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High-p T spectra and correlations from Cu+Cu and Au+Au collisions in STAR

High-p T spectra and correlations from Cu+Cu and Au+Au collisions in STAR. Marco van Leeuwen, LBNL for the STAR collaboration. Outline. Basic jet energy loss phenomenology is well-established New results from larger data samples allow to explore energy loss in quantitative detail:

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High-p T spectra and correlations from Cu+Cu and Au+Au collisions in STAR

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  1. High-pT spectra and correlations from Cu+Cu and Au+Au collisions in STAR Marco van Leeuwen, LBNL for the STAR collaboration

  2. 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 Dh excess ‘the ridge’

  3. Baryon enhancement Au+Au 0-10% Au+Au 0-10% p+p p+p • 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 L/K0s Au+Au 0-5% p+p Fragmentation not dominant for pT <~ 7 GeV

  4. Identified Particle RCP Intermediate pT enhancement similar for strange and non-strange baryons

  5. Jet-like di-hadron correlations New results, year-4 Year-2 results, published 8 < pT,trig < 15 GeV 4 < pT,trig < 6 GeV pT,assoc > 2 GeV Phys Rev Lett 91, 072304 No measurable away-side yield in Au+Au • Larger data sample (year-4) allows higher pT: • Emergence of the away side peak • Background negligible at higher pT,assoc Larger pT allows quantitative analysis of jet energy loss

  6. Di-hadron correlations: centrality dependence Fit scaledby x2 8 < pT,trig < 15 GeV/c Away-side: Increasing suppression with centrality Near side yields essentially unmodified

  7. Di-hadron fragmentation Scaling factors ~0.54 ~0.25 8 < pT,trig < 15 GeV/c Near side fragmentation unmodified Away-side: strong suppression,but shape similar above zT≈0.4

  8. A closer look at azimuthal peak shapes 8 < pT(trig) < 15 GeV/c pT(assoc)>6 GeV p Df No away-side broadening Large energy loss without observable modification of longitudinal and azimuthal distributions Observations constrain on energy loss fluctuations and geometrical bias

  9. Discussion of di-hadron results  = STAR preliminary Strong suppression (factor 4-5, similar to inclusive hadron suppression) without modification of longitudinal and azimuthal fragmentation shapes Observation: In contrast to several model expectations Majumder, Wang, Wang, nucl-th/0412061 Vitev, hep-ph/0501225 Broadening due to fragments of induced radiation Near-side enhancementdue to trigger bias Induced acoplanarity (BDMPS): Hydro: Hirano, Nara PRL 91, 083302

  10. Surface and other bias effects  = STAR preliminary Partonic spectrum Ejet Nuclear geometry L Energy loss DE(Ejet) Fragmentation D(Ejet,DE) General form:    PQM: Dainese, Loizides and Paic ‘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, Gyulassynucl-th/0512076 Are we selecting pairs, events with small energy-loss? X-N Wang, PLB 595, 165 (2004) Alternative: Shape of di-hadron fragmentation changes little if underlying partonic spectrum shape unmodified This calculation underpredicts suppression Note also: possible low-z enhancement from fragmentation of induced gluons. Outside measured range, awaits confirmation Need full calculations, a la PQM Different observables probe different parts of convolution

  11. Path-length dependence PQM: Dainese, Loizides, Paic RAA scales smoothly from Au+Au through Cu+Cu to p+p (Approximate) scaling of RAA is also expected in PQM: L2 scaling confirmed? I. Vitev, hep-ph/0603010: same conclusion 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?

  12. Medium response to recoiling jet Dh shape at lower pT,trig Lower pT,trig Lower pT,assoc 3 < pT,trig < 6 GeV2 < pT,assoc < pT,trig PRL 95, 152301 PRL 90, 082302 4< pT,trig < 6 GeV pT,assoc > 0.15 GeV Au+Au, 0-5% • Additional effects seen at lower pT • - Away-side Dj broadening • Near-side enhancement at large Dh

  13. Analysis methods  J = near-side jet-like corrl. R = “ridge”-like corrl. 2 (J) Method ||<0.7 (J) method 1 2  (J+R) - (R) (J+R) Method ||<1.7 Au+Au 20-30%

  14. Ridge yield extraction (J+R) method (J) method (J) method preliminary yield,) Npart 3 < pt,trigger < 4 GeV and pt,assoc. > 2 GeV Subtraction of ridge in Dh and Djgive consistent results No centrality dependence of j-h symmetric jet part • Definition of “ridge yield”: i) “ridge yield” := yield(J+R))-yield(J) ii) “relative ridge yield” := (yield(J+R))-yield(J) yield(J))

  15. Ridge yield in Au+Au pt,assoc > 2 GeV relative ridge yield absolute ridge yield preliminary preliminary Ridge yield contribution can be large compared to jet yield Ridge yield persists to large pT,trig=7 GeV

  16. Ridge yield in Au+Au pT,assoc > 3 GeV relative ridge yield absolute ridge yield preliminary preliminary Strong decrease of ridge yield for higher pT,assoc > 3 GeV Relative ridge yield also decreases: ridge softer than jet ?

  17. Dh 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

  18. System size dependence of ridge pt,assoc. > 2 GeV relative ridge yield relative ridge yield preliminary preliminary At the same Npart the relative ridge yield is similar in Cu+Cu and Au+Au Note: only partial statistics for Cu+Cu. Large uncertainties in flow subtraction due to non-flow. Work in progress

  19. More medium response 2.5 < pT,trigger < 4.0 GeV 1.0 < pT,assoc < 2.5 GeV QM05: N.Grau, talk, J Jia, poster PHENIX STAR Preliminary Strong away-side broadening seen at low pT,assoc Existence of ‘the dip’ so far only shown for intermediate pT,trigger and low pT,assoc Need to establish jet-related origin: does it persist to high pT,trigger? Also note: systematic uncertainties should not be disregarded

  20. 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 Dh ‘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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