Heavy Ion Physics at the LHC What's new ? What's next ?

1. Heavy Ion Physics at the LHCWhat's new ? What's next ? • Selected Results from the LHC • New light on old puzzles • Towardsprecision measurements of QGP properties • Discoveries • Some VERY preliminary Stuff • Rocky Horror Picture Show

2. Collider of'Large Hadrons' CMS LHCb ATLAS ALICE Large 'Hadron Collider' Design Energy: 14 TeV (pp) 1150 TeV (PbPb) Lake Geneva p + p Pb + Pb 4 Large Experiments, all participate in HI program (pA, AA)

3. What is different ? What is new ? • What is different ? Same physics with .. • different 'matter:'increased energy (up to factor ~30 in √s ) • QGP will be 'hotter - larger - longer living' • large cross section for 'hard probes' : high pT, jets, heavy quarks,… • different experiments: new generation, large acceptance state-of-the-art detectors • Atlas, CMS, Alice, [LHCb, for pA] • Where is progress@LHC ? (very limited & personal selection) • New Light on Old Problems (NLOP) • hadronisation/particle production • quarkonia suppression • Towards Precision Measurements(TPM) of QGP parameters • elliptic flow: viscosity h/s • jet quenching: opacity q^ • Discoveries • the 'Ridge': long range correlations in pp & pA • What seems odd ? • New/preliminary results which don't fit easily

4. NLOP I: Particle Production • Hadronisation is non-pertubative • phenomenological models ('event generators') with many parameters • strings or clusters, PPAR(x), x=1,..n • Statistical ('thermal') models: • particle with mass m produced in 'heat bath T' according to phase space • P(m) ~ e-(m/T)Tch Temperature (ch='chemical')mbBaryo-chemical potential (baryon conservation)gs Strangeness suppression accurate to O(10%)(no 'a priori' justification) Tch: 160-170 MeVgs : 0.9-1 (AA), 0.5-0.6 (pp) strangeness enhancement = QGP signal ? SPS Pb-Pb particles created per collision RHIC pp Data Model

5. Dynamical Origin of Thermal Ratios ? • 'born into equilibrium' (e+e-, pp, AA) at the QCD phase boundary Tc≈Tch • yields ~ QCD x (hadronic) phase space many channels => PS dominates • ppgs < 1 : QCD, ms • ?: which dynamics makesstrangeness enhancement in AA (gs≈1) ? • 'evolving into equilibrium' (AA) thermodynamics in parton/hadron phase • arbitrary (eg pp-like) initial hadron ratios + inelastic reactions • reach equilibrium close to phase boundary (Tch~ Tc) A + B <=> C + D + E • ?:Parton dynamics: why close tohadron rather than parton equilibrium ratios ? • non-equilibrium models with additional 'unpredictable' free parameters • ?:Hadron dynamics (expansion & cooling) => expect sequential freeze-out ? • hadrochemistry timing miracle: long enough to reach equilibrium, short enough for sudden, fixed T freeze-out In this picture, statistical models work - in AA by design (thermodynamics), - in pp/e+e- by accident (dominance of PS). Conceivable, but limited direct evidence.. Deviations from equilibrium would be most informative !

6. Particle Ratios at LHC • pp: Less well described than at lower energies ? • fits 2 ratios with 2 parameters (T, mB) • maybe finally 'hard QCD processes' (ME, dynamics) become visible over PS ? pp fit should be updated better & more hadrons available now RHIC pp

7. ? Particle Ratios Pb-Pb Strangeness is enhanced +30% (K), > factor 3 (W) gs = 1,like thermal model p/p off by factor > 1.5 from predictions ! suppressed, not enhanced, compared to pp initially very surprising result (safest prediction for LHC !) K(s) X(ss) W(sss) (Prediction, no fit)

8. NLOP I: Current Explanations • SM ok, but accuracy of experiment and/or model limited to 20-30% ? • Joverall not too bad, including d & 3HL • Lspreads misery amongst many particles • poor chi2 • what about RHIC results ? http://arxiv.org/abs/1311.4662 I expected that the SM could do better (<10%)

9. NLOP I: Current Explanations add inelastic hadronic reactions • Sequential Freeze-out ('afterburner') • J improved fit (also RHIC/SPS) • centrality dependence at LHC • L more art than science.. • many unknown s (e.g. L + W -> np + mK) • detailed balance (e.g. p + p <=> 5 p) • centrality dependence at RHIC, X • 'Parton-Equilibrium' Statistical Model J • L 2 more free parameters gQ, gs • non-intuitive values, d & 3HL (gqn)? • would be a major change of paradigm

