Heavy flavor and direct photon measurements at RHIC

1. Heavy flavor and direct photon measurements at RHIC David Silvermyr, ORNL DNP/JPS ’05 Kapalua, September 2005

2. Outline • Introduction /Motivation • J/production results for p+p, d+Au, Au+Au and Cu+Cu • Direct photon results for p+p, d+Au and Au+Au • Summary and Outlook

3. q: fast color triplet Induced gluon radiation? g: fast color octet Q: slow color triplet Energy Loss? QQbar: slow color singlet/octet Dissociation? Virtual photon: colorless Controls Real photon: colorless Unknown Medium A Pallet of Prompt Probes A general way to classify QCD probes is by speed and color multiplet; different combinations give rise to different classes of high-Q2 observables: Next talk Previous talk This talk Will start with slow and move on to colorless.. (from P. Stankus)

4. Lattice QCD calculation V(r)/ r Heavy Quarkonia Lattice QCD results show that the confining potential between heavy quarks is screened at high temperature. This screening should suppress bound states such as J/y. However, recent lattice results indicate that the J/y spectral functions only show modest modification near the critical temperature, and thus may not be suppressed until higher T. See e.g. C-Y Wong DD6, P. Petreczky KH8.

5. Observation at CERN SPS (NA50/60) J/y normalized to Drell-Yan vs “Centrality” NOTE: D-Y is not the optimal normalization, closed/open charm is better. • Pb+Pb collisions show suppression in excess of "normal" nuclear suppression • Recent news: NA60 observed very similar trend in In+In collisions. Expectation Suppression

6. T. Affolder et al., Phys. Rev. Lett. 85, 2886. F. Abe et al., Phys. Rev. Lett. 79, 572. CDF pp (s = 1.8 TeV) results • Color singlet model underpredicts high-pT yield. • Color octet model overpredicts transverse polarization at high pT.

7. J/y @ RHIC: Physics Plan • pp collisions • Reference, Initial production mechanism • pA (or dA) collisions • Shadowing • Initial state energy loss • Cold medium absorption • AA + Light ion collisions • Modify path length through medium • Most efficient way to dial in Nbinary. • Energy scans • Modify energy density • More difficult (both luminosity & cross-sections fall quickly w/ energy) Many competing effects: - Reference data essential!

8. p-p J/Psi – PHENIX 200GeV R. Vogt: EKS98 shadowing. 3mb absorption X2 X1 J/ in South y < 0 Rapidity J/y rapidity distribution in p+p and d+Au Collisions Total cross section in p+p (nucl-ex/0507032): 2.61+/-0.20(fit)+/-0.26(abs) µb

9. 1.2 1.0 0.8 RdA 0.6 0.4 0.2 0 Rapidity and Ncoll Dependence of RdAu: Gluon Shadowing and Nuclear Absorption Rapidity • Data favor weak shadowing and weak nuclear absorption effect: Calc. with 1-3 mb most successful at describing the data. [Shape reminiscent to what’s seen for dNch/dh (PHOBOS)] • More suppression for more central events(?)

10. Phenix muon arm 1st Upsilons at RHIC ! Recent RUN5 News PHENIX accumulated ~3pb-1 p+p collision during 2005 run. Will give order of magnitude stat. improvement for reference for d+Au and Au+Au. Different Quarkonia states test the degree of color screening and measure the temperature. Significant yields (>hundreds) at RHIC-II ? Beauty measurements will be quiteinteresting.

11. 0-20% 20-40% 40-93% Example Mass-plots: • Background subtracted using event mixing • Cu+Cu signal is similar to Au+Au peripheral, with much larger statistics Heavy Ions: J/ signal in Au+Au J/e+e- PHENIX J/-

12. 0-80% Au+Au STAR Preliminary 12M events STAR Preliminary J/y production : STAR Signal also seen in STAR for J/ψ in Au+Au! [di-electron measurement at mid-rapidity.] => Should have plenty of stat’s for Run-5 Cu+Cu and p+p too! The J/ yield is lower than statistical coalescence model prediction (red-dashed curve, A. Andronic)  Extreme enhancement scenario ruled out

13. J/ymm muon arm 1.2 < |y| < 2.2 J/yee Central arm -0.35 < y < 0.35 AuAu mm 200 GeV/c CuCu mm 200 GeV/c AuAu ee 200 GeV/c CuCu ee 200 GeV/c dAu mm 200 GeV/c CuCu mm 62 GeV/c RAA vs Ncoll About a factor 3 suppression for most central Au+Au points Band around 1.0 refers to the uncertainty of the p+p reference. [and sometimes has a global sys. error added for the dataset in question]

14. RAA vs Npart : Comparison with NA50 data NA50 data is normalized to NA50 p+p point. Suppression level is rather similar between the two experiments, although the collision energy is 10+ times higher at RHIC (200 GeV vs 17 GeV). Note: size of error bars not negligible!

