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A-A collisions in the LHC!

John Jowett - LHC

Federico Antinori - EPS-HEP 2011 - Grenoble - 25 July 2011

Contents

- Introduction
- Selection of results (hard choices were required…)
- Particle multiplicity
- Correlations
- High pT suppression
- Identified particles
- Quarkonia
- Jets
- Heavy flavours
- Conclusions

Ultrarelativistic AA Collisions

Federico Antinori - EPS-HEP 2011 - Grenoble - 25 July 2011

basic 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)

Lattice QCD

3 flavours; (q-q)=0

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

- zero baryon density, 3 flavours
- e changes rapidly around Tc
- Tc = 170 MeV:

ec = 0.6 GeV/fm3

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

Federico Antinori - EPS-HEP 2011 - Grenoble - 25 July 2011

Big Bang

LHC

Early Universe

RHIC

Quark-Gluon

Plasma

Tc ~ 170 MeV

SPS

AGS

Hadron gas

Temperature

Nuclear

Neutron Star

matter

~ 5 - 10 nuclear

r

Baryon density

ec ~ 1 GeV/fm3

QCD phase diagramFederico Antinori - EPS-HEP 2011 - Grenoble - 25 July 2011

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

b

peripheral

central

Federico Antinori - EPS-HEP 2011 - Grenoble - 25 July 2011

Particle multiplicity

ALICE: PRL105 (2010) 252301

log extrapolationfails (finally!)

2.2 x central Au+Au

(√sNN=0.2 TeV)

1.9 x pp (NSD)

(√sNN=2.36 TeV)

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

2 dNch/dη / <Npart> = 8.3 0.4 (sys.)

Federico Antinori - EPS-HEP 2011 - Grenoble - 25 July 2011

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

Centrality dependence

ALICE: PRL 106 (2011) 032301

Helena Santos - ATLAS

Federico Antinori - EPS-HEP 2011 - Grenoble - 25 July 2011

- 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

Azimuthal asymmetry

- 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

Federico Antinori - EPS-HEP 2011 - Grenoble - 25 July 2011

… 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

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?

ALICE: PRL 105 (2010) 252302

Federico Antinori - EPS-HEP 2011 - Grenoble - 25 July 2011

Structures in (Δη,Δφ)

near side jet peak

two shoulders

on away side

(at 120 and 240 )

aka “the Mach cone”

long range structure

in η on near side

aka “the ridge”

Helena Santos – ATLAS

Federico Antinori - EPS-HEP 2011 - Grenoble - 25 July 2011

Fluctuations v3

Matt Luzum (QM 2011)

v2

v3

ALICE: PRL 107 (2011) 032301

Federico Antinori - EPS-HEP 2011 - Grenoble - 25 July 2011

- “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…

Long-η-range correlations

Andrew Adare – ALICE (QM2011)

Federico Antinori - EPS-HEP 2011 - Grenoble - 25 July 2011

- “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

Nuclear modification factor (RAA)

Yen-Jie Lee – CMS

CMS: PAS HIN-10-005

Federico Antinori - EPS-HEP 2011 - Grenoble - 25 July 2011

- 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!

Suppression vs event plane

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

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

Alexandru Dobrin – ALICE (QM2011)

Federico Antinori - EPS-HEP 2011 - Grenoble - 25 July 2011

pT spectra vs hydrodynamics

Roberto Preghenella - ALICE

Michele Floris – ALICE (QM2010)

Federico Antinori - EPS-HEP 2011 - Grenoble - 25 July 2011

- 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)

v2 vs hydrodynamics

Raimond Snellings

ALICE (QM2010)

Federico Antinori - EPS-HEP 2011 - Grenoble - 25 July 2011

- 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 protons?

Quarkonium suppression

Federico Antinori - EPS-HEP 2011 - Grenoble - 25 July 2011

- 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)
- cc and bb states with radius > λD will not bind their production will be suppressed

J/ψ @ LHC: forward y, low pT

- LHC: 2.5 < y < 4, pT > 0 (ALICE)

Livio Bianchi – ALICE

Federico Antinori - EPS-HEP 2011 - Grenoble - 25 July 2011

- less suppression than

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

- ~ as suppressed as

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

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)

Catherine Silvestre – CMS

CMS: PAS HIN-10-006

Helena Santos – ATLAS

ATLAS: PLB 697 (2011) 294

Federico Antinori - EPS-HEP 2011 - Grenoble - 25 July 2011

Federico Antinori - EPS-HEP 2011 - Grenoble - 25 July 2011

Federico Antinori - EPS-HEP 2011 - Grenoble - 25 July 2011

Di-jet imbalance

Frank Ma - CMS

Federico Antinori - EPS-HEP 2011 - Grenoble - 25 July 2011

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

AJ

- with increasing centrality:
- enhancement of asymmetric di-jets
- with respect to pp & HIJING + PYTHIA

Helena Santos - ATLAS

ATLAS: PRL105 (2010) 252303

Federico Antinori - EPS-HEP 2011 - Grenoble - 25 July 2011

Di-jet Δφ

e.g.: Frank Ma – CMS

CMS: arXiv:1102.1957

Federico Antinori - EPS-HEP 2011 - Grenoble - 25 July 2011

no visible angular decorrelation in Δφ wrt pp collisions

Jet nuclear modification factor

- suppression of jet production
- in central Pb-Pb wrt binary-scaled peripheral
- very weak centrality dependence

Helena Santos - ATLAS

Federico Antinori - EPS-HEP 2011 - Grenoble - 25 July 2011

Frank Ma – CMS

CMS: arXiv:1102.1957

Federico Antinori - EPS-HEP 2011 - Grenoble - 25 July 2011

c and b quenching @ LHC

- substantial suppression of heavy flavour production
- beauty, too!

Diego Stocco – ALICE

Catherine Silvestre - CMS

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

Federico Antinori - EPS-HEP 2011 - Grenoble - 25 July 2011

[compilation courtesy of Andrea Dainese]

D vs π

- charm ~ as suppressed as pions
- maybe a hint of RAAD > RAAπ at low pT …

to be continued with higher statistics…

- 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

Diego Stocco – ALICE

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

Federico Antinori - EPS-HEP 2011 - Grenoble - 25 July 2011

Conclusions

Federico Antinori - EPS-HEP 2011 - Grenoble - 25 July 2011

- 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 first, limited data samples
- death of ridge and Mach cone?
- anomalies in proton yields & momentum distributions
- pattern of jet and heavy flavour suppression challenges Eloss models
- the future looks very bright
- precision measurements constrain dynamic and coupling properties of medium
- look out for surprises!!!

Merci…

Federico Antinori - EPS-HEP 2011 - Grenoble - 25 July 2011

- … aux organisateurs

for the invitation to present this plenary talk

- … à:

Peter Braun-Munzinger, Brian Cole, Andrea Dainese, David d’Enterria, Paolo Giubellino, Raphaël Granier de Cassagnac, Barbara Jacak, Peter Jacobs, John Jowett, Karel Šafařík, Carlos Salgado, Jürgen Schukraft, Edward Shuryak, Peter Steinberg, Xin-Nian Wang, Urs Wiedemann, Nu Xuand many others…

- … à vous tous

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