1. Dunlop, WW 2006 1 What More Can Be Learned from High Pt Probes at RHIC? James Dunlop Brookhaven National Laboratory

2. Dunlop, WW 2006 2 Suppression Suppression an established probe of the density of the medium The medium is dense: what more can be said? J. Adams et al, Phys. Rev. Lett. 91 (2003) 072304 Binary collision scaling p+p reference

3. Dunlop, WW 2006 3 Central R AA Data Increasing density The Limitations of R AA: “Fragility” Surface bias leads effectively to saturation of R AA with density Challenge: Increase sensitivity to the density of the medium K.J. Eskola, H. Honkanken, C.A. Salgado, U.A. Wiedemann, Nucl. Phys. A747 (2005) 511 A. Dainese, C. Loizides, G. Paic, Eur. Phys. J. C38(2005) 461

4. Dunlop, WW 2006 4 Black and White Medium extremely black to hadrons, limiting sensitivity to density Medium transparent to photons (white): no sensitivity Is there something grey? S.S. Adler et al, Phys. Rev. Lett. 94, 232301 (2005)

5. Dunlop, WW 2006 5 Baryon enhancement Large enhancement in baryon/meson ratios in central Au+Au collisions –Maximum at p T ~3 GeV/c, after which approach towards p+p Indication of dominant non- fragmentation contribution At what p T is this contribution no longer dominant? STAR QM05, Barannikova  /K 0 s Au+Au 0-10% p+p Au+Au 0-10% p+p Au+Au 0-5% p+p

6. Dunlop, WW 2006 6 Identified Particle R CP All particles consistent for p T >~5 GeV: dominance of fragmentation? In principle, different contributions from gluons and quarks: where is the 9/4? My speculation: quarks also deep into saturation of R AA, no sensitivity

7. Dunlop, WW 2006 7 STAR Preliminary s-quark 0-5% Au+Au p+p R AA of Strange Particles A note: R AA ≠ R CP While R CP common across baryons, clear separation in R AA, increasing with increasing strangeness content Related to canonical strangeness suppression in p+p? Other mechanisms? In any case, not known if this disappears at high p T or not STAR QM05, S. Salur, nucl-ex/0509036

8. Dunlop, WW 2006 8 Charm and Beauty In principle, single electrons are sensitive to charm and beauty –But, relative fraction of b/c uncertain, and calculations do not reproduce p+p data Hope is that heavier mass implies less energy loss (of all kinds) Figure from N. Armesto et al, nucl-ex/0511257 Calculation from M. Cacciari et al, Phys. Rev. Lett. 95, 122001 (2005) Wicks et al, nucl-ex/0512076

9. Dunlop, WW 2006 9 Suppression of single electrons Despite expectations, electrons suppressed at the same level as hadrons

10. Dunlop, WW 2006 10 Suppression of single electrons Current set of theoretical calculations moving towards data –Extreme densities (“violating entropy”) or addition of ignored elastic component –Uncertainty in b/c contribution limits strongly conclusions that can be made Side note: elastic contribution may have different “fragility” Whatever solution, clear that the c quark must behave as u,d quarks: last hope for more weakly interacting probe is the b quark N. Armesto et al, nucl-ex/0511257 b c Wicks et al, nucl-ex/0512076 Data from QM05 STAR, J. Bielcik nucl-ex/0511005 PHENIX S.S. Adler et al, nucl-ex/0510047

11. Dunlop, WW 2006 11 Beyond single particle spectra Overlap zone has ellipticity –path length dependence of suppression creates v 2 –path length dependence can be probed with dihadron correlations Dihadron correlations introduce different geometric biases –Surface bias in trigger hadrons: longer pathlengths –No surface bias in trigger photons: full pathlength distribution?

12. Dunlop, WW 2006 12 A note on v 2 : “non-flow” p+p  jet+jet (STAR@RHIC) nucleon parton jet If one naively measures v 2 in p+p collisions, how big a signal do you see? (Hint: it’s not 0) STAR, Phys. Rev. Lett. 93(2004) 252301

13. Dunlop, WW 2006 13 v 2 at “High” p T At intermediate p T, v 2 far too strong for any quenching model At higher p T and for lighter systems, strong dependence on methods: large systematic errors (though see D. Winter for PHENIX’s view) QM05 STAR, G. Wang, nucl-ex/0510034 STAR, Phys. Rev. Lett. 93 (2004) 252301

14. Dunlop, WW 2006 14 Dihadrons vs. Reaction Plane Indications of path length dependence in dihadron correlations To do: extend to higher p T (and better understand non-flow) STAR, Phys. Rev. Lett. 93 (2004) 252301

15. Dunlop, WW 2006 15 Dijets from dihadrons At high trigger p T, high associated p T : clear jet-like peaks seen on near and away side in central Au+Au STAR QM05, D. Magestro, nucl-ex/0510002 8 < p T (trig) < 15 GeV/c STAR Preliminary p T (assoc)>6 GeV

16. Dunlop, WW 2006 16 Yields of away-side peaks Away-side yield strongly suppressed to level of R AA No dependence on z T in measured range How does this relate to surface/tangential biases? STAR QM05, D. Magestro, nucl-ex/0510002

17. Dunlop, WW 2006 17 STAR Preliminary Changing the probe: towards  -jet in Au+Au Direct  does not couple to medium or fragment into jets – remove from trigger both surface bias, fragmentation uncertainty in Q 2 Correlations triggered on  clear near and away-side peaks Strong contamination remains from  0 decay daughters –Work in progress to separate out direct  STAR QM05, T. Dietel, nucl-ex/0510046 From 30 ub -1 Full year Au+Au run with final calorimeters, RHIC+ luminosity: ~0.5-1 nb -1 RHIC II: 30 nb -1 /year

18. Dunlop, WW 2006 18 Future programs RHIC: go rarer and rarer, in search for a weak probe –Beauty: last hope for a “grey” probe; needs detector upgrades to both STAR and PHENIX to isolate from charm –  -jet: needs higher luminosity LHC: new energy frontier –Cross-sections much higher but  -jet backgrounds extend to higher p T ; running time and luminosity lower –Initial-state effects may be stronger

19. Dunlop, WW 2006 19 Conclusions The medium is dense Progress occuring towards more quantitative statement –Strong quenching implies sensitivity dominated by geometry –Predictions of weaker quenching for single electrons from charm and bottom not borne out by the data: active theoretical investigation Moving towards full tomography in the future –Significant samples of  -jet events available with future luminosities at RHIC and energies at LHC –Separation of charm, and most importantly for tomography, beauty with upgraded RHIC detectors, and at the LHC