15. RAA vs Npart: Comparison with cold nuclear effects Mid rapidity Forward rapidity Prediction from pQCD calculations, including 3mb nuclear absorption and shadowing. Seems to underestimate the suppression somewhat. Note: sabs somewhat too high wrt d+Au data; Should have 1 mb curve also.

16. RAA vs Npart: Comparison with predictions without regeneration Models which approx. reproduce NA50 data, with J/ suppression only. (no regeneration mechanism) Over-estimates J/y suppression at RHIC!

17. RAA vs Npart : Comparison with predictions w. regeneration Models using suppression + various regeneration mechanisms; Better matching with data points, but note that all model calculations should be checked to use up-to-date charm and J/y p+p cross-sections! (reduced exp. errors on those quantities would also help)

18. Update : Comparison with a prediction w. regeneration After timely update from Rapp: agreement with data points rather similar to that of absorption calculation (with 3 mb sigma).

19. Test of Npart scaling Alternative looks at data may help to break gridlock.. Can the results be explained by some other scenario? Geometry and surface effects or scaling a la soft processes? [also argued for NA50 data by e.g. Gazdzicki, Braun-Munzinger et al.]

20. More variables : Rapidity • Rapidity distribution of recombined J/y is supposed to be peaked at y=0 (e.g. R.L. Thews & al., nucl-th/0505055) • True IF charm distribution ~ J/y in p+p ! • But Au+Au charm rapidity distributions might be very flat! (previous talk) mainly off-diagonal (with recomb.) p+p data pQCD, adjust <kT2> diagonal

21. Invariant yield vs pt Cu+Cu (|y|[1.2,2.2]) Au+Au (|y|[1.2,2.2]) We fit the pt spectrum using to extract <pt2>

22. Mean transverse momentum vs Ncoll • all PHENIX J/ data • Added Thews predictions for Au+Au and Cu+Cu collisions @200GeV. • Solid (dashed) is with (without) regeneration. Errors are from the fits only. Should probably try alternative parameterizations too.

23. J/y Conclusions Data exhibits a factor 3 suppression for most central events in Au+Au collisions. Suppression vs Npart rather similar to what was seen at SPS. Comparison with models suggests that 1) Models with only cold nuclear matter effects tend to under-predict the suppression 2) Models with color screening or comovers and without recombination have too much suppression 3) Models with recombination are in rather reasonable agreement with the data Not clear if recombination is the explanation though. Mixed evidence for recombination from other variables: Con(?): The rapidity dependence of the J/ yield shows no dramatic change in shape with increasing Npart. Pro(?): <pT2> is also consistent with flat behaviour, but large error bars.

24. J/y Action Items • Need more work on theory; e.g. updated recombination curves, absorption curves based on favourite RdAu parameterization.. New ideas? • Need more work on data; reduce size of errors and go to final results. Using the statistically superior Run5 p+p dataset for reference should be helpful. • Flow? - J/y v2 studies started; no results yet. Statistically very challenging analysis. • Question: Do we see (suppression + recombination) or just not so much suppression to start with..? • [‘soft’ scaling and similarity with NA50 suppression pattern - somewhat surprising and hard to overlook. Just coincidences?] The jury is still out.. Some re-thinking may be required

25. Direct Photons in 200 GeV p+p, d+Au, Au+Au The fast and colorless control probe..

26. p+p: Test of QCD Reduce uncertainty on pQCD photons in A+A d+Au Study nuclear effects Why Direct Photons? • A+A • Photons don’t strongly interact with produced medium • Hard photons • Allow test of Ncoll scaling for hard processes • Important for interpretation of high-pT hadron suppression at RHIC • Thermal photons • Carry information about early stage of collision • QGP potentially detectable via thermal photon radiation Pragmatic Definition : photons not from hadron decays Difficult : large backgrounds from p0 and h decays to subtract off. • STAR has inclusive (not direct) • photon measurements so far • [PRL95 (2005) 062301, PRC70(2004)044902.] • All direct photon results here are from PHENIX. • Also see Lin/KG13: STAR direct photon HBT (a la WA98 analysis)

27. thermal: Decay photons hard: Schematic Photon Spectrum in Au+Au

28. Hard Photons p+p, d+Au, Au+Au

29. good agreement with NLO pQCD Important baseline for Au+Au PbSc Direct Photons in p+p

