Ultrarelativistic Nucleus-Nucleus Collisions at the LHC

Federico AntinoriINFN, Padova, Italy and CERN, Geneva, Switzerland

中国合肥 - 中国科学技术大学 – 2012年 3 月29号

Contents

• Introduction• Multiplicity• Correlations

• High pT suppression

• Identified particles• Strangeness enhancement• Quarkonia• Jets • Heavy Flavours• Conclusions

Federico Antinori – 科大 – 2012年 3 月 29号 2

Introduction

Ultrarelativistic AA Collisionsbasic idea: compress large amount of energy in a

very small volume produce a “fireball” of hot matter:

temperature O(1012 K)– ~ 105 x T at centre of Sun

– ~ T of universe @ ~ 10 µs after 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)

Federico Antinori – 科大 – 2012 年 3 月 29 号 4

5

Lattice QCD

• zero baryon density, 3 flavours

changes rapidly around Tc

• Tc = 170 MeV:

c = 0.6 GeV/fm3

• at T~1.2 Tc settles at about 80% of the Stefan-Boltzmann value for an ideal gas of q,q g (SB)

3 flavours; (q-q)=0

in lattice QCD, non-perturbative problems are treated by discretization on a space-time lattice cure ultraviolet divergences

Federico Antinori – 科大 – 2012 年 3 月 29 号

QCD phase diagram

Federico Antinori – 科大 – 2012 年 3 月 29 号 6

Tc ~ 170 MeV

~ 5 - 10 nuclear

Quark-Gluon Plasma

Hadron gas

Nuclearmatter

Neutron Star

SPSAGS

Early Universe LHCRHIC

Baryon density

Tem

per

atur

e

c ~ 1 GeV/fm3

~ 10 s after Big Bang

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 (red):– 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

7Federico Antinori – 科大 – 2012 年 3 月 29 号

b

Multiplicity

Particle multiplicity

for the most central collisions: ~ 1600 charged particles per unit of η

Federico Antinori – 科大 – 2012年 3 月 29号 9

√sNN=2.76 TeV Pb+Pb, 0-5% central, |η|<0.5

2 dNch/dη / <Npart> = 8.3 0.4 (sys.)

log extrapolation fails (finally!)

2.2 x central Au+Au

(√sNN=0.2 TeV)

1.9 x pp (NSD)

(√sNN=2.36 TeV)

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

• very similar centrality dependence at LHC & RHIC

– once corrected for difference in absolute values

Federico Antinori – 科大 – 2012年 3 月 29号 10

ALICE: PRL 106 (2011) 032301

Helena Santos – ATLAS (EPS 2011)

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

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… in the transverse momentum

distribution of produced particles• why is it important?• non-central collisions are

asymmetric in azimuth azimuth = angle in the plane of the screen

Federico Antinori – 科大 – 2012年 3 月 29号 12

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

same hydrodynamical behaviour?

Federico Antinori – 科大 – 2012年 3 月 29号 13

ALICE: PRL 105 (2010) 252302

Structures in (Δη,Δφ)

Federico Antinori – 科大 – 2012年 3 月 29号 14

near side jet peak

long range structure in η on near sideaka “the ridge”

two shoulders on away side

(at 120 and 240 )aka “the Mach cone”

Fluctuations v3

• “ideal” shape of participants’ overlap is ~ elliptic

– in particular: no odd harmonics expected– participants’ plane coincides with event

plane

• but fluctuations in initial conditions:– participants plane event plane

v3 (“triangular”) harmonic appears

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

• and indeed, v3 0 !

• v3 has weaker centrality dependence than v2

• when calculated wrt participants plane, v3 vanishes

– as expected, if due to fluctuations…

Federico Antinori – 科大 – 2012年 3 月 29号 15

Matt Luzum (QM 2011)

ALICE: PRL 107 (2011) 032301

v2

v3

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 any need for 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 of new data suggests very natural alternative explanation in terms of hydrodynamic response to initial state fluctuations

Federico Antinori – 科大 – 2012年 3 月 29号 16

Andrew Adare – ALICE (QM2011)

High pT suppression

Nuclear modification factor (RAA)• RAA: particle yield in AA relative

to pp scaled with number of binary nucleon-nucleon collisions

• RAA(pT) for charged particles produced 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!

