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Hard Probes and Heavy Flavor
from STAR
Saskia Mioduszewski
for the STAR Collaboration
Texas A&M University
Rencontres de Moriond: QCD and High
Energy Interactions
14 March, 2008
Why Hard Probes?
• Only photons decouple from medium upon creation
• Large-mass quarks provide particularly good probe of
medium produced
– Created early in the collision
– mc ~ 1.3 GeV, mb ~ 4.8 GeV >> Tc, LQCD  less affected than
light quarks
– Perhaps more direct connection to transport properties of
medium
What Questions Can be Addressed via Hard Probes?
• g-jet : true (modified) fragmentation function
• Open Charm/Beauty: energy loss mechanism, degree of
thermalization
• Quarkonium: deconfinement (dissociation in QGP),
degree of thermalization
One of the most surprising results from
RHIC
• Heavy flavor suppression
as large as for light quarks
R AA 
Yield AuAu / Nbinary  AuAu
Yield pp
STAR PRL, 98, 192301 (2007)
• No dependence of energy
loss on flavor
• Do we understand energy
loss mechanism?
• Where is Beauty
contribution?
Quark vs. Gluon Energy Loss
Baryon & meson RAA
Mechanism of energy loss :
Medium-induced gluon radiation
Effect of color charge:
Anti-particle/particle
E g
9
~
Eq 4
Factor 9/4 Color effects not
observed up to pT ~ 12 GeV/c
- Not sensitive?
- In-medium conversions, qg?
(W. Liu, R.J. Fries, arXiv:0801.0453)
Theory: X.-N. Wang,
PRC 70 (2004) 031901
Data: PRL 97 (2006) 152301
PLB 655 (2007) 104
- Energy loss mechanism not
understood
From Single-Particle to 2-Particle Correlations
Single inclusive hadrons
Suppression quantified by
RAA =
YieldAA/(Yieldpp *<Nbinary>pp)
Single particle is leading hadron
of jet

Probe of density of medium, but
Strong surface bias for “trigger”
Trigger
particle
Di-jets (hadron-hadron
correlations)
Suppression quantified by
IAA =
Jet-assoc. YieldAA/Jet-assoc. Yieldpp
Hadrons associated with high-pT
trigger particle

More differential probe of energy loss
Surface bias reduced, but not
removed
Trigger Biases
Single inclusive hadrons
Renk and Eskola, hep-ph/0610059
Escaping Jet
“Near Side”
Di-jets (hadron-hadron
correlations)
Renk and Eskola, hep-ph/0610059
Trigger
particle
Trigger
particle
Lost Jet
“Far Side”
q
g
• Photon-jet measurement is, in
principle, sensitive to full medium
• True measure of the Energy (no
energy loss for direct photon)
g-Jet: “Golden Probe” of QCD Energy Loss
Wang et al., Phys.Rev.Lett. 77 (1996) 231-234
g
q
g
g
q
QCD analog of
Compton Scattering
h
• g emerges unscathed from the medium
- This probe is valuable for comparison with di-hadron correlations
- Full reconstructed kinematics: real fragmentation function D(z)
7
Jet Suppression measured via direct photon trigger
g-jet yield
Away-side hadrons
T. Renk, PRC74, 034906
Theoretical calculation showing sensitivity to medium
8
First measure of away-side IAA for g-h
A. Hamed, QM2008
Ejet = Eg = E trig
trig
D AA (z T ,ET )

trig
Dpp (z T ,ET )
IAA

T. Renk and K. Eskola PRC75:054910,2007
Associated Yield AA collisions
Associated Yield pp collisions
Good agreement
between theory
and measurement
Suppression similar level to inclusives in central collisions
Away-side Yields Relative to Peripheral Au+Au
A. Hamed, QM2008
trig
ICP 