10. Particle Production: What next ? • Is there indeed a significant change from RHIC to LHC ? • clear sign of thermo-dynamics in hadron production ! • revisit early RHIC results (decay corrections, ..), reduce syst, errors @ RHIC & LHC • Hadro-modification (sequential freeze-out) of other particles ? • K*, D++, L(1520), X* , r, … • everything with large width/cross section and few-particle channels • broad resonances are usually 'excused'; but we can't we rather use them ? - decay & rescattering ? - late freeze-out (T ~ 140 MeV) ?

11. Particle Production: What next ? • Strangeness (gs -> 1) enhancement in pPb ? in high multiplicity pp ?? • use, rather than ignore, approach to strangeness equilibrium.. Thermal Model T = 164 MeV If indeed we reach (approximate) chemical equilibrium in pPb: => compare small (pPb) and large (PbPb) systems => 'finite size' (lifetime) effects Thermal Model T = 164 MeV - K/p & L/p reach central PbPb - p/p stays well above ! - X,W to come..

12. NA38 ~ 1986 NLOP II: Quarkonium suppression • J/Y, the HP par excellence: 'well calibrated (pQCD) smoking gun' Matsui & Satz, 1986: NA50 ~ 1996 light ions (O,S): nomalous (CNM) heavy ions (Pb): Anomalous (QGP?) Nuclear Matter Effect !!

13. J/Y @ SPS Deconfinement of Onset ?? In-In Pb-Pb Onset of Deconfinement ! Deset of Onconfinement ??? Nuclear absorption measured at 158 GeV NA50 ~ 1996 Including nuclear shadowing Onset of Deconfinement ? NA60 ~ 2006 NA60 ~ 2009

14. Expectation: decisive answer from RHIC. And we even got it ! (~) all predictions were wrong ! RHIC (y=0) ≈ SPS (y=0) ; RHIC(y≠0)<RHIC(y=0)!! Two broad classes of explanations why RHIC ≈ SPS: equal by IntelligentDesign: sequential melting (Y', cc suppression only) equal by miraculous Accident: suppression cancelled by recombination Expectation: decisive answer from LHC (+ Y @ RHIC) different sensitivity to melting (Tc) and recombination (charm density) J/Y @ RHIC Déjà vu ? J/Y measured over expected arxive:1010.582 arxive:1010.582 RHIC midrapidity SPS Predictions 2005 Miracle or ID ? RHIC large rapidity O

15. CMS vs STAR mid-y, high pt J/Y:RHIC ≠ LHC (finally) ALICEvsPHENIX m+m-fwd-y, all pt STAR CMS LHC ≠ LHC Alice ee ALICE mm ALICE vsPHENIX e+e- y=0, all pt CMS high ptmm

16. J/YRAA: ptdependence RHIC ≠ LHC LHC = LHC ! J/Y suppression IS different @ LHC unless CNM plays very dirty,unexpected tricks ALICE e+e- ALICE mm CMS mm PHENIX e+e- PHENIX mm

17. NLOP II: J/Y Regeneration J/YRAA:centrality dependence J/YRAA:pT dependence melting regeneration On Recombination/Coalesence: Purist: Another dirt effect which obscures deconfinement (CNM, HotMatterRecombination) Pragmatist: Another deconfimenet signal ! Deconfinement colour conductivity 'partons can roam freely over large distance' That's what the primordial charm quarks have to do to recombine

18. sequential suppression <-> suppression & regeneration compelling evidence for recombination at RHIC ? is SPS/RHIC compatible with no J/Y, only Y', ccsuppression ? measure Y', cc! (proposed at SPS) All clear ? Miracle or ID ? Let's wait a bit longer and see - give TH a chance to digest - Y' & Upsilons @ RHIC/LHC - measure CNM with p-PB J/Y RAA: √s (in)dependence

19. ConSequential Y suppression • Y(1S), ~ 50% direct • complete higher state suppression ?? • Y(2S) (~ J/Y) • up to 5 x stronger than Y(1S) • Y(3S) goneY(3S)/Y(1S) < 0.1 (95%CL) Sequential suppression as expected from deconfinement !! Caveats: - Feed-downs from the many bb states to be sorted out - Initial State Effects ( to be estimated from pA)

