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

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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

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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

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(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

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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

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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

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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 ?

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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

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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

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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

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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