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Review of Heavy-Ion Results from the CERN LHC. David Evans The University of Birmingham. Strong and Electroweak Matter 2012 Swansea, UK, 10 th – 13 th July 2012. Outline of Talk. Introduction The LHC Experiments Selection of results from ALICE, ATLAS, & CMS Particle spectra & ratios - PowerPoint PPT Presentation
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Review of Heavy-Ion Results from the CERN LHC
David EvansThe University of Birmingham
Strong and Electroweak Matter 2012 Swansea, UK, 10th – 13th July 2012
Outline of Talk• Introduction• The LHC Experiments• Selection of results from ALICE, ATLAS, & CMS
– Particle spectra & ratios– Flow– Jet and high pT suppression (RAA)– Heavy Flavour
• Summary
2
Aims of Heavy-Ion Programme
Study strongly interacting matter at extreme energy densities over large volumes and long time-scales (study of QCD at its natural scale, QCD ~ 200 MeV). Study the QCD phase transition from nuclear matter to a deconfined state of quarks and gluons - The Quark-Gluon Plasma. Study the physics of the Quark-Gluon Plasma (QCD under extreme conditions).
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Phases of Strongly Interacting Matter
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Lattice QCD, B = 0Lattice QCD, B = 0
Both statistical and lattice QCD predict that nuclear matter will undergo a phase transition, in to a deconfined state of quarks and gluons – a quark-gluon plasma, at a temperature of, T ~ 170 MeV and energy density, ~ 1 GeV/fm3.
Heavy-Ion collisions far exceed these temperatures & densities at the LHC.
colour
Heavy Ion Collisions
pre-equilibration QGP hadronisation freeze out
Colliders: AGS, SPS, RHIC, LHC
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Create QGP by colliding ultra-relativistic heavy ions
SNN (GeV) = 5.4 19 200 2760 (5500)
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Observables
Jets
Open charm, beauty 6
7 7
LHC Detectors
ALICE
CMS
ATLAS
Centrality Selection – Glauber Model
• Assume particle production is factorises into number of participants Npart number of collisions Ncoll
• AssumeNegative Binomial Distribution
for both f*Npart + (1-f) Ncoll
• Relation of percentile classes to Glauber values (<Npart>, etc) purely geometrical by slicing in impact parameter
Ener
gy in
ZD
C (a
.u.)
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Charged Particle MultiplicitydNch/d versus sdNch/d (Pb-Pb) Theory
dNch/d = 1584 4 (stat.) 76 (syst.)
Larger than most model predictions – good agreement between LHC experiments
Rises with s faster than ppFits power law: Pb-Pb ~ s0.15
Bjorken formula estimates energy density, > 15 GeV/fm3 (3 x RHIC) 9
Phys. Rev. Lett. 105, 252301 (2010)Phys. Rev. Lett. 106, 032301 (2011)
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Bose–Einstein correlations(HBT)
• medium size: 2x larger than at RHIC– RoutRsideRlong ~ 300 fm3
• medium life time: 40% longer than at RHIC– ~ 10-11 fm/c
ALICE, Phys. Lett. B 696 (2011) 328.
QM enhancement of identical Bosons at small momentum difference
enhancement of e.g. like-sign pions at low momentum difference qinv=|p1-p2|,
Particle spectraLHC results agree well between experiments
Very significant changes in slope compared to RHICGood agree between experiments e.g. ALICE/ATLAS in this case
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Identified Particle spectra
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Very significant changes in slope compared to RHICMost dramatically for protonsVery strong radial flow, ≈ 0.66 even larger than predicted by most recent hydro
Blast Wave Fit
RHIC
Hydro Prediction
RHIC
Strangeness Enhancement
Multi-strange baryons enhanced with respect to pp. baryons enhanced more than s.
