Latest Results from the LHC: Heavy-Ion Physics

Latest Results from the LHC: Heavy-Ion Physics

Jan Fiete Grosse-OetringhausCERN

2nd August 2011, CERN

Latest Results from the LHC: Heavy-Ion Physics - Jan Fiete Grosse-Oetringhaus 2

Contents

• Introduction• Selection of recent heavy-ion physics results on

– Particle multiplicity– Correlations– Identified particles

– High pT suppression

– Heavy flavours– Quarkonia– Jets

I would like to give credit to many EPS speakers from which I took slides, in particular Federico Antinori


Introduction


Ultrarelativistic A-A Collisions

• Basic idea: compress large amount of energy in a very small volume Produce a “fireball” of hot matter– Temperature: O(1012 K)– ~ 105 times T at centre of Sun– ~ T of universe about 10 µs after the Big Bang

• How does matter behave under such extreme conditions?– Study the fireball properties

– QCD predicts state of deconfined quarks and gluons (Quark-Gluon Plasma)

• LHC can accelerate ions (up to Pb) up to sNN = 5.5 TeV


Geometry of a Pb-Pb Collision• Central collisions

– Small impact parameter b– High number of participants high multiplicity

• Peripheral collisions– Large impact parameter b– Low number of participants low multiplicity

• For example: sum of the amplitudes in the ALICE V0 scintillators

• Reproduced by Glauber model fit– Random relative position of nuclei in

transverse plane– Woods-Saxon distribution inside nucleus – Deviation at very low amplitude

expected due to non-nuclear (electromagnetic) processes

centralperipheral

b


Multiplicity


Particle Multiplicity

• For the 5% most central collisions: • 1584 ± 80 (syst) charged particles per unit of η (|η|<0.5)

• 2 dNch/dη / <Npart> = 8.3 ± 0.4 (syst)

• 2.2 x central Au+Au (√sNN=0.2 TeV) 1.9 x pp (NSD) (√sNN=2.36 TeV)

• log extrapolation fails (finally!)

ALICE: PRL105 (2010) 252301


Centrality Dependence

• Model comparisons– DPMJET (with string fusion)– HIJING 2.0 (no quenching)

• Centrality-dependent gluon shadowing

• Tuned to multiplicity in 0-5%

– Saturation models

(sometimes too much?)

• Very similar centrality dependence at LHC & RHIC

– Once corrected for difference in absolute values

ALICE: PRL 106 (2011) 032301

Helena Santos - ATLAS


Correlations


Reactionplane

In-planeOut

-of-

plan

e

Y

XFlow

Flow

Reactionplane

In-planeOut

-of-

plan

e

Y

XFlow

Flow

Reactionplane

In-planeOut

-of-

plan

e

Y

XFlow

Flow

Why is it important?• Non-central collisions are

asymmetric in azimuth Transfer of this asymmetry to

momentum space provides a measure of the strength of collective phenomena

• Large mean free path – Particles stream out isotropically, no memory of the asymmetry – Extreme: ideal gas (infinite mean free path)

• Small mean free path– Larger density gradient larger pressure gradient larger momentum – Extreme: ideal liquid (zero mean free path, hydrodynamic limit)

Quantified by second coefficient (v2) of Fourier expansion of azimuthal distribution

Azimuthal Asymmetry


Elliptic Flow v2

• v2 still large at the LHC

System still behaves very close to ideal liquid (low viscosity)

• v2(pT) very similar at LHC and RHIC

Similar hydrodynamical behaviour?