30. p+p and d+Au spectra compared to NLO pQCD Direct g in d+Au • ratio to NLO pQCD • consistent with 1 • No indication for nuclear effects 2

31. Direct Photons in Au+Au p0 suppression caused by medium created in Au+Au collisions Recentlypublished PRL 94, 232301 Expectation for Ncoll scaling of direct photons holds for all centrality classes

32. Direct Photons in Au+Au : RAA RAA consistent with 1 for direct photons

33. Thermal Photons Au+Au

34. Low pT • No significant excess at low pT • Stay tuned for more improvements

35. e+ Compton e- g* q g q p0 e+ g* e- Background from Dalitz decay g • PHENIX features • Low conversion rate • Excellent mass resolution • High statistics in Run4 (2004) A New Approach.. Use lepton pairs to measure virtual g Any source of real g emits virtual g with very low mass

36. g g p0 p0 e+ g* g e- The Idea • Start from Dalitz decay • Calculate invariant mass distribution of Dalitz pairs invariant mass of Dalitz pair invariant mass of Dalitz pair invariant mass of virtual photon invariant mass of virtual photon form factor form factor phase space factor phase space factor [Kroll-Wada, ’55] • Now direct photons • Any source of real g produces virtual gwith very low mass • Rate and mass distribution given by same formula • No phase space factor for mee<< pT photon

37. Method • Material conversion pairs removed by analysis cut • Combinatorics removed by mixed events 0-30 90-140 200-300 140-200 MeV Rdata ÷ ÷ ÷ • Calculate ratios of various (for cross-checks) Minv bins to lowest one: Rdata • If no direct photons: ratios correspond to Dalitz decays • If excess::direct photons

38. calculated from Dalitz formula Rdirect g g h h p0 Rh Rp0 ÷ Rdata S/B=~1 p0 measured

39. calculated from Dalitz formula ~25 % systematic error : ~20 % from measured h/p0 ratio ~10 % from g inclusive ~5 % acceptance Rdirect g g h h p0 Rh What we are after.. Rp0 measured with EMCal ÷ Rdata S/B=~1 p0 measured

40. g*direct/g*inclusive 0-20 % Significant 10% excess of very-low-mass virtual direct photons

41. more peripheral Centrality Dependence Indication for centrality dependence

42. ( + 1 ) Comparison to Conventional result New result consistent with conventional method, but with significantly smaller errors!

43. gdirect

44. The Spectrum Compare to published Run2 result: PRL94 232301 New result consistent with conventional method, but with significantly smaller errors!

45. Compare to NLO pQCD • L.E.Gordon and W. Vogelsang • Phys. Rev. D48, 3136 (1993) • excess above pQCD The Spectrum

46. Compare to NLO pQCD • L.E.Gordon and W. Vogelsang • Phys. Rev. D48, 3136 (1993) • excess above pQCD The Spectrum Compare to thermal model • D. d’Enterria, D. Perresounko • nucl-th/0503054 2+1 hydro T0ave=360 MeV(T0max=570 MeV) t0=0.15 fm/c • data above thermal at high pT

47. Compare to NLO pQCD • L.E.Gordon and W. Vogelsang • Phys. Rev. D48, 3136 (1993) • excess above pQCD The Spectrum Compare to thermal model • D. d’Enterria, D. Perresounko • nucl-th/0503054 2+1 hydro T0ave=360 MeV(T0max=570 MeV) t0=0.15 fm/c • data above thermal at high pT Compare to thermal + pQCD • data consistent with thermal + pQCD

48. A word of caution.. Are these thermal photons? The rate is above pQCD calculation and could provide the first direct measurement of the initial temperature of the matter. T0max ~ 500-600 MeV !? T0ave ~ 300-400 MeV !?

49. Direct Photon Conclusions • Hard direct photons pT>4GeV/c • p+p: • Spectrum consistent with pQCD calculations • d+Au: • No apparent nuclear effects • Au+Au: • Confirms Ncoll scaling for hard processes • Thermal (?) direct photons 1<pT<4GeV/c • New EMCal measurement with reduced systematics • Stay tuned for further improvements • New measurement through very-low-mass virtual photons • Significant 10% direct photon excess above decay photons • Spectrum consistent with thermal model • Reference measurement needed • pp and dAu (and CuCu) • Same analysis method

50. Outlook • Many recent interesting results on J/y and direct photons! • Interpretations and cross-checks, and getting the results in final form, are being worked on. Key issues: J/y recombination or not? J/y v2? Thermal photon signal real? Which limits can we place on the temperature?! • We’ve come a long way in the last few years; a lot of new interesting results are sure to follow in the coming years.