1. 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

2. 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

3. 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?
STAR PRL, 98, (2007)

4. Quark vs. Gluon Energy Loss
Baryon & meson RAA
Mechanism of energy loss : Medium-induced gluon radiation
Effect of color charge:
Anti-particle/particle
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: )
- Energy loss mechanism not understood
Theory: X.-N. Wang,
PRC 70 (2004)
Data: PRL 97 (2006)
PLB 655 (2007) 104

5. From Single-Particle to 2-Particle Correlations
Di-jets (hadron-hadron correlations)
Single inclusive hadrons
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
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” particle
Trigger

6. Di-jets (hadron-hadron correlations) Single inclusive hadrons
Trigger Biases
Di-jets (hadron-hadron correlations)
Single inclusive hadrons
Escaping Jet
“Near Side”
Lost Jet
“Far Side”
Renk and Eskola, hep-ph/
Renk and Eskola, hep-ph/
Trigger particle
Trigger particle
Photon-jet measurement is, in principle, sensitive to full medium
True measure of the Energy (no energy loss for direct photon) g q

7. -Jet: “Golden Probe” of QCD Energy Loss
Wang et al., Phys.Rev.Lett. 77 (1996)  q  g q
QCD analog of
Compton Scattering h
I really think you need this as a forward looking talk
 emerges unscathed from the medium
- This probe is valuable for comparison with di-hadron correlations
- Full reconstructed kinematics: real fragmentation function D(z) 7

8. Jet Suppression measured via direct photon trigger
-jet yield
Away-side hadrons
T. Renk, PRC74,
Theoretical calculation showing sensitivity to medium 8

9. First measure of away-side IAA for g-h
A. Hamed, QM2008
Ejet = E = E trig
Good agreement between theory and measurement
T. Renk and K. Eskola PRC75:054910,2007
Suppression similar level to inclusives in central collisions

10. Peripheral Au+Au ~ p+p = vacuum
Away-side Yields Relative to Peripheral Au+Au
A. Hamed, QM2008
Peripheral Au+Au ~ p+p = vacuum
Icp of -jet exhibits same suppression on the
away-side yield per trigger of the associated particles (3-8GeV/c). 10

11. 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?
STAR PRL, 98, (2007)

12. Electron-tagged correlations to obtain bottom contributionK- b b B-
D*0 D0 +
e e- B+ - K+
D0
Experimental approach
- non-photonic electrons from semi- leptonic charm decays are used to trigger on c-c̅, b-b̅ pairs
back-2-back D0 mesons are reconstructed via their hadronic decay channel (probe)‏
Underlying production mechanism can be identified using second charm particle
essentially from B decays only
like-sign e-K pairs
e –D0 correlation with
75% from charm
25% from beauty
A. Mischke, QM 2008
heavy quark production g c c c c g
0

flavor creation gluon splitting/fragmentation

13. Heavy quark production in p+p collisions
e-h
e-D0 correlation agree with e-h results
- the B contribution to non-photonic electrons is ~50% at pT~5 GeV/c, based on e-h and e-D correlations
A. Mischke, S. Sakai, G. Wang, QM2008

14. Data from SPS, showing J/Y suppression
We expect a suppression of bound states due to color screening in the
Quark Gluon Plasma. (Matsui & Satz, 1986)
Data from SPS, showing J/Y suppression
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)

15. Heavy Quarkonium Production (and Survival)
Two Component Approach: X. Zhao and R. Rapp, hep-ph/
Ads/CFT Calculation of Survival
J/Y Production in p+p collisions
Most models expect a decrease in
RAA as function of pT
H. Liu, K. Rajagopal and U.A. Wiedemann
PRL 98, (2007) and hep-ph/
Z. Tang, QM2008
Next step the  - almost there
Z. Tang Session XVII, D. Das Session XXII

16. 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/
AdS/CFT:
H. Liu, K. Rajagopal and U.A. Wiedemann,
PRL 98, (2007) and hep-ph/
Contribution from beauty decays - better agreement

17. 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

18. Extra Slides

19. Elliptic flow v2 – NPE from HF decays
PHENIX Run4
PRL, 98, (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.

20. J/Ψ – hadron correlations in p+p
Associated hadron spectra with leading J/ 1)
no near side correlation 2)
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/

21. No energy loss for triggered di-jets!
Triggering on Di-Jets
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
“jet-axis”
trigger (T2)
primary
trigger (T1)
Di-jets are suppressed.
Once select di-jets, away-side associated particles NOT suppressed.
Shapes of near- and away-sides similar.
Central Au+Au ~ d+Au.
O. Barannikova, QM2008
No energy loss for triggered di-jets!
Tangential di-jets (or punch-through without interactions).

22. Heavy quark production in p+p collisions
e-h
D* - jet correlation
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