20. Upsilon at RHIC Déjàvu ? J/Y: RHIC = SPS • STAR • RAA(1+2+3) = 0.56+0.22-0.26 • CMS • RAA(1+2+3) = 0.32 STAR CMS 1 Apple & 3 Apples RHIC -> LHC comparison is crucial - we need to raise the temperature

21. needs PDF + DE + sabs ? pt not very well described ! Y' stronger suppressed than J/Y ! final state absorption ?? RHIC dAu data also not easily described by theory ! CNM in pPb: J/YCNM = Currently Not Mastered J/YRAA versus y J/YRAA versus pt Double Ratio Y'/Y in pPb / pp

22. RpPb: U≈J/Y? (watch the errors !!) higher states more suppressed ≈ 20-30% for Y(2,3)/Y(1,)≈ 50% for Y'/Y CNM absorption (sabs) ? formation time => should disappear@LHC? HNM (comover) final state interaction ??? RpPblhcB ~ PDF(LO!) CNM in pPb: U RpPb Y versus y Plenty of food for thought .. and calculations.. Double Ratio U(n)/Y in pPb / pp RpPb J/Y from B decay vs y Pb-Pb

23. QM 2011: The Flow Renaissance • overwhelming and unambiguous evidence: complex structures from E-b-E hydro flow v2 v3 v4 v3 for p/K/p 23

24. A most amazing Discovery • The 'face' of the collision zone,(state-of-art for 20 years) including 'warts & wrinkles' of each event • Why did it take so long to see the obvious ? • could not be overlooked or mistaken at LHC • large dNch/dy => large signal, excellent resolution • large acceptance detectors Cone Event Plane Resolutions versus centrality 1 = perfect resolution Ridge

25. TPM-I: Quantum Jump in Exp. & Theory • From Leading Order .. • elliptic flow v2 • .. to NLO … • higher harmonic flows v3,…v6 • .. to NNLO … • correlation between harmonic symmetry planes • via geometryconstraints • via non-linear interactions in the hydro evolution(mode mixing) • leading to better limits on h/s Y2-Y4 • Better limit, but not yet good enough. • Aim for measurement (<30% ?) of h/s ! • h/S < 1/4p => conjectured limit is wrong • h/S > 1/4p => measure s in QGP • h/S ≈ 1/4p => AdS/CFT quantum corrections ? O(10-30%) • h/S(RHIC) ≠h/S(LHC) => T dependence J. Jia http://arxiv.org/pdf/1005.0645.pdf post-LHC 4ph/s < 2-3 pre-LHC 4ph/s < 3-6

26. E-b-E flow s(v3) s(v2) Can one use fluctuations as 'E-b-E' selection tool (like centrality) ?? E-b-E v2 distributions v2{n} versus centrality • Fluctuations are Bessel-Gaussian • 2 parameters: <v2> and • v{4} = v{6} = v{8} = …. • no additional information (if BG) • central events (< 5%) and (maybe) v3 are not BG !

27. E-b-E flow as tool normalized pt fluctuations vs Nch Ratio of p/K/p pt spectra: large v2 over all v2 s(v2)/v2 • Whatever it is that makes v2 in 3 < pt < 8 GeV: s(v2)/v2 = constant (5-30%) • trivial for hydro: s(v2)/v2 ~ s(e2)/e2 v2 ~ e2 (almost) indept. of pt • less trivial for quenching: (density weighted pathlength integral)n ~ e • Select v2 at fixed Nch: (compare large v2 & small v2 events) • azimuthal HBT, CME effect, …. • f-integrated pt distribution shows modification: • larger v2 => larger <pt> • radial pressure fluctuations ?

28. Quark Scaling ? v2p/K/p/L versus pt 1 = nq scaling Phenix:1203.2644 • There is no quark scaling < 0.7-1 GeV/nq. Nor should there be ! • We have Hydro mass scaling ! • > 1 GeV/nq: scaling not very good either.. • scaling violation also seen at RHIC • f@RHIC closer to p/K than to p • nq or hadronic 'afterburner' effect ??

29. Decoupling from Flow ? PbPb: p/K/p pt spectra BlastWave fit • Deviation from Hydro at similar pt for radial and elliptic flow • light particles follow radial/elliptic flow up to 1.5 - 2 GeV • heavy particles follow up to > 3-4 GeV • very natural consequence of hydro-push ! • waiting for f at LHC • where will it 'decouple' in pt and v2 ?

