1. Recent STAR results on high-pT probes
Marco van Leeuwen,
for the STAR collaboration

2. 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

3. 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

4. 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

5. Associated baryon/meson ratios
Au+Au: 2 < pTtrig < 3 GeV/c
Cu+Cu: 3 < pTtrig < 6 GeV/c
C. Suarez et al, QM08
pTtrig > 4.0 GeV/c
2.0 < pTAssoc < pTtrig
p+p / p++p-
M. Lamont et al, QM05 J. Bielcikova et al, WWND07, C. Nattrass et al, QM08
Baryon/meson ratio in ridge close to inclusive, in jet close to p+p
Different production mechanisms for ridge and jet?

6. Ridge event-by-event: 3-particle Dh-Dh
1
2
trigger
No acceptance
No acceptance
Idea: Distinguish
and
1 jet-like associated, 1 ridge-like:
Bands along axes
Ridge particles on either side:
uniform Dh-Dh
Both ridge particles on one side: diagonal structure

7. Dh-Dh results Dh-Dh structure: central peak + uniform
P. Netrakanti et al, QM08
Radial distribution
3<pTTrig<10 GeV/c 1<pTAsso<3 GeV/c
STAR Preliminary
dAu
dAu
dAu
Shaded :
sys. error.
Line :
v2 uncer.
STAR Preliminary
STAR Preliminary
AuAu
0-12%
AuAu
0-12%
STAR Preliminary
AuAu
0-12%
Note: 2x1 backgrounds subtracted
STAR Preliminary
Dh-Dh structure: central peak + uniform
No substructure in ridge: no gluon fragments?
Note absence of excess along axes: no jet+ridge events?
Need to estimate expected signal strengths

8. Dependence on angle with reaction plane0º
90º
3<pTtrig<4GeV/c
in-plane fS=0
out-of-plane fS=90o
A. Feng et al, QM08
STAR Preliminary
1 < pTasso < 1.5GeV/c
1.5 < pTasso < 2 GeV/c
|Dh|>0.7
Ridge
STAR Preliminary
Jet

9. Reaction plane dependence II
3<pTtrig<4
Away side core/shoulder
Near side: ridge/jet
core
shoulder
A. Feng et al, QM08
A. Feng et al, QM08
20-60%
jet part
ridge part
1.5<pTassoc<2.0 GeV/c
1.0<pTasso<1.5GeV/c
Yield in shoulder region independent of fs
Jet yield approx no dependence on φs
Yield in peak region decreases with fs
Ridge yield decreases with φs
Shape change due to core, not shoulder?
Ridge larger for shorter path length?

10. System size dependence of di-hadron yields
6 < pT trig < 10 GeV/c
pT assoc >3 GeV
Near Side
Away Side
O. Catu et al, QM08
O. Catu et al, QM08
Modified frag: ZOWW, PRL 98,
PQM: C. Loizides, Eur. Phys. J. C 49, (2007)
PQM: BDMPS-SW + rcoll scaling
Mod Frag Model: ZOWW Higher twist + rpart scaling
Need more model calculations to disentangle effects from energy loss model and density scaling/profile

11. Away-side IAA vs system size
6< pT trig < 10 GeV
O. Catu et al, QM08
O. Catu et al, QM08
Theory - X.N.Wang private communication
H.Zhang, J.F. Owens, E. Wang, X.N. Wang – PRL 98,
zT=pT,assoc/pT,trig
Away-side suppression constant for zT >0.3
Results agree with ZOWW modified fragmentation
2.8 ± 0.3 GeV2/fm for central Au+Au

12. Medium density from di-hadron measurement
J. Nagle, WWND2008
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:
Result depends on use of DAA vs IAA; theory curve does not match d+Au
Current exp uncertainty driven by d+Au; more stats from run-8, BEMC p0
Data: STAR PRL 95,

13. Fundamental quantity P(E)
Radiation spectrum
Radiation in realistic medium
Salgado and Wiedemann, Phys. Rev. D68,
~15 GeV
Renk, Eskola, PRC75,
In realistic systems, energy loss is a broad distribution P(DE)
Single-hadron and di-hadron observables fold production spectra with P(DE)
Need more differential measurement to probe P(DE)
Three techniques:
g-jet, A. Hamed’s talk
Jet-finding, J. Putschke, S. Salur, M. Heinz talks
Multi-particle correlations, this talk

