Review of Heavy-Ion Results from the CERN LHC

Review of Heavy-Ion Results from the CERN LHC

David EvansThe University of Birmingham

Strong and Electroweak Matter 2012 Swansea, UK, 10th – 13th July 2012

Outline of Talk• Introduction• The LHC Experiments• Selection of results from ALICE, ATLAS, & CMS

– Particle spectra & ratios– Flow– Jet and high pT suppression (RAA)– Heavy Flavour

• Summary

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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).

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Phases of Strongly Interacting Matter

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Lattice QCD, B = 0Lattice 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/fm3.

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

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Create QGP by colliding ultra-relativistic heavy ions

SNN (GeV) = 5.4 19 200 2760 (5500)

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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

• AssumeNegative Binomial Distribution

for both f*Npart + (1-f) Ncoll

• Relation of percentile classes to Glauber values (<Npart>, etc) purely geometrical by slicing in impact parameter

Ener

gy in

ZD

C (a

.u.)

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Charged Particle MultiplicitydNch/d versus sdNch/d (Pb-Pb) Theory

dNch/d = 1584 4 (stat.) 76 (syst.)

Larger than most model predictions – good agreement between LHC experiments

Rises with s faster than ppFits power law: Pb-Pb ~ s0.15

Bjorken formula estimates energy density, > 15 GeV/fm3 (3 x RHIC) 9

Phys. Rev. Lett. 105, 252301 (2010)Phys. Rev. Lett. 106, 032301 (2011)

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Bose–Einstein correlations(HBT)

• medium size: 2x larger than at RHIC– RoutRsideRlong ~ 300 fm3

• medium life time: 40% longer than at RHIC– ~ 10-11 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 spectraLHC results agree well between experiments

Very significant changes in slope compared to RHICGood agree between experiments e.g. ALICE/ATLAS in this case

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Identified Particle spectra

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Very significant changes in slope compared to RHICMost dramatically for protonsVery 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.

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Thermal Models

Thermal model: A.Andronic et al., Phys. Lett. B673:142-145, 2009

Low Tch suggested by proton spectra excluded by ,

If exclude p, data agree with thermal GC model prediction Tch = 164 MeV

Thermal models can not fit all data simultaneously14

'Baryon anomaly': /K0

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Baryon/Meson ratio still strongly enhanced x 3 compared to pp at pT = 3 GeV/c

- Enhancement slightly larger than at RHIC 200 GeV

- Maximum shift very little in pT compared to RHIC despite large change in underlying spectra !

Ratio at Maximum

RHIC/K0

x 3

Recent EPOS model calculation describes the data well: (K. Werner, arXiv: 1204.1394) (not decribed by coalescence model)

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Does QGP have elliptic Flow?

Ed 3Nd 3p

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d 2NpTdpTdy

1 2vn cos n r n1

Angle of reaction planeFourier coefficient

pT

Equal energy density contours

P

v2

v2 cos2

x

z

y

V1 = directed flow. V2 = elliptic flow.

Study angular dependence of emitted particles

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Elliptic Flow

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v2 shape is identical to

RHIC v 2 ~30% larger than

RHIC: larger <pT>,K reproduced by hydro calculation p ok for semi-peripheral but is a remaining challenge for central collisions

ALICE, Phys. Rev. Lett. 105 (2010) 252302.

Hydro-calcs.: Shen, Heinz, Huovinen and Song, arXiv:1105.3226.

Elliptic Flow – Higher Orders

ATLAS

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.

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Phys. Rev. Lett. 107, 032301 (2011)

Flow vs Centrality

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

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V2 at High PT

V2 drops to zero at high PT 20

Full Harmonic Spectrum

CMS

Measure multi Vn - Over-constrain HydroModels to get /s, initial conditions etc.