ALICE: PRL 105 (2010) 252302


Closer Look at v2

• v2 as function of

• Evolution with consistent with RHIC

CMS: PAS HIN-10-002PHOBOS: PRC72,051901

PHOBOS, 200 GeV


Structures in (Δη,Δφ)

Near-side jet peak

Long-range structure in η on near side

called “Ridge”

Two shoulders on away side

(at 120 and 240)called “Mach cone”

Helena Santos – ATLAS


Fluctuations and v3

• “Ideal” shape of participants’ overlap is elliptic, then

– No odd harmonics expected– Participants’ plane coincides with reaction

plane

• Fluctuations in initial conditions– Participants' plane reaction plane

v3 (“triangular”) harmonic appears

[B Alver & G Roland, PRC81 (2010) 054905]

• Indeed v3 0 measured!

• v3 has weaker centrality dependence than v2

• When calculated w.r.t. reaction plane, v3 vanishes

– as expected, if due to fluctuations…

Matt Luzum (QM 2011)

ALICE: PRL 107 (2011) 032301

v2

v3

v3/RP


Long η-range Correlations

• “Ultra-central” events: dramatic shape evolution in a very narrow centrality range

• Double-hump structure on away-side appears on 1% most central– Visible without v2 subtraction!

• First five harmonics describe shape at 10-3 level“Ridge” and “Mach cone”– Explanations based on medium

response to propagating partons were proposed at RHIC

– Fourier analysis at LHC suggests natural alternative explanation in terms of hydrodynamic response to initial state fluctuations

Andrew Adare – ALICE (QM2011)

?


Identified Particles


• Identified particle spectra modified by radial flow

• Comparison to hydrodynamical predictions– (calculations by C Shen et al.: arXiv:1105.3226 [nucl-th])

OK for π and K, but p seem to “misbehave” (less yield, flatter spectrum)

pT Spectra and Hydrodynamics

Roberto Preghenella - ALICE

p: feed-down corrected


v2 and Hydrodynamics

• Comparison of identified particles v2(pT) with hydro prediction

– (calculation by C Shen et al.: arXiv:1105.3226 [nucl-th])

again, protons are off… what’s going on with the protons?

Raimond Snellings ALICE (QM2010)


High pT Suppression


Nuclear Modification Factor (RAA)

• RAA: particle yield in AA relative to pp scaled with number of binary nucleon-nucleon collisions– Without modification RAA = 1

• RAA(pT) for charged particles in 0-5% centrality range

– Minimum (~ 0.14) for pT ~ 6-7 GeV/c

– Then slow increase at high pT

– Still significant suppression at pT ~ 100 GeV/c

• Essential quantitative constraint for parton energy loss models

)(Yield

)(Yield)(

ppAA

AAAA

T

TT pNbin

ppR

Yen-Jie Lee – CMSCMS-PAS-HIN-10-005


Suppression vs Event Plane

• Assess path-length dependent quenching• Significant effect, even at 20 GeV• Further constraints to energy-loss models

Path-length dependence of energy lossAlexandru Dobrin – ALICE (QM2011)


• Near-side of central events slightly enhanced IAA ~ 1.2 medium effects also visible for near-side

• Away-side of central events suppressed: IAA ~ 0.6 expected from in-medium energy loss

• Peripheral events consistent with unity

Jet-Yield Modification Factor IAA

pp

AA

AA Y

YI

d

dN

NY assoc

trig

1)(

JFGO - ALICE (QM 2011)


Heavy Flavours


c and b Quenching at LHC

• Study RAA of charm and beauty mesons and compare to light mesons– Parton mass and

colour charge dependence of energy loss

• Substantial suppression of heavy flavour production at LHC– Beauty suppressed too

Diego Stocco – ALICECatherine Silvestre – CMS

[compilation: Andrea Dainese]


D vs π

• Quarks expected to couple weaker than gluons– Colour factor (and other effects)

– E.g. at pT ~ 8 GeV, factor ~ 2 less suppression expected for D than for light hadrons in BDMPS-ADSW

• Charm suppression similar to pion suppression– Hint of RAA

D > RAAπ at low pT?

