Review of Heavy-Ion Results from the CERN LHC David Evans The University of Birmingham Strong and Electroweak Matter 2012 Swansea, UK, 10 th – 13 th July 2012

Outline of Talk Introduction The LHC Experiments Selection of results from ALICE, ATLAS, & CMS – Particle spectra & ratios – Flow – Jet and high p T suppression (R AA ) – Heavy Flavour Summary 2

Aims of Heavy-Ion Programme  Study strongly interacting matter at extreme energy densities over large volumes and long time-scales (study of QCD at its natural scale,  QCD ~ 200 MeV).  Study the QCD phase transition from nuclear matter to a deconfined state of quarks and gluons - The Quark-Gluon Plasma.  Study the physics of the Quark-Gluon Plasma (QCD under extreme conditions). 3

Phases of Strongly Interacting Matter 4 Lattice QCD,  B = 0 Both statistical and lattice QCD predict that nuclear matter will undergo a phase transition, in to a deconfined state of quarks and gluons – a quark-gluon plasma, at a temperature of, T ~ 170 MeV and energy density,  ~ 1 GeV/fm 3. Heavy-Ion collisions far exceed these temperatures & densities at the LHC. colour

Heavy Ion Collisions pre-equilibration  QGP  hadronisation  freeze out Colliders: AGS, SPS, RHIC, LHC 5 Create QGP by colliding ultra-relativistic heavy ions  S NN (GeV) = (5500)

6 Observables Jets Open charm, beauty 6

7 7 LHC Detectors ALICE CMS ATLAS

Centrality Selection – Glauber Model Assume particle production is factorises into number of participants Npart number of collisions Ncoll Assume Negative Binomial Distribution for both f*Npart + (1-f) Ncoll Relation of percentile classes to Glauber values (, etc) purely geometrical by slicing in impact parameter Energy in ZDC (a.u.) 8

Charged Particle Multiplicity dN ch /d  versus  sdN ch /d  (Pb-Pb) Theory dN ch /d  = 1584  4 (stat.)  76 (syst.) Larger than most model predictions – good agreement between LHC experiments Rises with  s faster than pp Fits power law: Pb-Pb ~ s 0.15 Bjorken formula estimates energy density,  > 15 GeV/fm 3 (3 x RHIC) 9 Phys. Rev. Lett. 105, (2010) Phys. Rev. Lett. 106, (2011)

10 Bose–Einstein correlations (HBT) medium size: 2x larger than at RHIC – R out R side R long ~ 300 fm 3 medium life time: 40% longer than at RHIC –  ~ fm/c ALICE, Phys. Lett. B 696 (2011) 328. QM enhancement of identical Bosons at small momentum difference enhancement of e.g. like-sign pions at low momentum difference qinv=|p1-p2|,

Particle spectra LHC results agree well between experiments Very significant changes in slope compared to RHIC Good agree between experiments e.g. ALICE/ATLAS in this case 11

Identified Particle spectra 12 Very significant changes in slope compared to RHIC Most dramatically for protons Very strong radial flow,  ≈ 0.66 even larger than predicted by most recent hydro Blast Wave Fit RHIC Hydro Prediction RHIC

Strangeness Enhancement Multi-strange baryons enhanced with respect to pp.  baryons enhanced more than  s. 13

Thermal Models Thermal model: A.Andronic et al., Phys. Lett. B673: , 2009 Low T ch suggested by proton spectra excluded by ,  If exclude p, data agree with thermal GC model prediction T ch = 164 MeV Thermal models can not fit all data simultaneously 14

'Baryon anomaly':  /K 0 15 Baryon/Meson ratio still strongly enhanced x 3 compared to pp at p T = 3 GeV/c - Enhancement slightly larger than at RHIC 200 GeV - Maximum shift very little in p T compared to RHIC despite large change in underlying spectra ! Ratio at Maximum RHIC  /K 0 x 3  Recent EPOS model calculation describes the data well: (K. Werner, arXiv: ) (not decribed by coalescence model)

16 Does QGP have elliptic Flow? Angle of reaction planeFourier coefficient  pTpT Equal energy density contours P v2v2 x z y V 1 = directed flow.V 2 = elliptic flow. Study angular dependence of emitted particles 16

Elliptic Flow 17  v 2 shape is identical to RHIC  v 2 ~30% larger than RHIC: larger  ,K reproduced by hydro calculation  p ok for semi- peripheral but is a remaining challenge for central collisions ALICE, Phys. Rev. Lett. 105 (2010) Hydro-calcs.: Shen, Heinz, Huovinen and Song, arXiv:

Elliptic Flow – Higher Orders ATLAS Higher-order coefficients (n>2) do not show strong centrality dependence – consistent with fluctuations in initial state geometry being driving force. Superposition of flow harmonics, including higher orders, is sufficient to describe Ridge and Cone effects without implying medium response. 18 Phys. Rev. Lett. 107, (2011)

Flow vs Centrality Flow strongest for semi-peripheral collisions (central collisions too symmetric). 19

V 2 at High P T V 2 drops to zero at high P T 20

Full Harmonic Spectrum CMS Measure multi Vn - Over-constrain Hydro Models to get  /s, initial conditions etc. 21

Elliptic Flow for Identified Particles  intermediate p T : dep. on particle species  high p T : v 2 and v 3 small: hydrodinamic flow negligible  Indication for non zero D meson v 2 (3σ in 2 < p T < 6 GeV/c)  Comparable with charged hadrons elliptic flow 22

