Heavy Ion Physics at the LHC Whats new

Heavy Ion Physics at the LHC What's new ? What's next ? l Short pre-LHC history l Reminder of main SPS/RHIC results l Selected Results from the LHC l New light on old puzzles l Towards measuring QGP properties l Discoveries 1 l Open Issues/Questions May 2013 Stockholm J. Schukraft

QGP: Matter under extreme conditions - Macroscopic partonic matter ('QCD thermodynamics') - Deconfinement/Color Conductivity - Chiral Symmetry Restoration 2

http: //press. web. cern. ch/press-releases/2000/02/new-state-matter-created-cern Based on a (unpublished) 'common assessment' of results from ~ half dozen experiments collected & published over the course of the SPS Pb program (1994 - 2000) http: //arxiv. org/abs/nucl-th/0002042 v 1 The collected data from the experiments gives compelling evidence that a new state of matter has been created. This state of matter found in heavy ion collisions at the SPS features many of the characteristics of theoretically predicted quark-gluon plasma. . 3 in today's LHC speak: '. . a QGP-like state. . '

Main Results from SPS l strangeness enhancement l 'anomalous' J/Y suppression ð in general: thermal particle production ð predicted for thermal system (partonic ? ) ð predicted as deconfinement signal Hyperon production pp x 20 p. A J/Y over Drell-Yan - the experimental results have x 5 stood the test of time - interpretations may have changed over time 2 & is correct (in my view) - essence of the statements xwas Pb. Pb ð evidence for a new state of matter at the SPS energy range e+e- pair yield however, today more 'compelling' than in 2000 ! (later SPS results, RHIC energy scan, LHC) l low mass lepton pair enhancement ð'rho melting', sign of chiral symmetry restoration ? 4 May 2013 Stockholm J. Schukraft

http: //www. bnl. gov/newsroom/news. php? a=1303 Based on a (published) comprehensive (re)analysis of the first years of RHIC (2000 - 2004) Nucl. Phys. A 757: 1 -284, 2005 . . created a new state of hot, dense matter out of the quarks and gluons. . , but it is a state quite different and even more remarkable than had been predicted. in today's LHC speak: not '. . the QGP. . ' but '. . a QGP. . ': s. QGP: strongly interacting QGP 5

Main Results from RHIC l high p. T suppression 'jet-quenching' l strong elliptic flow (scaled) Flow ð ~ maximum possible i. e. 'ideal liquid' (h/s ≈ 0) ð mostly produced in the early phase (partonic? ) ð very strongly interacting (large energy loss) RHIC SPSNote: s. QGP is described by its experimental properties AGS Also here, all experimental results AND their implied QGP characteristics stood the test of time: More remarkable than predicted ! ð Multiplicity l direct 'thermal' photons => 'hot matter' ð data: inverse slope T ~ 220 ± 20 Me. V model dependent T 0: 300 - 600 Me. V 6 May 2013 Stockholm J. Schukraft

Collider of 'Large. Collider' Hadrons' Large 'Hadron Design Energy: 14 Te. V (pp) 1150 Te. V (Pb. Pb) Lake Geneva CMS p+p Pb + Pb LHCb ALICE 7 ATLAS 4 Large Experiments, 3 participate in HI program May 2013 Stockholm J. Schukraft

What is left to do at LHC ? l What is different ? Same physics with. . ð different 'matter: ' increased energy (up to factor ~30 in √s ) µ QGP will be 'hotter - larger - longer living' µ large cross section for 'hard probes' : high p. T, jets, heavy quarks, … ð different experiments: new generation, large acceptance state-of-the-art detectors µ Atlas, CMS, Alice, [LHCb, for p. A] l Where is progress@LHC ? (very limited & personal selection) ð New Light on Old Problems (NLOP) µ hadronisation/particle production Q#1 µ quarkonia suppression Q#2 ð Towards Precision Measurements (PM) of QGP parameters µ elliptic flow: viscosity h/s µ jet quenching: opacity q^ ð Discoveries Q#3 µ the 'Ridge': long range correlations in pp & p. A 8 May 2013 Stockholm J. Schukraft

NLOP I: Particle Production l Hadronisation is non-pertubative ð phenomenological models ('event generators') with many parameters µ strings or clusters, PPAR(x), x=1, . . n l Statistical ('thermal') models: accurate to O(10%) (no 'a priori' justification) ð particle with mass m produced in 'heat bath T' according to phase space ð P(m) ~ e-(m/T) Tch: 160 -170 Me. V gs : 0. 9 -1 (AA), 0. 5 -0. 6 (pp) Tch Temperature (ch='chemical') strangeness enhancement = QGP signal ? mb Baryo-chemical potential (baryon conservation) gs Strangeness suppression particles created per collision RHIC pp Data SPS Pb-Pb 9 Model May 2013 Stockholm J. Schukraft

