Transcript Prospects for understanding energy loss in hot nuclear matter

Prospects for understanding energy
loss in hot nuclear matter
Marco van Leeuwen, Utrecht University
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
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What can we learn from RAA?
p0 spectra
Nuclear modification factor
PHENIX, PRD 76, 051106, arXiv:0801.4020
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 10-20 GeV parton at RHIC
Note: slope of ‘input’ spectrum changes with pT: use experimental reach to exploit this
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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
(e.g. eikonal approximation not
valid in principle)
Significant probability
to lose no energy
+ geometry. Nuisance, or tool?
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RAA at LHC
GLV
BDMPS
T. Renk, QM2006
RHIC
RHIC
S. Wicks, W. Horowitz, QM2006
LHC: typical parton energy > typical DE
Expected rise of RAA with pT depends on energy loss formalism
Nuclear modification factor RAA at LHC sensitive to radiation spectrum P(DE)
Independence of RAA on pT at RHIC due to fine-tuning/interplay of different trends?
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Parton energy from g-jet and jet reconstruction
second-generation measurements at RHIC
Qualitatively:
dN
dpT

hadr
dN
dE
 P(DE )  D( pT ,hadr / E jet )
jets
known
pQCDxPDF
extract
`known’ from e+e-
Full deconvolution large uncertainties (+ not transparent)
Fix/measure Ejet to take one factor out
g
Two approaches:
 g-jet
- Jet reconstruction
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Direct-g prospects at RHIC
Projected uncertainties
Expected recoil for various P(DE)
(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
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Rates for g-jet at RHIC and LHC
g-jet rates
g/p-ratio larger at RHIC than LHC
P. Jacobs, M. van Leeuwen
B. Jacak, W. Vogelsang
Need plot
With |h| < 1,
reach 70-80 GeV for g-jet at LHC
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Jets at LHC
(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:
Large hard process yields:
Jets to > 200 GeV
Light, heavy hadrons to 100 GeV
Need more theoretical or theory/experiment
exploration of capabilities
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Are there other sensitive observables?
PTt1
One idea that is around:
pTt1
pTa1
T.Renk,arXiv:0804.1204
2 density models
pTt2
Idea: use back-to-back hadron
pair to trigger on
di-jet and study assoc yield
Tune/control fragmentation bias
and possibly geometry/energy loss bias
PTt2
Does this constraint P(DE) better
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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
Meson
pT=2pT,parton
Baryon
pT=3pT,parton
Baryon
pT=3pT,parton
Meson
pT=2pT,parton
and/or
Hot matter
Hot matter
Hard parton
No jet structure/associated yield
Expect reduced associated yield
with baryon triggers 3<pT<4 GeV
(Hwa, Yang)
Expect large baryon/meson ratio
associated with high-pT trigger
Current experimental results not conclusive (limited pT-range and large uncertainties)
Worth pursuing!
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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)?
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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)
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g
Direct-g recoil suppression
8 < ET,g < 16 GeV Expected
recoil for various P(DE)
J. Frantz, Hard Probes 2008
T. Renk
A. Hamed, Hard Probes 2008
2 < pTassoc < 10 GeV
STAR Preliminary
IAA(zT) =
ET,g
DAA (zT)
Dpp (zT)
Large suppression for away-side: factor 3-5
Measurement sensitive to energy loss distribution P(DE)
Results
agree with
model predictions
Need precision
to distinguish
scenarios
Uncertainties still sizable
Some improvements expected for final results
Future improvements with increased RHIC luminosity
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Au+Au vs d+Au comparison
T1: pT>5 GeV/c, T2: pT>4 GeV/c, A: pT>1.5 GeV/c
3
T
1
STAR Preliminary
Au+Au
d+Au
O. Barannikova, F. Wang, QM08
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_dN_
Ntrig d(Df )
200 GeV Au+Au & d+Au
2
1
T.Renk,arXiv:0804.1204
2 density models
0
-2
-1
0
1
Df
2
3
4
5
Au+Au similar to d+Au
Di-hadron trigger selects jet pairs
with little or no energy loss in Au+Au
T2
Model calculation:
DE smallest when PTT1~PTT2
(still DE>0)
To do: increase PTT1-PTT2
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Energy loss in QCD matter
radiated gluon
QCD bremsstrahlung
(+ LPM coherence effects)
m2
Transport coefficient qˆ 
l
Energy loss
m2
propagating parton
l
DEmed ~  S qˆL2
Energy loss probes:
Density of scattering centers:
l
1

Nature of scattering centers, e.g. mass: radiative vs elastic loss
Or no scattering centers, but fields  synchrotron radiation?
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