1. Hot and dense matter: Hard probes from RHIC to LHC
M. van Leeuwen, Utrecht University

2. Hard probes of QCD matter
Use ‘quasi-free’ partons from hard scatterings
Calculable with pQCD
to probe ‘quasi-thermal’ QCD matter
Interactions between parton and medium:
Radiative energy loss
Collisional energy loss
Hadronisation: fragmentation and coalescence
Quasi-thermal matter: dominated by soft (few 100 MeV) partons
Use the strength of pQCD to explore QCD matter
Sensitive to medium density, transport properties

3. Energy loss in QCD matter
radiated gluon
propagating parton
QCD bremsstrahlung (+ LPM coherence effects) m2
Transport coefficient l
Energy loss
Energy loss probes:
Density of scattering centers:
Nature of scattering centers, e.g. mass: radiative vs elastic loss
Or no scattering centers, but fields  synchrotron radiation?

4. Relativistic Heavy Ion Collider
Au+Au sNN= 200 GeV
PHENIX
STAR
RHIC: variety of beams: p+p, d+Au, Au+Au, Cu+Cu
Two large experiments: STAR and PHENIX
Smaller experiments: PHOBOS, BRAHMS decomissioned
Recent years: Large data samples, reach to high pT

5. Energy loss in QCD matter
Yield per collision
Nuclear modification factor
: RAA = 1
C. Vale, K. Okada, Hard Probes 2008
0: RAA ≈ 0.2
D. d’Enterria
: no interactions
Hadrons: energy loss
RAA = 1
RAA < 1
Hard partons lose energy in the hot matter

6. What can we learn from RAA?
p0 spectra
Nuclear modification factor
PHENIX, PRD 76, , arXiv:
This is a cartoon!
Hadronic, not partonic energy loss
No quark-gluon difference
Energy loss not probabilistic P(DE)
Ball-park numbers: DE/E ≈ 0.2, or DE ≈ 2 GeV for central collisions at RHIC
Note: slope of ‘input’ spectrum changes with pT: use experimental reach to exploit this

7. Energy distribution from theory
TECHQM ‘brick problem’
L = 2 fm, DE/E = 0.2
E = 10 GeV
‘Typical for RHIC’
ASW: Armesto, Salgado, Wiedemann
WHDG: Wicks, Horowitz, Dordjevic, Gyulassy
Not a narrow distribution:
Significant probability for DE ~ E
Conceptually/theoretically difficult
T. Renk
Significant probability to lose no energy
For nuclear collisions, need to fold in geometry

8. Determining the medium density
PHENIX, arXiv: , J. Nagle WWND08
PQM (Loizides, Dainese, Paic), Multiple soft-scattering approx (Armesto, Salgado, Wiedemann) Realistic geometry
For each model:
Vary parameter and predict RAA
Minimize 2 wrt data
Models have different but ~equivalent parameters:
Transport coeff.
Gluon density dNg/dy
Typical energy loss per L: e0
Coupling constant aS
GLV (Gyulassy, Levai, Vitev), Opacity expansion (L/l), Average path length
WHDG (Wicks, Horowitz, Djordjevic, Gyulassy) GLV + realistic geometry
ZOWW (Zhang, Owens, Wang, Wang) Medium-enhanced power corrections (higher twist) Hard sphere geometry
AMY (Arnold, Moore, Yaffe)
Finite temperature effective field theory (Hard Thermal Loops)

9. Medium density from RAA^
+2.1
- 3.2
PQM <q> = GeV2/fm
+270
- 150
+0.2
- 0.5
GLV dNg/dy = 1400
ZOWW e0 = GeV/fm
+200
- 375
+0.016
WHDG dNg/dy = 1400
AMY as = 0.280
Quantitative extraction gives medium density to 10-20%
Method extracts medium density given the model/calculation Theory uncertainties need to be further evaluated
e.g. comparing different formalisms, varying geometry
Different models approximately agree – except PQM, high density
Density 30-50x cold nuclear matter

10. Heavy quark suppression
Using non-photonic electrons
Expected energy loss
light
M.Djordjevic PRL 94
Wicks, Horowitz et al, NPA 784, 426
PHENIX nucl-ex/ , STAR nucl-ex/
Expect: heavy quarks lose less energy due to dead-cone effect
Most pronounced for bottom
Measured suppression of non-photonic electrons larger than expected
Djordjevic, Phys. Lett. B632, 81
Armesto, Phys. Lett. B637, 362
Radiative (+collisional) energy loss not dominant? E.g.: in-medium hadronisation/dissociation (van Hees, et al)

