1. Prospects for understanding energy loss in hot nuclear matter
Marco van Leeuwen, Utrecht University

2. Goals for hard probes In my opninion, we aim to
1. Understand energy loss and in-medium fragmentation
and use it to
2. Measure the medium density (properties) in heavy ion collisions
1. is interesting in its own right as long as we can make contact with good QCD theory
2. is of obvious interest for heavy ion physics
Two questions, so probably need two or more observables and a combined fit

3. What can we learn from RAA?
p0 spectra
Nuclear modification factor
PHENIX, PRD 76, , arXiv:
Too-simple model favors DE/E constant Renk et al: favor DE constant
Ball-park numbers: DE/E ≈ 0.2, or DE ≈ 2 GeV not small compared to GeV parton at RHIC
Note: slope of ‘input’ spectrum changes with pT: use experimental reach to exploit this

4. Energy loss distribution from theory
Energy loss distribution P(DE) is where we can constrain/compare to QCD theory (BDMPS-Z, GLV, AMY)
‘Typical for RHIC’
TECHQM ‘brick problem’
L = 2 fm, DE/E = 0.2
E = 10 GeV
Not a narrow distribution:
Significant probability for DE ~ E
Conceptually/theoretically difficult
Significant probability to lose no energy
(e.g. eikonal approximation not valid in principle)
+ geometry. Nuisance, or tool?

5. 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)
Independence of RAA on pT at RHIC due to fine-tuning/interplay of different trends?

6. 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

7. Direct-g prospects at RHIC
Expected recoil for various P(DE)
Projected uncertainties
(with stochastic cooling at RHIC)
T. Renk
ET,g
Measurement sensitive to energy
loss distribution P(DE)
Need precision to distinguish scenarios
Large luminosities expected at RHIC, precision increases
But still ET,trig ~ ET,jet ~ 10 GeV 7

8. Rates for g-jet at RHIC and LHC
g-jet rates
B. Jacak, W. Vogelsang
Need plot
P. Jacobs, M. van Leeuwen
g/p-ratio larger at RHIC than LHC
With |h| < 1, reach GeV for g-jet at LHC

9. Jets at LHC Simulated result
(Jets at RHIC covered by Peter Jacobs)
Simulated result
ALICE EMCal TDR
Jet fragmentation should be sensitive to energy loss model. e.g.ALICE jet quenching code with Ejet = 175 GeV
However:
Need more theoretical or theory/experiment exploration of capabilities
Large hard process yields:
Jets to > 200 GeV
Light, heavy hadrons to 100 GeV

10. Are there other sensitive observables?
One idea that is around:
PTt1
pTa1
pTt1
pTt2
T.Renk,arXiv:
2 density models
Idea: use back-to-back hadron pair to trigger on di-jet and study assoc yield
PTt2
Tune/control fragmentation bias and possibly geometry/energy loss bias
Does this constraint P(DE) better

11. Associated yields from coalescence
Importance of particle identification?
If coalescence/recombination really plays a role, we should see:
‘Shower-thermal’ recombination
Recombination of thermal (‘bulk’) partons
Baryon pT=3pT,parton
Meson pT=2pT,parton
Baryon pT=3pT,parton
Meson pT=2pT,parton
and/or
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
Current experimental results not conclusive (limited pT-range and large uncertainties)
Worth pursuing!

12. Summary Energy loss distribution P(DE) is key to hard probes of QGP
Integrating observables (single- and di-hadron suppression) only sensitive with large dynamic range (e.g. LHC)
g-jet should be sensitive, statistics limited
Jet fragmentation (spectra) should be sensitive
Currently, limits of sensitivity etc not explicit
Need theory-experiment work to clarify
Can we identify other observables (e.g. 2+1 hadrons)?

13. Projected performance for g-hadron measurement
Outlook II: RHIC
Accelerator upgrades
Stochastic cooling
Detector upgrades
Vertex detectors
PHENIX
Add rate plot?
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)

14. 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 14

15. Au+Au vs d+Au comparison
T1: pT>5 GeV/c, T2: pT>4 GeV/c, A: pT>1.5 GeV/c
Au+Au
d+Au Df -1 -2 1 2 3 4 5
_dN_
Ntrig d(Df )
STAR Preliminary
200 GeV Au+Au & d+Au T1
T.Renk,arXiv:
2 density models
O. Barannikova, F. Wang, QM08 T2
Au+Au similar to d+Au
Model calculation:
DE smallest when PTT1~PTT2
(still DE>0)
Di-hadron trigger selects jet pairs with little or no energy loss in Au+Au
To do: increase PTT1-PTT2

16. 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?