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Thermal Models
Thermal model: A.Andronic et al., Phys. Lett. B673:142-145, 2009
Low Tch suggested by proton spectra excluded by ,
If exclude p, data agree with thermal GC model prediction Tch = 164 MeV
Thermal models can not fit all data simultaneously14
'Baryon anomaly': /K0
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Baryon/Meson ratio still strongly enhanced x 3 compared to pp at pT = 3 GeV/c
- Enhancement slightly larger than at RHIC 200 GeV
- Maximum shift very little in pT compared to RHIC despite large change in underlying spectra !
Ratio at Maximum
RHIC/K0
x 3
Recent EPOS model calculation describes the data well: (K. Werner, arXiv: 1204.1394) (not decribed by coalescence model)
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Does QGP have elliptic Flow?
Ed 3Nd 3p
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d 2NpTdpTdy
1 2vn cos n r n1
Angle of reaction planeFourier coefficient
pT
Equal energy density contours
P
v2
v2 cos2
x
z
y
V1 = directed flow. V2 = elliptic flow.
Study angular dependence of emitted particles
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Elliptic Flow
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v2 shape is identical to
RHIC v 2 ~30% larger than
RHIC: larger <pT>,K reproduced by hydro calculation p ok for semi-peripheral but is a remaining challenge for central collisions
ALICE, Phys. Rev. Lett. 105 (2010) 252302.
Hydro-calcs.: Shen, Heinz, Huovinen and Song, arXiv:1105.3226.
Elliptic Flow – Higher Orders
ATLAS
ATLAS
Higher-order coefficients (n>2) do not show strong centrality dependence – consistent with fluctuations in initial state geometry being driving force.
Superposition of flow harmonics, including higher orders, is sufficient to describe Ridge and Cone effects without implying medium response.
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Phys. Rev. Lett. 107, 032301 (2011)
Flow vs Centrality
Flow strongest for semi-peripheral collisions (central collisions too symmetric).
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V2 at High PT
V2 drops to zero at high PT 20
Full Harmonic Spectrum
CMS
Measure multi Vn - Over-constrain HydroModels to get /s, initial conditions etc.
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Elliptic Flow for Identified Particles
intermediate pT: dep. on particle specieshigh pT: v2 and v3 small: hydrodinamic flow negligibleIndication for non zero D meson v2 (3σ in 2 < pT < 6 GeV/c)Comparable with charged hadrons elliptic flow
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Jets in heavy ion collisions• Studying deconfinement with jets
di-quark
quark
Interaction at the quark (parton) level The same interaction at the hadron level
pT
pL
pTOT
jet
soft beam jet
Fragmentation
radiatedgluons
heavy nucleus
key QCD prediction: jets are quenched
X.-N. Wang and M. Gyulassy, Phys. Rev. Lett. 68 (1992) 1480
Free quarks not allowedwhen quark, anti-quark pairs are produced they fly apart producing back-to-back ‘jets’ of particles
Quarks remain free in QGP and lose energy while travelling through 23
JETS
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Charged Jets in Pb-Pb
Large suppression of jets seen in central collisions
192 GeV168 GeV
10-20% peripheral
bin size: 0.1x0.147 GeV
102 GeV
0-10% central
Back-to-back high-energy jets seen in peripheral collisions
25
Jet Suppression
Significant imbalance between leading and sub-leading jets for central collisions26
Jet Suppression
Also seen by CMS27
Where is Jet Energy?
Jet energy unbalanced inside cone, but unbalanced in opp direction outside cone- Overall balance is achieved.
Another way to study this effect is to look at high pT suppression.
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Nuclear Modification Factor
• particle interaction with medium
• stronger suppression at LHC• ~factor 7 at 7 GeV/c
29
TDppcoll
TDAA
TDAA dpdN
dpdpR
/
/)(
Leading
Hadron
quark-gluon plasma
Divide pT spectra in Pb-Pb by p-p (with suitable normalisation factor)
RAA vs centrality
• Suppression increases with centrality.