11. Di­hadron correlations
8 < pTtrig < 15 GeV
Combinatorial background
associated
pTassoc > 3 GeV 
trigger
Near side
Away side
Use di-hadron correlations to probe the jet-structure in p+p, d+Au
and Au+Au

12. Medium density from di-hadron measurement
J. Nagle, WWND2008
8 < pT,trig < 15 GeV
associated 
trigger
d-Au
IAA constraint
DAA constraint
DAA + scale uncertainty
Au-Au
Medium density from away-side suppression and single hadron suppression agree
Theory: ZOWW, PRL98,
e0=1.9 GeV/fm
single hadrons
However:
Theory curve does not match d+Au: need to evaluate systematics
Experimental uncertainties will decrease in near future
Data: STAR PRL 95,
zT=pT,assoc/pT,trig

13. Lowering pT: gluon fragments/bulk response
d+Au, 200 GeV 
trigger
Au+Au 0-10%
STAR preliminary
Jet-like peak
associated
3 < pt,trigger < 4 GeV
pt,assoc. > 2 GeV
Long. flow
J. Putschke, M. van Leeuwen, et al
`Ridge’: associated yield at large 
dN/d approx. independent of 
Strong - asymmetry suggests effect of longitudinal flow or underlying event

14. Associated yields from coalescence
Recombination of thermal (‘bulk’) partons
‘Shower-thermal’ recombination
Baryon pT=3pT,parton
Meson pT=2pT,parton
Baryon pT=3pT,parton
Meson pT=2pT,parton
Hot matter
Hot matter
Hard parton
(Hwa, Yang)
No jet structure/associated yield
Expect large baryon/meson ratio
associated with high-pT trigger
Expect reduced associated yield
with baryon triggers 3 < pT < 4 GeV

15. Associated baryon/meson ratios
pTtrig > 4.0 GeV/c
2.0 < pTAssoc < pTtrig
C. Suarez et al, QM08
p+p / p++p-
Inclusive spectra
Associated yields
Au+Au: Baryon enhancement
Ridge (large Dh):
Baryon enhancement
p+p, d+Au: B/M  0.3
Jet (small Dh)
B/M  0.3
Baryon/meson ratio in ridge close to Au+Au inclusive, in jet close to p+p
Different production mechanisms for ridge and jet?

16. Parton energy from g-jet and jet reconstruction
second-generation measurements at RHIC
Qualitatively:
known
pQCDxPDF
extract
`known’ from e+e-
Full deconvolution large uncertainties (+ not transparent)
Fix/measure Ejet to take one factor out 
Two approaches:
g-jet
Jet reconstruction

17. Direct-g recoil suppression
Expected recoil for various P(DE)
T. Renk
Measurement sensitive to energy loss distribution P(DE)
Need precision to distinguish scenarios
8 < ET,g < 16 GeV
2 < pTassoc < 10 GeV
J. Frantz, Hard Probes 2008
A. Hamed, Hard Probes 2008
STAR Preliminary
ET,g
DAA (zT)
IAA(zT) =
Dpp (zT)
Large suppression for away-side: factor 3-5
Results agree with model predictions
Uncertainties still sizable Some improvements expected for final results Future improvements with increased RHIC luminosity 17

18. Jet reconstruction in heavy ion eventsη
pt per grid cell [GeV]
STAR preliminary
~ 21 GeV
Jets clearly visible in heavy ion events at RHIC
Quantitative analysis requires:
Good jet-finding algorithm
Combinatorial background subtraction
Use different algorithms to estimate systematic uncertainties:
Cone-type algorithms simple cone, iterative cone, infrared safe SISCone
Sequential recombination algorithms kT, Cambridge, inverse kT
FastJet:Cacciari, Salam and Soyez; arXiv:

19. Jet spectrum – RAA of jets
Energy resolution
Pythia jet+Heavy ion background
Effect of resolution on spectrum
ET [GeV]
Nbin scaled p+p
Au+Au 0-10%
 MB-Trig
O HT-Trig
R=0.4
pT cut =1 GeV
Seed=4.6 GeV
LOHSC
Statistical Errors Only
dNJet/dET (per event)
dNJet/dET (per event)
PyDet
PyEmbed
PyTrue
LOHSC
seed=4.6 GeV
R=0.4
STAR Preliminary
Counts
pTcut =1.0 GeV
Seed=4.6 GeV
LOHSC
pTcut =1 GeV R=0.4
ET=35±5 GeV
S. Salur, Hard Probes 2008
STAR Preliminary ∆E
E = EPyDet – EPyTrue E = EPyEmbed - EPyTrue E = EPyEmbed - EPyDet
No significant suppression of jet production in AA
 Jet reconstruction recovers unbiased jet sample

