High-p T spectra and correlations from Cu+Cu and Au+Au collisions in STAR Marco van Leeuwen, LBNL for 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 p T –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 p T : Large baryon/meson enhancement in central Au+Au collisions Baryon/meson ratios in Au+Au approach p+p values at p T ~ 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 p T <~ 7 GeV

4Marco van Leeuwen, WWND06, San Diego Identified Particle R CP Intermediate p T enhancement similar for strange and non-strange baryons

5Marco van Leeuwen, WWND06, San Diego Jet-like di-hadron correlations Larger p T allows quantitative analysis of jet energy loss Phys Rev Lett 91, < p T,trig < 6 GeV p T,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 p T : -Emergence of the away side peak -Background negligible at higher p T,assoc 8 < p T,trig < 15 GeV

6Marco van Leeuwen, WWND06, San Diego Di-hadron correlations: centrality dependence Fit scaled by x2 8 < p T,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 ~ < p T,trig < 15 GeV/c Scaling factors Near side fragmentation unmodified Away-side: strong suppression, but shape similar above z T ≈0.4

8Marco van Leeuwen, WWND06, San Diego A closer look at azimuthal peak shapes 8 < p T (trig) < 15 GeV/c p T (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 results Strong suppression (factor 4-5, similar to inclusive hadron suppression) without modification of longitudinal and azimuthal fragmentation shapes In contrast to several model expectations Broadening due to fragments of induced radiation Induced acoplanarity (BDMPS): Hydro: Hirano, Nara PRL 91, = STAR preliminary Near-side enhancement due to trigger bias Majumder, Wang, Wang, nucl-th/ Observation: Vitev, hep-ph/

10Marco van Leeuwen, WWND06, San Diego Surface and other bias effects PQM: 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/ 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 spectrum E jet Nuclear geometry L Energy loss  E(E jet ) Fragmentation D(E jet,  E) General form:   Need full calculations, a la PQMDifferent observables probe different parts of convolution

11Marco van Leeuwen, WWND06, San Diego Path-length dependence R AA scales smoothly from Au+Au through Cu+Cu to p+p (Approximate) scaling of R AA is also expected in PQM: L 2 scaling confirmed? R AA values at low N part only marginally consistent: need to explore sensitivity of model Expect di-hadron results for Cu+Cu in near future. Will I AA ≈ R AA also hold in Cu+Cu? PQM: Dainese, Loizides, Paic I. Vitev, hep-ph/ : same conclusion

12Marco van Leeuwen, WWND06, San Diego Medium response to recoiling jet 4< p T,trig < 6 GeV p T,assoc > 0.15 GeV Additional effects seen at lower p T - Away-side  broadening -Near-side enhancement at large  Au+Au, 0-5% 3 < p T,trig < 6 GeV 2 < p T,assoc < p T,trig PRL 90, PRL 95, Lower p T,assoc Lower p T,trig  shape at lower p T,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 yield ,  ) N part 3 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+Au p t,assoc > 2 GeV preliminary relative ridge yieldabsolute ridge yield Ridge yield persists to large p T,trig =7 GeV Ridge yield contribution can be large compared to jet yield

16Marco van Leeuwen, WWND06, San Diego Ridge yield in Au+Au p T,assoc > 3 GeV preliminary relative ridge yieldabsolute ridge yield Strong decrease of ridge yield for higher p T,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 p T,trigger for p T,assoc > 2 GeV Ridge yield persists to high p T,trigger ≈ 7 GeV Scenarios:  Parton radiates energy before fragmenting and couples to the longitudinal flow Armesto et al, nucl-ex/  Heating of the medium Chiu & Hwa Phys. Rev. C72:034903,2005 –Radial flow + jet-quenching Voloshin nucl-th/ Armesto et al, nucl-ex/ Need theory calculations of centrality and p T dependence to compare to data Future direction: explore species dependence of ridge effect

18Marco van Leeuwen, WWND06, San Diego System size dependence of ridge p t,assoc. > 2 GeV preliminary At the same N part the relative ridge yield is similar in Cu+Cu and Au+Au 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 p T,trigger and low p T,assoc Strong away-side broadening seen at low p T,assoc Need to establish jet-related origin: does it persist to high p T,trigger ? Also note: systematic uncertainties should not be disregarded 2.5 < p T,trigger < 4.0 GeV 1.0 < p T,assoc < 2.5 GeV

20Marco van Leeuwen, WWND06, San Diego Conclusions Di-hadron correlations at high p T show strong suppression but no change of longitudinal or azimuthal shapes R AA scales with N part in Cu+Cu and Au+Au –Stay tuned for more Cu+Cu results to test models Medium response: –Clear  ‘ridge’, persists to high p T,trig –Azimuthal broadening so far only shown at lower p T,trig We are narrowing down the mechanisms for parton energy loss Quantitative analysis is ongoing and should yield bounds on medium density