Dynamical Origin of Thermal Ratios ? l 'born into equilibrium' (e+e-, pp, AA) ð yields ~ QCD x (hadronic) phase space µ pp gs < 1 : QCD, ms µ AA gs ≈ 1 : thermo-dynamics in parton phase at the QCD phase boundary Tc ≈ Tch many channels => PS dominates ð BUT: e+e-, pp: why Tc ≈ Tch AA: why don't we see parton equilibrium ? l 'evolving into equilibrium' (AA) thermodynamics in hadron phase ð arbitrary (eg pp-like) initial hadron ratios + inelastic reactions µ reach equilibrium close to phase boundary (Tch < Tc) A + B <=> C + D + E ð BUT: dynamic system (expansion & cooling) => expect sequential freeze-out ? µ hadrons with large inelastic s should freeze out later (lower T) ! 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. . 10 May 2013 Stockholm J. Schukraft

Particle Ratios at LHC l pp: Less well described than at lower energies ! ð fits 2 ratios with 2 parameters (T, m. B) ð maybe finally 'hard QCD processes' (ME, dynamics) become visible over PS ? RHIC pp 11 May 2013 Stockholm J. Schukraft

Particle Ratios Pb-Pb Strangeness is enhanced +30% (K), > factor 3 (W) gs = 1, like thermal model initially very surprising result (safest prediction for LHC !) p/p off by factor > 1. 5 from predictions ! suppressed, not enhanced, compared to pp K(s) X(ss) ? W(sss) (Prediction, no fit) 12 May 2013 Stockholm J. Schukraft

NLOP I: Current Explanations l Sequential Freeze-out add inelastic hadronic reactions ð hadronic final state reactions ð but: more art than science. . µ many unknown s (e. g. L + W -> np + m. K) µ detailed balance (e. g. p + p <=> 5 p) Questions 1: Hadronisation Unanticipated deviations at LHC from the Statistical Model (pp&AA) l 'Parton-Equilibrium' Statistical Model ð 2 more free parameters g. Q, gs - Evidence against the SM ? ? µ non intuitive values, supercooling - Make 'mysteriously successful' SM more reasonable by ð would be athe major change of paradigm showing expected/conceivable deviations ? (NLO-corrections) - In either case, can we use these results to make progress ? 13 May 2013 Stockholm J. Schukraft

NLOP II: Quarkonia Suppression Heavy Flavor cc / bb hard collisions (p. QCD@t ≈ 0) 'diffuse' (colour conductivity) 'melt' (deconfinement) RAA: J/Y measured / expected SPS ≈ RHIC l J/Y suppression similar at RHIC and SPS ! ð should depend on Energy (Temperature/Density) µ 1) No J/ Y melting at both SPS & RHIC (T > 1. 5 – 2 Tc )? only weakly bound states (Y’ & c) melt. µ 14 2) More J/ Y melt at RHIC than at SPS but ‘by chance’ cancellation from regeneration (cc recombination) ? December 2012 Bad Honnef J. Schukraft

J/Y: Consistent with Melting + Regeneration ! J/Y RAA: centrality dependence J/Y RAA: p. T dependence LHC > RHIC melting regeneration central y≈0 forward y≈3 15 Regeneration more important at - central: ~ Nc 2 - low p. T : phase space Recombination/Coalesence: Purist: Dirt effect, obscures deconfinement Pragmatist: Deconfinement signal ! Deconfinement colour conductivity 'partons roam freely over large distance' That's what primordial charm quarks do to December 2012 recombine Bad Honnef J. Schukraft

Y suppression l Y(1 S), ~ 50% direct ð direct Y not suppressed ? l Y(2 S) (~ J/Y) ð up to 5 x stronger suppression than Y(1 S) l Y(3 S) ~ gone ð Y(3 S)/Y(1 S) < 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 p. A) 16 December 2012 Bad Honnef J. Schukraft

NLOP II: Quarkonium Suppression l J/Y, the HP par excellence: 'well calibrated (p. QCD) smoking gun' Matsui & Satz, 1986: Questions 2: Quarkonia Production - Regeneration (if confirmed): Dirt effect or Deconfinement signal ? - Sequential Y suppression: Settles the deconfinement case ? - If neither, what else would be needed from experiments ? (besides smaller errors) 17 June 2009 India J. Schukraft