14. Di-hadron triggered analysis
O. Barannikova, F. Wang, QM08
a.u.
T1T2
pTa1
pTt1
pTt2
Signal + Background
Background
Signal
A: pT>1.5 GeV/c
T1: pT>5 GeV/c
Au+Au 0-12%
Df T2A1 -1 -2 1 2 3 4 5
_dN_
Ntrig d(Df )
STAR Preliminary
T2A1_T1
T2A1
T1A1 2 4
Df (T1T2)
T2: pT>4 GeV/c
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
Subtract two combinatorial terms:
random T1, random T2

15. Di-hadron triggers in d+Au
O. Barannikova, F. Wang, QM08
_dN_
Ntrig d(Df )
T2A1_T1
T2A1
d+Au
200 GeV
Di-hadron trigger
pTa1
pTt1
pTt2 2 1
T1: pT>5 GeV/c, T2: pT>4 GeV/c, A: pT>1.5 GeV/c
STAR Preliminary -1 -2 1 2 3 4 5
Df (T2A1)
Single hadron trigger
Requiring away-side trigger increases yield
 Selects higher underlying jet energies
 Fragmentation bias changes
pTa1
pTt1

16. Di-hadron triggers in Au+Au
O. Barannikova, F. Wang, QM08
Di-hadron trigger
_dN_
Ntrig d(Df ) 4
pTa1
pTt1
pTt2
Au+Au 0-12%
T2A1_T1
T2A1 2
Single hadron trigger
pTa1
pTt1
STAR Preliminary -2 Df -1 -2 1 2 3 4 5
T1: pT>5 GeV/c, T2: pT>4 GeV/c
A: pT>1.5 GeV/c
Single trigger: broad away-side
Di-hadron trigger: jet peaks on both near and away side
Di-hadron trigger selects different events, has different bias

17. 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

18. Multi-hadron cluster triggers
Idea:
Reduce fragmentation bias by clustering hadrons
‘proto-jet’
0-12% Au+Au
PT,sec seed > 3 GeV/c
PT,sec seed > 4 GeV/c
STAR Preliminary
STAR Preliminary
Multi-hadron trigger
B. Haag WWND08 R
S/B=0.7
Seed
S/B=2.0
Secondary Seeds
R=0.3
R=0.3
Associated track
Combinatorial background in
multi-hadron cluster
can be tuned with pT,sec seed
Use cluster energy for trigger:
- R = 0.3
- pT,seed > 5 GeV/c
- pT,sec seed > 3 GeV/c
(not subtracted in present result)

19. Multi-hadron cluster trigger in Au+Au
Away-side spectrum Au+Au
Pythia
0-12% Au+Au
STAR Preliminary
B. Haag, WWND08
B. Haag, WWND08
PYTHIA
Single-hadron and multi-hadron triggers give similar result
Fragmentation bias does not change?
PYTHIA also predicts similar results for multi-hadron and single-hadron triggers

20. Conclusion Intermediate pT (1 < pT < 5 GeV/c)
Near side ridge and away-side double-peak
Continue to explore with more differential studies:
Identified particles: baryon/meson ratio larger in ridge than jet-like peak
3-particle Dh-Dh: no Dh substructure seen in ridge
Angle wrt reaction plane: ridge larger in-plane, double-peak more prominent out-of-plane
High-pT (pT > 6 GeV/c)
Suppression due to E-loss – Theory well-controlled?
Quantitative analyses being pursued
- Single hadron and di-hadron suppression consistent
- System size dependence indicates rpart scaling
New techniques: multi-hadron analyses
Back-to-back di-hadron trigger
Multi-hadron cluster trigger
Difficult to gain access to unbiased parton kinematics
 Need g-jet, jet-finding, see talks by A. Hamed and J. Putschke

21. What’s to come: Run-8 d+Au
Run-8 finished: d+Au + p+p
Final total: 49 nb-1
+ 660M FMS+fast-detector minbias events
Large EMCal coverage
New in Run-8 FMS: 2.5 < h < 4.0
+ 45M minbias TPC events
(~4x run-3 sample):
improve reference for high-pT di- and multi-hadrons
Use forward p0-p0 correlations to probe low-x in Au nucleus

22. Extra Slides

23. Extracting the transport coefficient
Di-hadron suppression
Di-hadrons
Inclusive hadrons
Zhang, H et al, nucl-th/
Inclusive hadron suppression
Di-hadrons provide stronger constrain on density
2-minimum narrower for di-hadrons
Extracted transport coefficient from singles and di-hadrons consistent
2.8 ± 0.3 GeV2/fm