30. Everything 'Flows' at LHC Atlas jets Alice open charm J/Y Can there be too much of a good thing ? some flow results are not easily accommodated by theory CMS high pt particles STAR Non Photonic Electrons Alice direct photons

31. jet quenching measures ‘stopping power' of QGP DE ~ f(m) xcqx q xLn x f(E) ^ TPM-II: Energy Loss ('Jet-quenching') MUCW: (my understanding of conventional wisdom) E-E': large, O(10-20 GeV@LHC), RHIC->LHC ~ as expected jet(E'): ~ unmodified vacuum fragmentation (pt > 4 GeV, R ~ 0.4) DE: multiple soft gluons, large angle & low pt (< 3-4 GeV) ^ Main Observables: 1) Inclusive single particles RAA (Nch, PID, HF) 2) Two(3) particle correlations Nch, PID, HF; near/away side 3) Inclusive single jets RAA, long/transverse frag. functions 4) Jet correlations (jet-jet, g(Z)-jet) Main Questions: 1) How much energy is lost ? measure jet imbalance E - E' 2) Shows expected scaling ? vary L, m, E, . 3) Where (and how) is it lost ? measure radiated energy DE .. ! !? ?? • high energy partons loose energy DE when traversing a medium • jet(E) → jet (E’ ) + soft particles(DE) (E’ = E-DE) • energy loss DE expected to depend on: • q : 'opacity ' = property of medium ('radiation length of QGP') • L: size of medium (~ L (elastic) ~ L2(radiative)~ L3(AdS/CFT)) • cq: parton type (gluon > quark) • f(m) : quark mass (light q > heavy Q) • f(E) : jet energy (DE = constant or ~ ln(E))

32. Alice 102 GeV 47 GeV Df Dh Observation of Jet Quenching Energy flow in h-f plane CMS Atlas • Observed at RHIC in 2001 • via suppression of 'leading fragments' (not enough energy to see jets) • qualitative clear effect, quantitative interpretation difficult & model dependent • Very striking at LHC • many unbalanced (E1 ≠ E2) jets and 'monojets'

33. CMS g-Jet :How much Energy is lost ? g - jet energy (im)balance Mean xJ= pt(jet)/pt(g) -Fraction without jet: RJ PT1 g-jet should lead, with improved statistics, to a precise determination of q ^ PT1 g looses no energy => calibrates jet before energy loss !

34. Where (& how) is it lost ? Energy versus jet cone radius r: PbPb/pp Very differential information to constrain Theory RAA of 'hump-back' plateau f(x) RCP of jet fragmentation functions f(z) ^ soft leading soft leading Jet Quenching = Modified Fragmentation Function

35. Mass & Color Charge Dependence RAA of p, D, B RAA inclusive jets and b-jets - Clear mass effect (D versus B), disappears at pt/m >> 1 - Where is the gluon versus quark difference ??

36. Discovery 'Near Side Ridge 'Away Side JET' 'Near Side JET' The 'Opera' defense ! • The first LHC Discovery (pp, Sept 2010) • long range rapidity 'ridge' in 2-particle correlations • visible in the highest multiplicity pp collisions • arguably still the most unexpected LHC discovery

37. Origin of the pp 'Ridge' PbPb 10-20% Flow • Spawned a large numberof different explanations • mostly rather ad hoc, very speculative, or outright weird • Color Glass Condensate CGC:'first principles' theory • classsical FT in high density limit (small x, small Q2) • 'new state of cold & dense parton matter' • some success describing aspects of ep, pp, eA: geometric scaling, low-x, particle production, .. • however, no 'smoking gun' so far… • Collective flow (Hydro) ? • vaguely similar correlations in nucleus-nucleus

38. Ridges everywhere.. 2 particles 4 particles ATLAS pPb 5 TeV Double Ridge It looks like a rose … CMS pPb 5 TeV ALICE pPb 5 TeV Stronger NSR • Ridge is much stronger in pPb (end 2012)! • and is, in fact, a 'double ridge' • even and odd components (v2, v3) • collective multiparticle v2{4} (i.e., not 'jet' like) • mass dependent v2(m,pt) • now also seen in dAu at RHIC ! (tbc) • strength ≈ as predicted by some hydro model