23 Jets in heavy ion collisions Studying deconfinement with jets di-quark quark Interaction at the quark (parton) level The same interaction at the hadron level pTpT pLpL p TOT jet soft beam jet Fragmentation radiated gluons heavy nucleus key QCD prediction: jets are quenched X.-N. Wang and M. Gyulassy, Phys. Rev. Lett. 68 (1992) 1480 Free quarks not allowed when quark, anti-quark pairs are produced they fly apart producing back-to-back ‘jets’ of particles Quarks remain free in QGP and lose energy while travelling through 23

JETS 24

25 Charged Jets in Pb-Pb Large suppression of jets seen in central collisions 192 GeV 168 GeV 10-20% peripheral   bin size: 0.1x GeV 102 GeV 0-10% central  Back-to-back high- energy jets seen in peripheral collisions 25

Jet Suppression Significant imbalance between leading and sub-leading jets for central collisions 26

Jet Suppression Also seen by CMS 27

Where is Jet Energy? Jet energy unbalanced inside cone, but unbalanced in opp direction outside cone - Overall balance is achieved. Another way to study this effect is to look at high p T suppression. 28

Nuclear Modification Factor particle interaction with medium stronger suppression at LHC ~factor 7 at 7 GeV/c 29 Leading Hadron quark-gluon plasma Divide p T spectra in Pb-Pb by p-p (with suitable normalisation factor)

R AA vs centrality Suppression increases with centrality. Minimum remains around 6-7 GeV/c 30

R AA for  s, Ws & Zs No suppression as expected 31

R AA for different particles For hadrons containing heavy quarks, smaller suppression expected Enhancement of  /K clearly seen At high p T (>8-10 GeV/c) R AA universality for light hadrons Strange 32

R cp of Jets Jets suppressed by factor 2 in central w.r.t. peripheral collisions. Suppression flat with p T. R cp of muons from heavy flavour decays about 0.4 for central collisions. Again, flat for 0.4 < P T < 14 GeV 33

R AA for Heavy Flavour ● H c,b → muons ● D mesons (charm) ● B → J/ψ (beauty) CMS arXiv: Charm and beauty: no evidence of mass effects yet Maybe a hint of hierarchy w.r.t pions! Charm 34

35 R AA J/  : Nuclear Modification Factor Inclusive J/  RAA =  (stat.)  (syst.) Rather small suppression and less than at RHIC (forward rapidity, P T > 0) ALICE p T > 0PHENIX p T > 0 almost no centrality dependence above Npart ~ 100 Unlike at RHIC

R AA J/  Greater suppression seen at LHC in mid-rapidity, at p T > 6.5 GeV And centrality dependent. At low-pt, ALICE RAA ≥ 0.5, no centrality dependence → behaviour clearly different to the one observed in PHENIX where a larger suppression and a strong centrality dependence is seen At high-pt, larger suppression seen both by ALICE and CMS → behaviour is similar to the one observed at low energy 36

Di-Muon Spectrum 37

ϒ Suppression N Υ(2S) /N Υ(1S) | pp = 0.56 ± 0.13 ± 0.01 N Υ(2S) /N Υ(1S) | PbPb = 0.12 ± 0.03 ± 0.01 N Υ(3S) /N Υ(1S) | pp = 0.21 ± 0.11 ± 0.02 N Υ(3S) /N Υ(1S) | PbPb < 0.07 Ratios not corrected for acceptance and efficiency 38

– ϒ(1S) RAA in 7 centrality bins – first results on ϒ(2S) RAA – clear suppression of ϒ(2S) – ϒ(1S) suppression consistent with excited state suppression (~50% feed down) ϒ(1S) and ϒ (2S) RAA 39

Summary We have started to piece together the standard model of heavy-ion collisions We have measured many of the global features – Phase transition temperature - Agrees with theory T c ~ 164 MeV – Energy density - Over 10x critical energy density,  > 15 GeV/fm 3 – size & lifetime of system ~ 5000 fm 3,  ~ fm/c Started to measure dynamical features – Very strong radial flow,  ≈ 0.66 – Strong elliptical flow – ideal liquid – Strong suppression of jets and high p T particles – Little evidence of quark mass dependence on energy loss (unlike predictions – although maybe a hint at this stage) – J/  suppressed less than RHIC at low P T - ϒ’ and ϒ’’ suppressed. We are at the early stages of a 10 year programme and have just scratched the surface. lots of work and exciting physics to look forward to. Thank you for listening 40

Backup Slides 41

Chiral Magnetic Effect ('strong parity violation') B + - Same charge correlations positive Opposite charge correlations negative RHIC ≈ LHC somewhat unexpected should decrease with N ch may decrease with √s RHIC : (++), (+-) different sign and magnitude LHC: (++),(+-) same sign, similar magnitude ? + - B ? RHIC Local Parity Violation in strong magnetic Field ? 42

Particle Identification TPC dE/dx ITS Silicon Drift/Strip dE/dx TOF dE/dx: 5% resolution Time-of flight, resolution: ~85 ps (Pb-Pb) ~110 ps (p-p) TPC dE/dx: separate p from K up to 1.1 GeV Time of flight: separate K from  up to ~ 2.2 GeV Ω  ΛΚ 43