ultrarelativistic nucleus-nucleus collisions at the lhc
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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!
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