Flow in Heavy Ion Collisions l Elliptic Flow v 2 ð Initial Conditions µ e. g. Geometry ð Fluid Properties µ e. g. shear viscosity h Fourier analysis: d. N/df = 1 + 2 v 2 cos(2 f) + … d. Nch/df (eccentricity e 2 = (y 2 -x 2)/(y 2+x 2)) usually use Viscosity/Entropy (h/s dimensionless) x y y x Z Azimuthal (f) pressure gradients Anisotropic particle density Py Pz Px Re ac tio n pl an e f Pressure Dpx > Dpy Y Spatial deformation 18 X Anisotropic flow December 2012 Bad Honnef J. Schukraft

QGP: The 'perfect Liquid' l Perfect liquid ® Viscosity h/s » 0 ð large interaction cross section s in the liquid l unexpected result ð QGP though to behave like a gas (i. e. weakly interacting) ð closest Theory prediction h/s > 1/4 p ≈ 0. 08 µ Ad. S/CFT: ('QCD analogue') (Conformal Field Theory in Anti-de Sitter Space) µ conjectured Quantum limit: h/s = 1/4 p mfp l ≈ Compton wavelength l Pre-LHC limit: h/s < (3 -6) x 1/4 p ð initial conditions (pressure/energy distribution) not known precisely enough 19 December 2012 Bad Honnef J. Schukraft

Initial Conditions l Around 2010 ð 2) Event-by-Event fluctuations → more complicated shapes (higher order ) µ suggested in 2010 but controversial, higher vn where not directly 'seen' in the data Fourier series: d. N/df = 1 + 2 v 1 cos(f) + 2 v 2 cos(2 f) + 2 v 3 cos(3 f) +… Triangular, v 3 20 Elliptic, v 2 December 2012 Bad Honnef J. Schukraft

2011: First LHC (& new RHIC) data l overwhelming and unambiguous evidence: complex structures from E-b-E hydro flow ð interference of different harmonics ð all characteristics as expected from hydro: Ridge h-f. ATLAS-CONF-2011 -074 Two Particle Correlation (Atlas) Valley µ strength, mass/centrality/momentum dependence Two Particle Correlation projection on f v 1+v 2+v 3+v 4+v 5 v 2 v 3 v 4 v 3 for p/K/p mass dependence typical for hydro ! 21 21 May 2013 Stockholm J. Schukraft

A most amazing Discovery l The 'face' of the collision zone, (state-of-art for 20 years) including 'warts & wrinkles' of each event ð progress in precision measurements of h/s µ higher harmonics large sensitivity to viscosity µ discriminate & constrain models & geometry 22 Hydro Calculation December 2012 Bad Honnef J. Schukraft

TPM-I: Quantum Jump in Exp. & Theory l From Leading Order. . ð elliptic flow v 2 Better l. . to NLO … limit, but not yet good enough. for measurement ð higher. Aim harmonic flows v 3, …v 6 (<30% ? ) of h/s ! Y 2 -Y 4 h/S l. . to NNLO …< 1/4 p => conjectured limit is wrong > 1/4 pharmonic => measure s in QGP ð correlationh/S between symmetry planes h/S ≈ 1/4 p => Ad. S/CFT quantum corrections ? O(10 -30%) ð via geometry constraints J. Jia http: //arxiv. org/pdf/1005. 0645. pdf ð via non-linear interactions in the hydro evolution (mode mixing) l leading to better limits on h/s pre-LHC 4 ph/s < 3 -6 23 post-LHC 4 ph/s < 2 -3 May 2013 Stockholm J. Schukraft

TPM-II: Jets & 'Jet-quenching' l partons loose energy DE when traversing a medium ð Jet(E) → Jet (E’ = E-DE) + soft particles(DE) ð QCD energy loss DE expected to depend on: ^ : 'opacity ' = property of medium ('radiation length of QGP') µ q µ L: size of medium (~ L (elastic) ~ L 2(radiative), L 3(Ad. S/CFT)) µ cq: parton type (gluon > quark) µ f(m) : quark mass (light q > heavy Q) µ f(E) : jet energy (DE = constant or ~ ln(E)) jet quenching measures ‘stopping power' of QGP DE ~ f(m) x cq x ^ q x Ln x f(E) 24 q q 1) How much energy is lost ? measure jet imbalance E - E' 2) Where (and how) is it lost ? measure radiated energy DE 3) Shows expected scaling ? December 2012 Bad Honnef J. Sc vary L, m, E, . . .