Federico Antinori – 科大 – 2012年 3 月 29号 18

)(Yield

)(Yield)(

ppAA

AAAA

T

TT pNbin

ppR

CMS: PAS HIN-10-005

Suppression vs event plane

• significant effect, even at 20 GeV!• further constraints to energy loss models

path-length dependence of energy loss (L2, L3, …)

Federico Antinori – 科大 – 2012年 3 月 29号 19

Alexandru Dobrin – ALICE (QM2011)

Identified particles

pT spectra vs hydrodynamics

• comparison of identified particle spectra with hydro 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)

Federico Antinori – 科大 – 2012年 3 月 29号 21

Michele Floris – ALICE (QM2011)

v2 vs hydrodynamics

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

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

again, protons…

Federico Antinori – 科大 – 2012年 3 月 29号 22

Raimond Snellings ALICE (QM2011)

Strangeness enhancement

Strangeness enhancement

• enhancement decreases with increasing √s strange/non-strange increases with √s in pp

Federico Antinori – 科大 – 2012年 3 月 29号 24

Maria Nicassio – ALICE (SQM2011)

Comparison with thermal model

• Reasonable grand-canonical description with Tch=164 MeV

except for p/π !

Federico Antinori – 科大 – 2012年 3 月 29号 25

Alexander Kalweit – ALICE (SQM2011)

Quarkonia

J/ψ @ LHC: central y, high pT

• LHC: |y| < 2.4, pT > 6.5 GeV/c (CMS)prompt J/ψ

more suppressed than

RHIC: |y| < 1. pT > 5 GeV/c (STAR)inclusive J/ ψ

• LHC |y| < 2.5, pT > 3 GeV/c (ATLAS)

Federico Antinori – 科大 – 2012年 3 月 29号 27

CMS: PAS HIN-10-006 ATLAS: PLB 697 (2011) 294

J/ψ @ LHC: forward y, low pT

less suppression than RHIC: 1.2 < y < 2.2, pT > 0 (PHENIX)

~ as suppressed as

RHIC: |y| < 0.35. pT > 0 (PHENIX)

Federico Antinori – 科大 – 2012年 3 月 29号 28

• LHC: 2.5 < y < 4, pT > 0 (ALICE)

Federico Antinori – 科大 – 2012年 3 月 29号 29Yen-Jie Lee – CMSYen-Jie Lee – CMS (EPS-HEP 2011)

Jets

Di-jet imbalance• Pb-Pb events with large di-jet imbalance observed by ATLAS & CMS

recoiling jet strongly quenched!

Federico Antinori – 科大 – 2012年 3 月 29号 31

Frank Ma – CMS (EPS-HEP 2011)

AJ

• with increasing centrality: enhancement of asymmetric di-jets

– with respect to pp & HIJING + PYTHIA

Federico Antinori – 科大 – 2012年 3 月 29号 32

8.2 4.0 R

ATLAS: PRL105 (2010) 252303

Di-jet Δφ

• no visible angular decorrelation in Δφ wrt pp collisions

Federico Antinori – 科大 – 2012年 3 月 29号 33

CMS: arXiv:1102.1957

Jet nuclear modification factor

suppression of jet production – in central Pb-Pb wrt binary-scaled peripheral

very weak centrality dependence

Federico Antinori – 科大 – 2012年 3 月 29号 34

PeripheralPeripheral

CentralCentralCP Nbin

NbinR

Yield

Yield

Helena Santos – ATLAS (EPS-HEP 2012)

Fragmentation function

• similar to fragmentation in vacuum?

Federico Antinori – 科大 – 2012年 3 月 29号 35

Heavy Flavours

c and b quenching @ LHC

Federico Antinori – 科大 – 2012年 3 月 29号 37

substantial suppression of heavy flavour production

– beauty, too!

study parton mass and colour charge dependence of interaction with medium!

D vs π

• charm ~ as suppressed as pions

• maybe a hint of RAAD > RAA

π at low pT …

to be continued with higher statistics…

Federico Antinori – 科大 – 2012年 3 月 29号 38

• quarks (CR = 4/3) expected to couple weaker than gluons (CR = 3)

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

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

Andrea Dainese – ALICE (QM 2011)

D0 elliptic flow• First direct measurement of D flow in heavy-ion collisions• Yield extracted from invariant mass spectra of K candidates in 2

bins of azimuthal angle relative to the event plane

Federico Antinori – 科大 – 2012年 3 月 29号 39

OUTIN

OUTIN

NN

NNv

42

Chiara Bianchin – ALICE (SQM 2011)

Conclusions

• in November 2010, the field of ultrarelativistic nuclear collisions has entered a new era with the start of heavy ion collisions at the LHC

– abundance of hard probes– state-of-the art collider detectors

• exciting results already from 2010 data sample– death of ridge and Mach cone?– anomalies in proton yields & momentum distributions– pattern of jet and heavy flavour suppression challenge to Eloss models

• and the future looks bright!– ~ 150/µb delivered by LHC in 2011– p-Pb run scheduled for 2012 precision measurements + handle on cold nuclear matter effects close in on dynamic and coupling properties of medium

and … look out for surprises!!!

Federico Antinori – 科大 – 2012年 3 月 29号 40

谢谢！