20. Jet fragmentation Fragmentation function
Fragmentation function ratio A+A/p+p
pt,jetrec. >30 GeV
STAR preliminary stat. errors only
J. Putschke, Hard Probes 2008
pthadron~10 GeV
Au+Au HT Et>7.5 GeV
p+p HT Et>5.4 GeV
Jet-energy uncorrected – needs study
No apparent modification in the fragmentation function with respect to p+p

21. Relating jets and single hadrons
High-pT hadrons from jet fragmentation
Qualitatively:
Measured suppression of single hadrons:
Suppression of jet yield (out-of-cone radiation)
Modification of fragment distribution (in-cone radiation)
First results from STAR show neither
Work in progress:
Better understanding of interplay between reconstruction biases (trigger bias, background subtraction) and jet-quenching needed
Requires theory-experiment collaboration

22. New development: E-loss generators
Event generators treat full in-medium QCD shower:
Theory/model advantage over analytic approach: conserve energy/respect kinematic bounds
Important experimental tool: can perform jet reconstruction (and other analysis) on model
2 models
JEWEL
K. Zapp, U. Wiedemann, arXiv:
Focus on collisional energy loss + medium response
Cluster hadronisation
q-PYTHIA
L. Cunqueiro, N. Armesto, C. Salgado, arXiv:
Focus on radiative energy loss
Modified shower evolution
Important step for the field

23. Transport and medium properties
Transport coefficient
2.8 ± 0.3 GeV2/fm
(model dependent)
e  23 ± 4 GeV/fm3
pQCD:
T  400 MeV
(Baier)
(Majumder, Muller, Wang)
 ~ GeV/fm3 T ~ MeV
Viscosity
t0 = 0.3-1fm/c
Total ET
From v2
(see previous talk: Steinberg)
(Bjorken)
Lattice QCD: h/s < 0.1
(Meyer)
Broad agreement between different observables, and with theory
A quantitative understanding of hot QCD matter is emerging

24. Outlook I: LHC
Simulated result
ALICE EMCal TDR
For example: Can measure jet fragmentation with Ejet = 175 GeV
Talks by T. Awes, M. Spousta, L. Sarycheva
Use kinematic reach (DE >> E) to determine energy dependence of DE
Large hard process yields:
Jets to > 200 GeV
Light, heavy hadrons to 100 GeV
Test/validate understanding gained from RHIC

25. Projected performance for g-hadron measurement
Outlook II: RHIC
Accelerator upgrades
Stochastic cooling
Detector upgrades
Vertex detectors
PHENIX
Projected performance for g-hadron measurement
STAR
Enables charm/bottom direct measurements
Ongoing data analysis:
Large sample Au+Au (run-7) and d+Au (run-8)

26. Conclusion Hard probes provide insight in nature of hot QCD matter
RHIC results:
‘Standard’ measurements extent to high pT, start to constrain theory in detail
Intermediate pT: interplay between hard and soft physics, e.g. ‘the ridge’
New large samples: experimental control over Eparton
Gamma-jet analysis
Jet reconstruction
DE ~ 2 GeV at RHIC, DE/E not small
Start systematic comparisons of different measurements with theory
 Assessment of systematics in theory and experiment
Progress towards ‘global analysis’ of heavy ion data
Future:
RHIC: larger luminosity, detector upgrades
LHC: Large rates at high pT, exploit DE < E
Theory: develop in-medium shower generators

27. Conclusion Hard probes provide insight in nature of hot QCD matter
Recent developments:
Large luminosity at RHIC
Extend to higher pT
Gamma-hadron analysis (fix parton energy)
Jet reconstruction
Near future at LHC
Large rates at high pT, exploit DE < E
Theory: develop in-medium shower generators
DE ~ 2 GeV at RHIC, DE/E not small
Start combining different measurements to improve understanding
- Assessment of systematics in theory and experiment
Progress towards ‘global analysis’ of heavy ion data