39. More on the Ridge ATLAS v2 v3 CMS v2{2}, v2{4}, v3 Hydro predictions ALICE v2 for p, K, p

40. (Mis)Use PbPb Glauber MC for pPb It smells like a rose … e{2}/e{4} ≈ 1.7 Npart = 25 e{2}/e{4} ≈1.5 Npart = 12 e{4} ≈e{6} v{2}/v{4} ≈1.4

41. .. there MUST be radial flow Where there is elliptic Flow.. 'Baryon Anomaly' L/K0S CMS <pt> versus Nch for p, K, p BlastWave Fit p, K, p Combined BW fit to pt and v2 It pricks like a rose …

42. Another prediction come true .. b versus Nch in pPb, PbPb BW fit (b,T)pPb and PbPb T versus Nch in pPb, PbPb smaller fireball, larger gradient, more radial flow ? as predicted (e.g.by Shuryak) ?? arXiv:1301.4470 [hep-ph] A. Ortez et al http://arxiv.org/abs/1303.6326v2

43. ? .. or too much of a good thing ? BW fit to high multiplicity pp BW fit (b,T)pPb and PbPb andpp We can not (longer) afford to ignore small systems: - similarity in particle ratios (smooth evolution of gs) - similarity in flow patterns (radial & azimuthal) ??? Where is the onset ??? All Options look bad: - Hydro everywhere (incl. low mult. pp) - Hydro nowhere (incl. central PbPb) - Looks like flow in ppbut isn't (pPb ??) Where are all the roses gone ...

44. pPb (pp): PantaRhei ? New State of Matter created at CERN which features many of the characteristics of the theoretically predicted Colour Glass Condensate. RHIC Scientists found "Colorful Glass" to serve the Perfect Liquid Rewrite the textbooks at least change the title from 'Heavy Ion physics' to .. • CGC ? • some results come natural, others need additional 'ad hoc' explanations • odd harmonics (v3), cumulants v2{4}, PID v2, • Collective 'Hydro-like' flow in pA (& pp) ?? • most results are 'natural' (at least within factor 2) if one assumes hydro.. • energy/particle density quiet comparable to AA (eg high Nchpp@LHC≈ Cu-Cu mid-central @RHIC) • system size only few fm3 ?? (presumably << 10 compared to >> 1000) • however, hydro has no intrinsic size, only ratio's: l/r, and l≈ 0 ! (from h/s) • a proton@LHC is more like a small nucleus(dozens of partons, MPI,..) ! • what other measurements are needed ?? • In either case, more than a curiosity • CGC • discovered a 'new state of matter' • smoking gun for new 'first principle' limit of QCD • Hydro stunning: a system the size of a single hadron behaves like 'macroscopic matter' • 'extra dimension' for QGP study: size ! • finite size effects => correlation & coherence length, time scales, ….

45. The rocky Horror Picture Show • A random collection of recent results (Hard Probes Conferences 2012/2013) • which don't fit easily anywhere • will fade away with a whimper • or disappear with a bang • or stay and have a big impact ?

46. Weird ! CMS charged particles vs ALICE/ATLAS jets CMS: RpPb at high pt ALICE charged ATLAS jets pt x 0.5 ALICE jets pt x 0.5 ' Anti-shadowing on steroids' Anti-doping squad is investigating; so far just about compatible with competition (Atlas/Alice)

47. Weirder !! CMS has discovered the 'anomalous Y' un-suppression' implications totally unknown, but definitely profound (one way or another) Some tension with ALICE high ptY' lower ptY' • "RAA" ; i.e. double ratio Y'/Y(Pb) / Y'/Y(pp) • high pt: "RAA"< 1 (Y'suppression stronger), small indicationrising with Npart ?? • lower pt: "RAA" > 1!!! (Y' suppression less strong !), rising with Npart !!

48. Weirdest !!! ATLAS has discovered 'Super jet-quenching' in central pPb collisions which however turns out to be 'Hyper-jet un-quenching' in peripheral pPb collisions ! jet RCP in p-Pb as function of centrality & y jet RpA(Phythia) in p-Pb as function of centrality & y jet RpA(Phythia) inMinBias Spectra biased by centrality selection ? as seen by ALICE in pt spectra

49. Weirdissimo !!!! After hadrochemistry & hydro-flow The final proof: 'Y suppression in pp' U(2S)/U(1S) versus ET(|h|>4) U(2S)/U(1S) versus Nch More likely a totally obvious & trivial effect left as an easy exercise to the reader hint: little/no dependence on Et(|h|)>4 in pp..

50. Abrupt End !