1. Hard vs. Soft Physics at RHIC - Insights from PHENIX l Why hard vs. soft? l Soft physics: thermal, flow effects l Hard processes at RHIC l Conclusion Barbara Jacak Stony Brook

2. Why hard vs. soft? l Soft physics : thermal system with T ~ 200 MeV collective flow boosts p t spectra l Hard processes happen early create useful probes of the system J/ , charm, jets  QGP signals calculable via pQCD sensitive to parton distribution evolution i.e. gluon saturation l Theoretical tools very different Gyulassy/Wang: boundary 1-2 GeV/c from parton structure functions from Glauber model Experimental challenge: constrain which to be used where!

4. PHENIX at RHIC 2 Central spectrometers 2 Forward spectrometers 3 Global detectors Philosophy: optimize for signals / sample soft physics

5. PHENIX configuration RUN1: DAQ recorded EACH of ~5M events RUN2: full central arms S muon arm (  running now!

6. Centrality selection In PHENIX “min bias” = 92% of geometric cross section Use Glauber model to calculate N part

7. Charged particle multiplicity: with quenching No jet quenching STAR PHENIX BRAHMS PRL86, 3500 (2001) Find A = 0.88  0.28 B = 0.34  0.12 hard: scales with Ncoll soft: scales with Npart hard processes are becoming significant Fit with

8. E T per charged particle E T grows because of particle number!   4.6 GeV/fm 3 PRL87, 052301 (2001) NB. Does not include mass of baryons Does include M B >50% higher than at CERN

9. TOF to identify the hadrons Measure momentum & flight time calculate particle mass

10. m T spectra l See mass dependence  collective radial expansion Integrate spectra, extrapolating to p T =0

11. antiproton dN/dy = 20! hadron dN/dy at y=0 per participant pair: K,p per participant rise with Npart more than  !

12. K/  at y=0 PHENIX preliminary  s=17 GeV Pb+Pb Phys.Lett.B 471, 6 (1999) Both K+/  and K-/  increase with Npart Peripheral collisions near pp value K+/  and K-/  do not diverge as at SPS,AGS K/  at  s=200 p+p Z.Phys. C41,179 (1988) (UA5)

13. Net baryons per participant PHENIX preliminary Mid-rapidity  Net baryon density increases somewhat with Npart  more stopping in central collisions  Pb+Pb at SPS: net baryon density per participant = 0.18 (NB: in PHENIX ~50% of  decays included)

14. Fit spectra with m t exponential T eff = T fo + m 2 T fo = 140 - 150 MeV  radial = 0.5 - 0.6 (higher for central collisions) was 0.4 at SPS less flow in peripheral collisions! range m T -m 0 < 1 GeV/c

15. HBT in PHENIX Identified pions in 15% central collisions : high resolution TOF in East arm EMCAL TOF in West arm See R parameters are rather small (5-6 fm)! very similar to those at SPS agreement with STAR Qualitatively to be expected if radial flow is large...

16. Many high p t baryons! Not been seen before! boosted by the collective outward expansion hydrodynamical calculation by Teaney agrees with data  soft physics to ~ 3 GeV/c p t !

17. increases with centrality Expect such trend from radial flow but also from partonic multiple scattering and gluon saturation final or initial state effect???

18. Turn now to hard probes beams of hard probes: jets, J/  …. vacuum QGP 1.dE/dx in QGP  jet quenching 2.Deconfinement  J/  suppression hadrons q q leading particle leading particle schematic view of jet production Jets: primarily from gluons at RHIC produced early Observed via fast leading particles or azimuthal correlations between them

19. PHENIX measures  0 in PbSc and PbGl calorimeters  0 ’s p T >2 GeV, asym<0.8 in PbSc excellent agreement!

20. Compare h  and  0 Peripheral collisions (60-80% of  geom ): ~ p-p scaled by = 20  6 central (0-10%): shape different (more exponential) below scaled p-p! ( = 905  96)

21. Compare central Au-Au to p-p

22. Comparing CERN-SPS Pb-Pb to p-p l R AA exhibits “Cronin effect” behavior X.N.Wang soft/hard transition? parton energy loss, if any, overwhelmed by initial state soft multiple scattering!

23. Is SPS-RHIC comparison fair? l Same p t implies different x! RHIC if p T(had) / p T(jet) ~ 1 then x T ~ x(parton) at y=0 x T =

24. Shadowing at RHIC? Zheng Huang, Hung Jung Lu, Ina Sarcevic: Nucl.Phys.A637:79-106,1998 (hep-ph/9705250 ) Shadowing of structure functions small in RHIC x range!! Gluon shadowing even less (according to theory) p t comparison is fair! deficit  shadowing! quark structure function

25. different systematics: instead of pp extrapolation depletion remains!  /h smaller in central vs. peripheral!

26. Identified electrons in PHENIX All tracks: 0.8>p>0.9 GeV/c Electron enriched sample (using RICH)  p Associate tracks with RICH and EMCAL

27. Inclusive electron p t spectrum

28. Coming this year l Statistics! Reach p T ~ 15 GeV/c l Fully instrumented central arms l South muon arm ready to go large acceptance for J/  l Selective L1, L2 triggers l Electron pairs

29. Conclusions l Soft physics at RHIC   4.6 GeV/fm 3, driven by more N ch K, p / part. increase in central collisions dN pbar /dy ~ 20 but net baryon density is low large radial flow baryons cross p at p T  2 GeV/c soft physics extends to nearly 3 GeV/c! l Hard processes contribute to particle production PHENIX observes a deficit at high p T vs hard scattering expectations likely NOT shadowing opposite direction from Cronin effect pA data will settle underlying physics l This run: spectra to p T ~ 15 GeV/c back-to-back correlations J/ , electron pairs, direct photons...

30. hadron dN/dy at y=0 statistical errors in table;  syst = 13%, 15%, 14%  K  p, pbar Integrate spectra, extrapolate to p T =0 PHENIX preliminary