could be fixed from energy density of nuclear matter • From lattice QCD we know that in the QGP so this suggests that Pocket formulas for dense fields

49. We can estimate the relative Qd growth from RHIC to LHC as total multiplicity will scale with the number of initial domains • 3D: → Guess 2D: • 3D: → Guess 2D: • 𝑑𝐸𝑇/𝑑𝜂 grows faster than dN/dη • Fundamental prediction of model when particle composition is fixed (be careful about RHIC) Predictions from dense fields:ET grows faster than dN/deta

50. In PYTHIA one has the scale pT0:that depends on and is of a similar value as Qs(y=0) • In the same way one would expect that if enery loss is due to an interaction with the medium (rather than a FF change) the dense field scale will screen • Initial screening parameter will therefore be ~40% larger at LHC than at RHIC • This is IMO also a fundamental property of this model Predictions from dense fields:quenching non-linear in density