• Minimum remains around 6-7 GeV/c
30
RAA for s, Ws & Zs
No suppression as expected 31
RAA for different particles
• For hadrons containing heavy quarks, smaller suppression expected
• Enhancement of /K clearly seen• At high pT (>8-10 GeV/c) RAA universality for
light hadrons
Strange
32
Rcp of Jets
Jets suppressed by factor 2 in central w.r.t. peripheral collisions.Suppression flat with pT.
Rcp of muons from heavy flavour decays about 0.4 for central collisions.Again, flat for 0.4 < PT < 14 GeV 33
RAA for Heavy Flavour
●Hc,b → muons●D mesons (charm)●B → J/ψ (beauty)
CMSarXiv:1201.5069
• Charm and beauty: no evidence of mass effects yet
• Maybe a hint of hierarchy w.r.t pions!
Charm
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RAA J/: Nuclear Modification Factor
Inclusive J/ RAA = 0.497 0.006 (stat.) 0.078 (syst.)
Rather small suppression and less than at RHIC (forward rapidity, PT > 0)
ALICE pT > 0 PHENIX pT > 0
almost no centrality dependence above Npart ~ 100Unlike at RHIC
RAA J/
Greater suppression seen at LHC in mid-rapidity, at pT > 6.5 GeV
And centrality dependent.
At low-pt, ALICE RAA ≥ 0.5, no centrality dependence→ behaviour clearly different to the one observed in PHENIX where a largersuppression and a strong centrality dependence is seenAt high-pt, larger suppression seen both by ALICE and CMS→ behaviour is similar to the one observed at low energy
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Di-Muon Spectrum
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ϒ Suppression
NΥ(2S)/NΥ(1S)|pp = 0.56 ± 0.13 ± 0.01 NΥ(2S)/NΥ(1S)|PbPb = 0.12 ± 0.03 ± 0.01NΥ(3S)/NΥ(1S)|pp = 0.21 ± 0.11 ± 0.02 NΥ(3S)/NΥ(1S)|PbPb < 0.07
Ratios not corrected for acceptance and efficiency 38
– ϒ(1S) RAA in7 centrality bins– first results on ϒ(2S) RAA– clear suppression of ϒ(2S)– ϒ(1S) suppressionconsistent with excitedstate suppression(~50% feed down)
ϒ(1S) and ϒ (2S) RAA
39
Summary• We have started to piece together the standard model of
heavy-ion collisions• We have measured many of the global features
– Phase transition temperature - Agrees with theory Tc ~ 164 MeV– Energy density - Over 10x critical energy density, > 15 GeV/fm3
– size & lifetime of system ~ 5000 fm3, ~ 10-11 fm/c• Started to measure dynamical features
– Very strong radial flow, ≈ 0.66 – Strong elliptical flow – ideal liquid– Strong suppression of jets and high pT particles– Little evidence of quark mass dependence on energy loss (unlike
predictions – although maybe a hint at this stage)– J/ suppressed less than RHIC at low PT - ϒ’ and ϒ’’ suppressed.
• We are at the early stages of a 10 year programme and have just scratched the surface.
• lots of work and exciting physics to look forward to.• Thank you for listening
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Backup Slides
41
Chiral Magnetic Effect ('strong parity violation')
RP 2cos
B
+-
Same charge correlations positiveOpposite charge correlations negativeRHIC ≈ LHCsomewhat unexpected
should decrease with Nch
may decrease with √s
RHIC : (++), (+-) different sign and magnitudeLHC: (++),(+-) same sign, similar magnitude?
cos
+ -
B
?
RHIC
RHIC
Local Parity Violation in strong magnetic Field ? 42
Particle Identification
TPC dE/dx
ITS Silicon Drift/Strip dE/dx
TOF
• dE/dx:5% resolution
• Time-of flight,resolution: ~85 ps (Pb-Pb)~110 ps (p-p)
• TPC dE/dx: separate p from K up to 1.1 GeV
• Time of flight: separate K from up to ~ 2.2 GeV
Ω ΛΚ
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