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Elliptic Flow for Identified Particles

intermediate pT: dep. on particle specieshigh pT: v2 and v3 small: hydrodinamic flow negligibleIndication for non zero D meson v2 (3σ in 2 < pT < 6 GeV/c)Comparable with charged hadrons elliptic flow

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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

pT

pL

pTOT

jet

soft beam jet

Fragmentation

radiatedgluons

heavy nucleus

key QCD prediction: jets are quenched

X.-N. Wang and M. Gyulassy, Phys. Rev. Lett. 68 (1992) 1480

Free quarks not allowedwhen 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

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Charged Jets in Pb-Pb

Large suppression of jets seen in central collisions

192 GeV168 GeV

10-20% peripheral

bin size: 0.1x0.147 GeV

102 GeV

0-10% central

Back-to-back high-energy jets seen in peripheral collisions

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Jet Suppression

Significant imbalance between leading and sub-leading jets for central collisions26

Jet Suppression

Also seen by CMS27

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 pT suppression.

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Nuclear Modification Factor

• particle interaction with medium

• stronger suppression at LHC• ~factor 7 at 7 GeV/c

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TDppcoll

TDAA

TDAA dpdN

dpdpR

/

/)(

Leading

Hadron

quark-gluon plasma

Divide pT spectra in Pb-Pb by p-p (with suitable normalisation factor)

RAA vs centrality

• Suppression increases with centrality.

• Minimum remains around 6-7 GeV/c

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RAA for s, Ws & Zs

No suppression as expected 31

RAA for different particles

• For hadrons containing heavy quarks, smaller suppression expected

• Enhancement of /K clearly seen• At high pT (>8-10 GeV/c) RAA universality for

light hadrons

Strange

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Rcp of Jets

Jets suppressed by factor 2 in central w.r.t. peripheral collisions.Suppression flat with pT.

Rcp of muons from heavy flavour decays about 0.4 for central collisions.Again, flat for 0.4 < PT < 14 GeV 33

RAA for Heavy Flavour

●Hc,b → muons●D mesons (charm)●B → J/ψ (beauty)

CMSarXiv:1201.5069

• Charm and beauty: no evidence of mass effects yet

• Maybe a hint of hierarchy w.r.t pions!

Charm

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RAA J/: Nuclear Modification Factor

Inclusive J/ RAA = 0.497 0.006 (stat.) 0.078 (syst.)

Rather small suppression and less than at RHIC (forward rapidity, PT > 0)

ALICE pT > 0 PHENIX pT > 0

almost no centrality dependence above Npart ~ 100Unlike at RHIC

RAA J/

Greater suppression seen at LHC in mid-rapidity, at pT > 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 largersuppression and a strong centrality dependence is seenAt high-pt, larger suppression seen both by ALICE and CMS→ behaviour is similar to the one observed at low energy

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Di-Muon Spectrum

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ϒ Suppression

NΥ(2S)/NΥ(1S)|pp = 0.56 ± 0.13 ± 0.01 NΥ(2S)/NΥ(1S)|PbPb = 0.12 ± 0.03 ± 0.01NΥ(3S)/NΥ(1S)|pp = 0.21 ± 0.11 ± 0.02 NΥ(3S)/NΥ(1S)|PbPb < 0.07

Ratios not corrected for acceptance and efficiency 38

– ϒ(1S) RAA in7 centrality bins– first results on ϒ(2S) RAA– clear suppression of ϒ(2S)– ϒ(1S) suppressionconsistent with excitedstate suppression(~50% feed down)

ϒ(1S) and ϒ (2S) RAA

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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 Tc ~ 164 MeV– Energy density - Over 10x critical energy density, > 15 GeV/fm3

– size & lifetime of system ~ 5000 fm3, ~ 10-11 fm/c• Started to measure dynamical features

– Very strong radial flow, ≈ 0.66 – Strong elliptical flow – ideal liquid– Strong suppression of jets and high pT 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 PT - 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

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Backup Slides

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Chiral Magnetic Effect ('strong parity violation')

RP 2cos

B

+-

Same charge correlations positiveOpposite charge correlations negativeRHIC ≈ LHCsomewhat unexpected

should decrease with Nch

may decrease with √s

RHIC : (++), (+-) different sign and magnitudeLHC: (++),(+-) same sign, similar magnitude?

cos

+ -

B

?

RHIC

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

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