28. Extra Slides

29. Direct-g recoil yields
Run GeV
M. Nguyen, Quark Matter 2006
A. Hamed, Hard Probes 2008
Direct-g–jet measurements being pursued by STAR and PHENIX
Requires large data samples
Suppression of away-side yield visible
Similar to di-hadrons, but now with selected parton energy

30. Away-side suppression
A. Hamed et al QM08
Away-side yield AuAu/pp
Model predictions tuned to hadronic measurements
First g-jet results from heavy ion collisions
Measured suppression agrees with theory expectations
Next step: measure pTassoc dependence to probe DE distribution

31. Intermediate pT: the ridge
3 < pt,trig< 4 GeV/c
Jet-like peak
4 < pt,trig < 6 GeV/c
pt,assoc. > 2 GeV/c
Au+Au 0-10%
STAR preliminary
Au+Au 0-10%
STAR preliminary
J. Putschke et al, QM06 
trigger
`Ridge’: associated yield at large , small Df
associated
Weak dependence of ridge yield on pT,trig
 Relative contribution reduces with pT,trig
Strong - asymmetry suggests coupling to longitudinal flow

32. RAA at LHC
GLV
BDMPS
T. Renk, QM2006
RHIC
RHIC
S. Wicks, W. Horowitz, QM2006
LHC: typical parton energy > typical E
Expected rise of RAA with pT depends on energy loss formalism
Nuclear modification factor RAA at LHC sensitive to radiation spectrum P(E)

33. Baryon/meson ratios Anti-Baryon/meson ratio p/p ratio
P. Fachini et al, QM08
Theory: X.-N. Wang, PRC 70,
Large baryon/meson ratio at intermediate pT
 Hadronisation by coalescence of quarks?
New data extend pT-range in p+p
Expect p mainly from gluons and DEg>DEq, no stronger suppression seen for p
New STAR results on baryon fragmentation in p+p, see M. Heinz

34. Heavy-light difference
Expect: dead-cone effect
M.Djordjevic PRL 94 (2004)
Wicks, Horowitz et al, NPA 784, 426
Armesto et al, PRD71,
light
Armesto plot
Below 10 GeV: charm loses 20-30% less energy than u,d
Bottom loses ~80% less
Expected suppression of D mesons ~0.5 times light hadrons

35. Heavy flavour: Discrepancy STAR/PHENIX spectra
PHENIX: A/(exp(-a pT - b pT2) + pT/p0)n:
A = 377 +/- 60 mb GeV^-2 c^3
a = / c/GeV b = / (c/GeV)^2
p0 = / (c/GeV) n = /- 0.04
Zhangbu Xu (STAR Collaboration) SQM08 35 35

36. Zhangbu Xu (STAR Collaboration) SQM08
e/p ratio
STAR Preliminary
STAR Preliminary
e+e-
STAR Preliminary
Conversion in detector material x10 reduced
Run8 is consistent with run3 NPE results
Run8: 0.55%X0 (beampipe 0.29%, air: 0.1%, wrap ~0.14% …) run3: 5.5%X0 (+ SVT…)
Detector Material is not the issue
Jin Fu, Parallel session 12.I
Zhangbu Xu (STAR Collaboration) SQM08 36 36

37. QCD and quark parton model
At high energies, quarks and gluons are manifest
At low energies, quarks are confined in hadrons
S. Bethke, J Phys G 26, R27
Asymptotic freedom
Running coupling:
s grows with decreasing Q2
Running coupling: from confinement to asymptotic freedom
QCD governs both extremes. Can we study/conceptualise the evolution?
Study emergent behaviour at large coupling: confinement, bulk QCD matter

38. Radiative energy loss in QCD
Energy loss process characterized by a single constant
kT~m
Transport coefficient l
Energy loss
Transport coefficient sets medium properties
pQCD expectation
(Baier et al)
Non-perturbative: is a Wilson loop
(Wiedemann)
e.g. N=4 SUSY:
From AdS/CFT
(Liu, Rajagopal, Wiedemann)
Transport coefficient is a fundamental parameter of QCD matter

39. Some open questions
Urs Wiedemann, Hard Probes 2008

40. Di-hadron suppression in Au+Au
d+Au
Au+Au 20-40%
Au+Au 0-5%
pTassoc > 3 GeV
pTassoc > 6 GeV
High-pT hadron production in Au+Au dominated by (di-)jet fragmentation
Suppression of away-side yield in Au+Au collisions
Measures energy loss in di-jet events
No detectable broadening or change of peak shape: fragmentation after energy loss