Observation of Jet Quenching l Observed at RHIC in 2001 ð via suppression of 'leading fragments' (not enough energy to see jets) µ qualitative clear effect, quantitative interpretation difficult & model dependent l Very striking at LHC ð many unbalanced (E 1 ≠ E 2) jets and 'monojets' RAA: measured / expected yield Alice 102 Ge. V Energy flow in h-f plane 47 Ge. V CMS Df Dh Atlas 25 December 2012 Bad Honnef J. Sc

1) How much Energy is lost ? l Di-Jet energy balance Aj Aj = (PT 1 -PT 2)/(PT 1+PT 2) PT 1 ≈ PT 2 PT 1 Quenching Model PT 1 >> PT 2 PT 1 <DE> ≈ 20 Ge. V (wide distribution) Medium is VERY strongly interacting ('opaque') (but within expectations) jet quenching DE ~. . . 2) Energy dependence roughly as expected (weak dep. on Energy) jet quenching DE ~ f(m) x cq x ^q x Ln x f(E) (CMS PLB 712 (2012) 176) 26 December 2012 Bad Honnef J. Sc

2) Where (& how) is it lost ? Di-jet angular correlation Df 12 Simulation pp Pb. Pb back-to-back Df 12=1800 E → E’ + DE ? Unexpected Result : - jets remain back-to-back like in pp (little additional broadening from radiated Energy) - radiated energy appears in low energy hadrons, 27 far away from the jet (CMS PRC 84 (2011) 024906) December 2012 Bad Honnef J. Sc

4) Mass & Color Charge Dependence l Measure Heavy Quarks (c, b) versus p RAA = measured/expected ~ AA/pp (gluon fragmentation dominates p at LHC) jet quenching DE ~ f(m) x cq x ^q x Ln x f(E) Expectation: DE(p) > DE(D) > DE(B) - qluon ↔ quark - light ↔ heavy Charm Mesons B Meson 28 (p+ + p-) RAA hints for the expected hierarchy less strong than naively expected Needs better statistics & quantitative comparison with models December 2012 Bad H

Discovery l The first LHC Discovery (pp, Sept 2010) ð long range rapidity 'ridge' in 2 -particle correlations µ visible in the highest multiplicity pp collisions µ arguably still the most unexpected LHC discovery 'Near Side Ridge 'Away Side JET' 'Near Side JET' 29 May 2013 Stockholm J. Schukraft

Origin of the pp 'Ridge' l Spawned a large number of different explanations ð mostly rather ad hoc, very speculative, or outright weird l Color Glass Condensate CGC: 'first principles' theory ð classsical FT in high density limit (small x, small Q 2) ð 'new state of cold & dense parton matter' ð some success describing aspects of ep, pp, e. A: geometric scaling, low-x, particle production, . . µ however, no 'smoking gun' so far… l Collective flow (Hydro) ? ð vaguely similar correlations in nucleus-nucleus 30 Pb. Pb 10 -20% Flow May 2013 Stockholm J. Schukraft

Ridges everywhere. . l Ridge is much stronger in p. Pb (end 2012)! 2 particles 4 particles ATLAS p. Pb 5 Te. V ð and is, in fact, a 'double ridge' ð even and odd components (v 2, v 3) ð collective multiparticle (i. e. , not 'jet' like) ð now also seen in d. Au at RHIC ! (tbc) ð strength ≈ as predicted by some hydro models CMS p. Pb 5 Te. V ALICE p. Pb 5 Te. V Stronger NSR Double Ridge 31 December 2012 Bad Honnef J. Sc

Panta Rhei ? l CGC in trouble ? ð mini-jet like correlation (4 part. << 2 part. ? ), no odd harmonics (v 3), … l Collective 'Hydro-like' flow in p. A (& pp) ? ? ð energy/particle density quiet comparable to AA (eg high Nch pp@LHC ≈ Cu-Cu mid-central @RHIC) ð system size only few fm 3 ? ? (presumably << 10 compared to >> 1000) µ however, hydro has no intrinsic size, only ratio's: l/r, and l ≈ 0 ! (from h/s) µ a proton@LHC is more like a small nucleus (dozens of partons, MPI, . . ) ! ð additional measurements should tell µ mass dependence of ridge (p, K, p) New State of Matter created at CERN which of thevia characteristics of the µ other collective signals (eg radial flow via p. T features spectra, many expansion HBT/BE correlations) l In either case, more than a curiosity theoretically predicted Colour Glass Condensate. ð CGC RHIC Scientists found "Colorful Glass" µ discovered a 'new state of matter' to serve the Perfect Liquid µ smoking gun for new 'first principle' limit of QCD ð Hydro stunning: a system the size of a single hadron behaves like 'macroscopic matter' µ 'extra dimension' for QGP study: size ! µ finite size effects => correlation & coherence length, time scales, …. Rewrite the textbooks 32 at least change the title from 'Heavy Ion. Stockholm physics' to. . May 2013 J. Schukraft

