1. Ultrarelativistic Nucleus-Nucleus Collisions at the LHC
Federico Antinori
INFN, Padova, Italy and CERN, Geneva, Switzerland
中国合肥 - 中国科学技术大学 – 2012年3月29号

2. 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

3. Introduction

4. Ultrarelativistic AA Collisions
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 ~ 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

5. Lattice QCD
in lattice QCD, non-perturbative problems are treated by discretization on a space-time lattice  cure ultraviolet divergences
3 flavours; (q-q)=0
zero baryon density, 3 flavours
e changes rapidly around Tc
Tc = 170 MeV:
 ec = 0.6 GeV/fm3
at T~1.2 Tc e settles at about 80% of the Stefan-Boltzmann value for an ideal gas of q,q g (eSB)
Federico Antinori – 科大 – 2012年3月29号

6. QCD phase diagram ec ~ 1 GeV/fm3 Tc ~ 170 MeV r ~ 5 - 10 nuclear
Quark-Gluon
Plasma
Hadron gas
Nuclear
matter
Neutron Star
SPS
AGS
Early Universe
LHC
RHIC
Baryon density
Temperature
ec ~ 1 GeV/fm3
~ 10 ms after
Big Bang
Federico Antinori – 科大 – 2012年3月29号

7. 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 b
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
peripheral
central
Federico Antinori – 科大 – 2012年3月29号

8. Multiplicity

9. Particle multiplicity
for the most central collisions: ~ 1600 charged particles per unit of η
ALICE: PRL105 (2010)
log extrapolation fails (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 – 科大 – 2012年3月29号

10. 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
ALICE: PRL 106 (2011)
Helena Santos – ATLAS (EPS 2011)
Federico Antinori – 科大 – 2012年3月29号

11. Correlations

12. 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
… 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

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

14. Structures in (Δη,Δφ) two shoulders near side jet peak on away side
(at 120 and 240 )
aka “the Mach cone”
near side jet peak
long range structure
in η on near side
aka “the ridge”
Federico Antinori – 科大 – 2012年3月29号

15. 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) ]
and indeed, v3  0 !
v3 has weaker centrality dependence than v2
when calculated wrt participants plane, v3 vanishes
as expected, if due to fluctuations…
Matt Luzum (QM 2011) v2 v3
ALICE: PRL 107 (2011)
Federico Antinori – 科大 – 2012年3月29号

16. 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
Andrew Adare – ALICE (QM2011)
Federico Antinori – 科大 – 2012年3月29号

17. High pT suppression

18. 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!
CMS: PAS HIN
Federico Antinori – 科大 – 2012年3月29号

19. Suppression vs event plane
Alexandru Dobrin – ALICE (QM2011)
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

20. Identified particles

21. pT spectra vs hydrodynamics
comparison of identified particle spectra with hydro predictions
(calculations by C Shen et al.: arXiv: [nucl-th])
OK for π and K, but p seem to “misbehave” (less yield, flatter spectrum)
Michele Floris – ALICE (QM2011)
Federico Antinori – 科大 – 2012年3月29号

22. v2 vs hydrodynamics
comparison of identified particles v2(pT) with hydro prediction
(calculation by C Shen et al.: arXiv: [nucl-th])
 again, protons…
Raimond Snellings
ALICE (QM2011)
Federico Antinori – 科大 – 2012年3月29号

23. Strangeness enhancement

24. Strangeness enhancement
enhancement decreases with increasing √s
 strange/non-strange increases with √s in pp
Maria Nicassio – ALICE (SQM2011)
Federico Antinori – 科大 – 2012年3月29号

25. Comparison with thermal model
Reasonable grand-canonical description with Tch=164 MeV
 except for p/π !
Alexander Kalweit – ALICE (SQM2011)
Federico Antinori – 科大 – 2012年3月29号

26. Quarkonia

27. 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)
CMS: PAS HIN
ATLAS: PLB 697 (2011) 294
Federico Antinori – 科大 – 2012年3月29号

28. J/ψ @ LHC: forward y, low pT
LHC: 2.5 < y < 4, pT > 0 (ALICE)
less suppression than
RHIC: 1.2 < y < 2.2, pT > 0 (PHENIX)
~ as suppressed as
RHIC: |y| < pT > 0 (PHENIX)
Federico Antinori – 科大 – 2012年3月29号

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

30. Jets

31. Di-jet imbalance
Pb-Pb events with large di-jet imbalance observed by ATLAS & CMS
recoiling jet strongly quenched!
Frank Ma – CMS (EPS-HEP 2011)
Federico Antinori – 科大 – 2012年3月29号

32. AJ with increasing centrality: enhancement of asymmetric di-jets
with respect to pp & HIJING + PYTHIA
ATLAS: PRL105 (2010)
Federico Antinori – 科大 – 2012年3月29号

33. Di-jet Δφ no visible angular decorrelation in Δφ wrt pp collisions
CMS: arXiv:
Federico Antinori – 科大 – 2012年3月29号

34. Jet nuclear modification factor
suppression of jet production
in central Pb-Pb wrt binary-scaled peripheral
very weak centrality dependence
Helena Santos – ATLAS (EPS-HEP 2012)
Federico Antinori – 科大 – 2012年3月29号

35. Fragmentation function
similar to fragmentation in vacuum?
Federico Antinori – 科大 – 2012年3月29号

36. Heavy Flavours

37. c and b LHC
substantial suppression of heavy flavour production
beauty, too!
study parton mass and colour charge dependence of interaction with medium!
Federico Antinori – 科大 – 2012年3月29号

38. Andrea Dainese – ALICE (QM 2011)
D vs π
charm ~ as suppressed as pions
maybe a hint of RAAD > RAAπ at low pT …
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
Andrea Dainese – ALICE (QM 2011)
N Armesto et al., PRD 71 (2005)
 to be continued with higher statistics…
Federico Antinori – 科大 – 2012年3月29号

39. Chiara Bianchin – ALICE (SQM 2011)
D0 elliptic flow
First direct measurement of D flow in heavy-ion collisions
Yield extracted from invariant mass spectra of Kp candidates in 2 bins of azimuthal angle relative to the event plane
Chiara Bianchin – ALICE (SQM 2011)
Federico Antinori – 科大 – 2012年3月29号

40. 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

41. 谢谢！