High-pT spectra and correlations from Cu+Cu and Au+Au collisions
in STARMarco van Leeuwen, LBNLfor the STAR collaboration
2Marco van Leeuwen, WWND06, San Diego
Outline
• Basic jet energy loss phenomenology is well-established• New results from larger data samples allow to explore
energy loss in quantitative detail:– Di-hadron correlations at high pT
– First Cu+Cu results to study path length dependence– Detailed analysis of near-side large excess ‘the ridge’
3Marco van Leeuwen, WWND06, San Diego
Baryon enhancement
• Intermediate pT: Large baryon/meson enhancement in central Au+Au collisions
• Baryon/meson ratios in Au+Au approach p+p values at pT ~ 7 GeV
/K
0 s
Au+Au 0-10%
p+p
Au+Au 0-10%
p+p
Au+Au 0-5%
p+p
Fragmentation not dominant for pT <~ 7 GeV
4Marco van Leeuwen, WWND06, San Diego
Identified Particle RCP
Intermediate pT enhancement similar for strange and non-strange baryons
5Marco van Leeuwen, WWND06, San Diego
Jet-like di-hadron correlations
Larger pT allows quantitative analysis of jet energy loss
Phys Rev Lett 91, 072304
4 < pT,trig < 6 GeVpT,assoc > 2 GeV
Year-2 results, published
No measurable away-side yield in Au+Au
New results, year-4
Larger data sample (year-4) allows higher pT:
- Emergence of the away side peak- Background negligible at higher pT,assoc
8 < pT,trig < 15 GeV
6Marco van Leeuwen, WWND06, San Diego
Di-hadron correlations: centrality dependence
Fit scaledby x2
8 < pT,trig < 15 GeV/c
Near side yields essentially unmodified
Away-side: Increasing suppression with centrality
7Marco van Leeuwen, WWND06, San Diego
Di-hadron fragmentation
~0.54
~0.25
8 < pT,trig < 15 GeV/c
Scalingfactors
Near side fragmentation unmodified
Away-side: strong suppression,but shape similar above zT≈0.4
8Marco van Leeuwen, WWND06, San Diego
A closer look at azimuthal peak shapes8 < pT(trig) < 15 GeV/c
pT(assoc)>6 GeV
Large energy loss without observable modification of longitudinal and azimuthal distributions
Observations constrain on energy loss fluctuations and geometrical bias
No away-side broadening
9Marco van Leeuwen, WWND06, San Diego
Discussion of di-hadron resultsStrong suppression (factor 4-5, similar to inclusive hadron suppression) without modification of longitudinal and azimuthal fragmentation shapes
jetTcs pN
dz
dE 2
8
In contrast to several model expectations
Broadening due to fragments of induced radiation
Induced acoplanarity (BDMPS):
Hydro: Hirano, Nara PRL 91, 083302
= STAR preliminary
Near-side enhancementdue to trigger bias
Majumder, Wang, Wang, nucl-th/0412061
Observation:
Vitev, hep-ph/0501225
10Marco van Leeuwen, WWND06, San Diego
Surface and other bias effectsPQM: Dainese, Loizides and Paic
X-N Wang, PLB 595, 165 (2004)
= STAR preliminary
Note also: possible low-z enhancement from fragmentation of induced gluons. Outside measured range, awaits confirmation
‘Surface bias’:- Trigger, associated selection favours short path lengths
Surface bias is not the only possibility:- Energy-loss fluctuations (at fixed path length) potentially large- Fragmentation bias Wicks, Horowitz, Djordjevic, Gyulassy
nucl-th/0512076
Are we selecting pairs, events with small energy-loss?
Alternative:Shape of di-hadron fragmentation changes little if underlying partonic spectrum shape unmodified
This calculation underpredicts suppression
Partonic spectrumEjet
Nuclear geometry
L
Energy lossE(Ejet)
FragmentationD(Ejet,E)
General form:
Need full calculations, a la PQM Different observables probe different parts of convolution
11Marco van Leeuwen, WWND06, San Diego
Path-length dependence
RAA scales smoothly from Au+Au through Cu+Cu to p+p
(Approximate) scaling of RAA is also expected in PQM: L2 scaling confirmed?