Questions 3: (assuming hydro explanation for ridge) Similar hadronisation (particle ratios), now signs of collectivity in pp ? pp, p. A, AA: What, if anything, is qualitatively different ? - Does this make AA more pp-like ? (no 'new state of matter) - Or pp more AA like ? (QGP 'matter' everywhere in dense systems > few fm 3) How small can it get ? - Is there another smoking gun for CGC in p. A at LHC ? (should be, x < 10 -3 -10 -5) - If not, is there still a science case for an electron-ion collider ? 33

The Questions: Questions 1: Hadronisation Unanticipated deviations at LHC from the Statistical Model (pp&AA) - Evidence against the SM ? - Make the 'mysteriously successful' SM more reasonable by showing expected/conceivable deviations ? (NLO-corrections) - In either case, can we use this to make progress ? Questions 2: Quarkonia Production - Regeneration (if confirmed): Dirt effect or Deconfinement signal ? - Sequential Y suppression: Settles the deconfinement case ? - If neither, what else would be needed from experiments ? (besides smaller errors) Questions 3: (assuming hydro explanation for ridge) Similar hadronisation (particle ratios), now signs of collectivity in p. A (& pp) ? pp, p. A, AA: What, if anything, is qualitatively different ? - Does this make AA more pp-like (no 'new state of matter) ? - Or pp more AA like (QGP 'matter' in all dense hadronic systems > few fm 3) ? - Is there another smoking gun for CGC in p. A at LHC (there should be, x < 10 -3 -10 -5)? -34 If not, is there still a science case for an electron-ion collider ? May 2013 Stockholm J. Schukraft

What's next for HI ? l short/medium term at LHC ( ≈ 5 y) ð complete (& solve ? ) the quarkonia puzzle(deconfinement) µ quantify 'other effects' (p. A), measure Y', better Y µ theory: needs progress in calculating melting temperatures ! ð Hydro: increased precision/sophistication (e. g. 30% in h/s ? ? ) ð solve the 'ridge' mystery (CGC vs Hydro vs ? ? ) µ any other sign of CGC in p. A@LHC ? (e. g. monojets at large y => LHCb) µ p. A at LHC could make (or break ? ) the science case for e. RHIC/EIC l medium/longer term LHC (≈ 10 y, including exp/LHC upgrades) ð comprehensive & precise energy loss ("jet-quenching') (needs HF, g-jet) ð chiral symmetry restoration ? e. g. much better low mass lepton pairs(NA 60) l Outside LHC (not part of this presentation) ð 'Phase Transition Line' & 'Tri-critical Point' ð matter at high baryon density (compression) µ FAIR@GSI, NICA@DUBNA µ RHIC energy scan, SPS fixed target 35 where starts the 'normal' (hadronic) matter ? May 2013 Stockholm J. Schukraft

The Questions: Questions 1: Hadronisation Unanticipated deviations at LHC from the Statistical Model (pp&AA) - Evidence against the SM ? - Make the 'mysteriously successful' SM more reasonable by showing expected/conceivable deviations ? (NLO-corrections) - In either case, can we use this to make progress ? Questions 2: Quarkonia Production - Regeneration (if confirmed): Dirt effect or Deconfinement signal ? - Sequential Y suppression: Settles the deconfinement case ? - If neither, what else would be needed from experiments ? (besides smaller errors) Questions 3: (assuming hydro explanation for ridge) Similar hadronisation (particle ratios), now signs of collectivity in p. A (& pp) ? pp, p. A, AA: What, if anything, is qualitatively different ? - Does this make AA more pp-like (no 'new state of matter) ? - Or pp more AA like (QGP 'matter' in all dense hadronic systems > few fm 3) ? - Is there another smoking gun for CGC in p. A at LHC (there should be, x < 10 -3 -10 -5)? -36 If not, is there still a science case for an electron-ion collider ? May 2013 Stockholm J. Schukraft

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Mapping the Phase Boundary QGP boundary measured 'particle freeze-out' boundary (e ≈ 0. 5 Ge. V/fm 3) 41 May 2013 Stockholm J. Schukraft