DC entral AA (z T ,ET )
trig
DP eripheral AA (z T ,ET )
Associated Yield Central AA
Associated Yield PeripheralAA
Peripheral Au+Au ~
p+p = vacuum
Icp of g-jet exhibits same suppression on the
away-side yield per trigger of the associated particles (3-8GeV/c).
10
One of the most surprising results from
RHIC
• Heavy flavor suppression as
large as for light quarks
• No dependence of energy
loss on flavor
• Do we understand energy
loss mechanism?
• Where is Beauty
contribution?
R AA 
Yield AuAu / Nbinary  AuAu
Yield pp
STAR PRL, 98, 192301 (2007)
Electron-tagged correlations to obtain bottom contribution
• Experimental approach
A. Mischke, QM 2008
- non-photonic electrons from semileptonic charm decays are used to
trigger on c-c̅, b-b̅ pairs
e –D0 correlation
with
0
- back-2-back
D mesons
like-sign
e-K pairs are
reconstructed via their hadronic decay
channel (probe)
• Underlying production mechanism
can be identified using second charm
c
particle
c
essentially from
B gdecaysc only
g
c 
75% from charm
g
g
25%
from beauty
0
g
g
flavor creation gluon splitting/fragmentation
heavy quark
production
Heavy quark production in p+p collisions
e-h
e-D0 correlation agree with e-h results
A. Mischke, S. Sakai, G. Wang, QM2008
- the B contribution to nonphotonic electrons is ~50%
at pT~5 GeV/c, based on e-h
and e-D correlations
J/Y Suppression
We expect a suppression of bound
states due to color screening in the
Quark Gluon Plasma. (Matsui & Satz, 1986)
c
c
Color Screening
• Charm cross-section larger at RHIC than
SPS – ~ 20 cc pairs produced per collision
• We have evidence that charm may be
partially thermalized at RHIC 
recombination of cc pairs to regenerate
J/Y ?
• Or sequential melting of charmonium
(Karsch, Kharzeev, Satz)
Data from SPS, showing
J/Y suppression
Heavy Quarkonium Production (and Survival)
Two Component Approach: X. Zhao and R. Rapp, hep-ph/07122407
Ads/CFT Calculation of Survival
J/Y Production in p+p collisions
Most models expect a decrease in
H. Liu, K. Rajagopal and U.A. Wiedemann
RAA182301(2007)
as function and
of phep-ph/0607062
PRL 98,
T
Z. Tang, QM2008
Next step the  - almost there
Z. Tang Session XVII, D. Das Session XXII
Heavy Quarkonium Production (and Survival)
J/Y in Cu+Cu Collisions at RHIC Z. Tang, QM2008
• Data consistent with no suppression at
high pT: RAA(pT > 5 GeV/c) = 0.9 ± 0.2
• While at Low-pT RAA: 0.5—0.6 (PHENIX)
• Indicates RAA increase from low pT to
high pT
• Most models expect a decrease RAA at
high pT (not including bottom decays):
Two Component Approach:
X. Zhao and R. Rapp, hep-ph/07122407
AdS/CFT:
H. Liu, K. Rajagopal and U.A. Wiedemann,
PRL 98, 182301(2007) and hep-ph/0607062
Contribution from beauty decays - better agreement
16
Conclusions
• Surprise at RHIC: heavy-quark energy loss does not
follow expectation – Do we really understand energy
loss mechanism?
• Exploring di-jet and photon-jet measurements to
constrain energy loss mechanism
• Beauty contribution to heavy flavor measurements is
~50% at pT~5 GeV/c, indicating that bottom must be
suppressed as well
• J/Y not suppressed at high pT, contrary to expectation
from theoretical calculation if no contribution from
Beauty decays
• Need direct measure of Charm/Beauty up to high pT –
will be possible with planned upgrades
Extra Slides
Elliptic flow v2 – NPE from HF decays
PHENIX Run4
PRL, 98, 172301 (2007)
Non-zero elliptic flow for electron from heavy flavor decays → indicates
non-zero D v2, partonic level collective motion.
Strongly interact with the dense medium at early stage of HI collisions.
Light flavor thermalization.
J/Ψ – hadron correlations in p+p
1)
ggg
 J /  g
Associated hadron spectra with leading J/
no near side correlation
2)
g  g bb
 Bhadron  X
 J /  X
strong near side correlation
Z. Tang, QM2008
No near side correlation seen!
Away side: consistent with leading charged hadron correlations
Near side: consistent with no associated hadron production
BJ/ not a dominant contributor to inclusive J/
Triggering on Di-Jets
T1: pT>5 GeV/c, T2: pT>4 GeV/c, A: pT>1.5 GeV/c
1
_dN_
Ntrig d( )
200 GeV Au+Au & d+Au
3
STAR Preliminary
Au+Au
d+Au
2
1
“jet-axis”
trigger (T2)
0
-2
•
•
•
•
-1
0
1

2
3
primary
trigger (T1)
4
5
Di-jets are suppressed.
O. Barannikova, QM2008
Once select di-jets, away-side associated particles NOT suppressed.
Shapes of near- and away-sides similar.
Central Au+Au ~ d+Au.
No energy loss for triggered di-jets!
Tangential di-jets (or punch-through without interactions).
Heavy quark production in p+p collisions
D* - jet correlation
*

0

D

D

K



e-h
mD*  mD 0  145MeV
e-D0 correlation agree with e-h results
A. Mischke, S. Sakai, G. Wang, QM2008
The B contribution to non-photonic
electrons is ~50% at pT~5 GeV/c, based
on e-h and e-D correlations