RAA values at low Npart only marginally consistent: need to explore sensitivity of model
Expect di-hadron results for Cu+Cu in near future. Will IAA ≈ RAA also hold in Cu+Cu?
PQM: Dainese, Loizides, Paic
I. Vitev, hep-ph/0603010: same conclusion
12Marco van Leeuwen, WWND06, San Diego
Medium response to recoiling jet
4< pT,trig < 6 GeVpT,assoc > 0.15 GeV
Additional effects seen at lower pT
- Away-side broadening- Near-side enhancement at large
Au+Au, 0-5%
3 < pT,trig < 6 GeV2 < pT,assoc < pT,trig
PRL 90, 082302PRL 95, 152301
Lower pT,assocLower pT,trig shape at lower pT,trig
13Marco van Leeuwen, WWND06, San Diego
(J+R)Method||<1.7
J = near-side jet-like corrl.
R = “ridge”-like corrl.
Analysis methods
1
Au+Au 20-30%
2
2
(J+R)- (R)
(J)
Method||<0.7
(J)method
14Marco van Leeuwen, WWND06, San Diego
Ridge yield extraction
Definition of “ridge yield”:
i) “ridge yield” := yield(J+R))-yield(J)
ii) “relative ridge yield” := (yield(J+R))-yield(J)yield(J)
preliminary(J+R) method
(J) method
(J) method
yiel
d
,)
Npart
3 < pt,trigger < 4 GeV and pt,assoc. > 2 GeV
Subtraction of ridge in and give consistent results
No centrality dependence of - symmetric jet part
15Marco van Leeuwen, WWND06, San Diego
Ridge yield in Au+Aupt,assoc > 2 GeV
preliminary preliminary
relative ridge yield absolute ridge yield
Ridge yield persists to large pT,trig=7 GeV
Ridge yield contribution can be large compared to jet yield
16Marco van Leeuwen, WWND06, San Diego
Ridge yield in Au+AupT,assoc > 3 GeV
preliminary preliminary
relative ridge yield absolute ridge yield
Strong decrease of ridge yield for higher pT,assoc > 3 GeV
Relative ridge yield also decreases: ridge softer than jet ?
17Marco van Leeuwen, WWND06, San Diego
Discussion• Relative ridge yield increases with centrality and decreases with pT,trigger
for pT,assoc > 2 GeV
• Ridge yield persists to high pT,trigger≈ 7 GeV
• Scenarios: Parton radiates energy before fragmenting
and couples to the longitudinal flow Armesto et al, nucl-ex/0405301
Heating of the medium Chiu & Hwa Phys. Rev. C72:034903,2005
– Radial flow + jet-quenching Voloshin nucl-th/0312065 Armesto et al, nucl-ex/0405301
Need theory calculations of centrality and pT dependence to compare to data
Future direction: explore species dependence of ridge effect
18Marco van Leeuwen, WWND06, San Diego
System size dependence of ridgept,assoc. > 2 GeV
preliminary
preliminary
At the same Npart the relative ridge yield is similar in Cu+Cu and Au+Au
relative ridge yield relative ridge yield
Note: only partial statistics for Cu+Cu. Large uncertainties in flow subtraction due to non-flow. Work in progress
19Marco van Leeuwen, WWND06, San Diego
More medium response
QM05: N.Grau, talk, J Jia, poster
STAR Preliminary
PHENIX
Existence of ‘the dip’ so far only shown for intermediate pT,trigger and low pT,assoc
Strong away-side broadening seen at low pT,assoc
Need to establish jet-related origin: does it persist to high pT,trigger?
Also note: systematic uncertainties should not be disregarded
2.5 < pT,trigger < 4.0 GeV1.0 < pT,assoc < 2.5 GeV
20Marco van Leeuwen, WWND06, San Diego
Conclusions
• Di-hadron correlations at high pT show strong suppression but no change of longitudinal or azimuthal shapes
• RAA scales with Npart in Cu+Cu and Au+Au– Stay tuned for more Cu+Cu results to test models
• Medium response:– Clear ‘ridge’, persists to high pT,trig
– Azimuthal broadening so far only shown at lower pT,trig
We are narrowing down the mechanisms for parton energy loss
Quantitative analysis is ongoing and should yield bounds on medium density
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