– To be continued with higher statistics…

Diego Stocco – ALICE

N Armesto et al., PRD 71 (2005) 054027


Quarkonia


Quarkonia Suppression

• Quark-Gluon Plasma "signature" proposed in the 80’s[T Matsui & H Satz, Phys. Lett. B 178 (1986) 416]

• In the plasma phase the interaction potential is expected to be screened beyond the Debye length λD (analogous to e.m. Debye screening)

• ccbar and bbbar pairs with radius > λD will not bind production suppressed

• Critical "melting" temperature for J/ψ, ψ', , ', '' etc.


Catherine Silvestre – CMS

Quarkonia at LHC


• In 2.5 < y < 4 at LHC • Less suppression

than at RHIC in 1.2 < y < 2.2

• as suppressed as at RHIC in |y| < 0.35

• pA reference needed for quantitative conclusions

J/ψ at LHC: Forward y, Low pT

Livio Bianchi – ALICE

LHC: 2.5 < y < 4, pT > 0 (ALICE)RHIC: 1.2 < y < 2.2, pT > 0 (PHENIX)RHIC: |y| < 0.35. pT > 0 (PHENIX)


J/ψ at LHC: Central y, High pT

LHC: |y| < 2.4, pT > 6.5 GeV/c prompt J/ψ (CMS) more suppressed than RHIC: |y| < 1. pT > 5 GeV/c inclusive J/ψ (STAR)

LHC |y| < 2.5, pT > 3 GeV/c (ATLAS)

Catherine Silvestre – CMS Helena Santos – ATLAS

RCP

PLB 697 (2011) 294


(2S+3S) Suppression

Yield of (2S+3S) relative to (1S) in Pb-Pb compared to pp

Yen-Jie Lee – CMSarXiv:1105.4894

)(03.0)(31.0

)1(/)32(

)1(/)32( 19.015.0 syststat

SSS

SSS

pp

PbPb


Jets


Di-jet Imbalance

• Pb-Pb events with large di-jet imbalance observed by ATLAS & CMS recoiling jet strongly quenched!

Frank Ma - CMS


Di-jet Asymmetry AJ

Enhancement of asymmetric di-jets with increasing centrality

– measured w.r.t. pp & HIJING + PYTHIA

8.2 4.0 R

Helena Santos - ATLASATLAS: PRL105 (2010) 252303

AJ


Jet Nuclear Modification Factor RCP

Suppression of jet production – In central Pb-Pb w.r.t. binary-

scaled peripheral

Weak centrality dependence

PeripheralPeripheral

CentralCentralCP Nbin

NbinR

Yield

Yield

Helena Santos - ATLAS


Di-jet Decorrelation in Δφ

• Although significant energy loss (large AJ), no visible angular decorrelation in Δφ w.r.t. pp collisions– Quenched jets still back-to-back

e.g.: Frank Ma – CMSCMS: arXiv:1102.1957

pT,1 > 120 GeV/cpT,2 > 50 GeV/c


Quenched Energy

• Measurement of missing pT w.r.t. leading jet direction

• Di-jet momentum difference balanced by low pT particles (spread far out of jet cone)

Yen-Jie Lee – CMSFrank Ma – CMS

CMS: arXiv:1102.1957


Conclusions

• Heavy-ion collisions at LHC have entered a new regime with abundant hard probes and powerful detectors

• Exciting results from first short (low L) run– Comprehensive explanation of ridge and Mach cone– Anomalies in proton yields & momentum distributions– Heavy-flavour suppression challenges energy-loss models – Strong jet quenching balanced by soft particles

• Precision measurements will constrain dynamics and transport properties of the medium



Backup


QCD Phase Diagram

Tc ~ 170 MeV

~ 5 - 10 nuclear

Quark-Gluon Plasma

Hadron gas

Nuclearmatter

Neutron Star

SPSAGS

Early Universe LHCRHIC

Baryon density

Tem

per

atu

re

c ~ 1 GeV/fm3

~ 10 s after Big Bang

Documents

Latest Results from the LHC: Heavy-Ion Physics