Non-photonic leptons and charm production at RHIC an experimental overview

1. Non-photonic leptons and charm production at RHICan experimental overview Alexandre Suaide University of São Paulo – Brazil

2. RHIC white papers: many new things… ... and still many questions. • RHIC has produced matter that behaves differently from anything we have seen previously... • Can we fully describe it? • Can we see the phase transition/critical point? • Lower energies, different system sizes? • ... is dense (many times cold nuclear matter density)... • ... is dissipative... • ... exhibits strong collective behavior... • Does dissipation and collective behavior both occur at the partonic stage? How partons interact with matter? • ... and seems to be thermally equilibrated • Is it?

3. How heavy flavors can help in this search? • Heavy quarks are ideal probes for medium created at RHIC • Two ways of doing that • Quarkonium investigation • Deconfinement • Medium thermometer • Open heavy flavor • Production mechanisms • thermalization • Interaction with the medium • tomography B. Mueller, nucl-th/0404015 D mesons , Y’, c

4. (or m) Open heavy flavors • Useful tool to probe the medium • Yield, spectra, correlations, jets… • How do we do it? • Hadronic reconstruction • Clean probe, but difficult in high multiplicity environments • Semi-leptonic decays • Easier, but depends on ‘magic’ to disentangle flavors

5. How do we measure it? • Designed for leptonic measurements • Low radiation length • Open heavy flavors • Electron measurements and muons • Quarkonia states • Large acceptance and efficiency • Good particle identification • dE/dx, EMC and ToF • Open heavy flavors • hadronic reconstruction, muons and electrons • Quarkonia states depend on special triggers

6. p m How do we measure it? • Phenix • Electrons • Electromagnetic calorimeter and RICH at mid rapidity • Muons • Muon arms at forward rapidities • STAR • Hadronic reconstruction of D-mesons • Muon identification with TPC/ToF • Electrons • ToF + TPC for low momentum (pT<4 GeV/c) • EMC + TPC for high momentum (pT>1.5 GeV/c)

7. PHENIX Not all electrons come from heavy flavor • Most of electrons are originated from sources other than heavy flavors • g e+ + e- (small for Phenix) • p0  g + e+ + e- • h, w, f, etc. • Phenix is almost material free, so their background is highly reduced when compared to STAR • Phenix applies two different methods with very good consistency between them • Converter method • Cocktail method • STAR has the advantage of being capable of measuring the background despite the amount of material So, keep in mind that electrons go through a lot of plastic surgery Mass (GeV/c2)

8. Knowing that experiments are capable of measuring heavy flavors at RHIC, lets go through some findings. 3 main topics to discuss 1. Total charm cross section 2. Interactions of heavy flavors with the medium 3. Separation of charm from bottom at RHIC

9. Production mechanisms • Charm quarks are believed to be produced at early stage by initial gluon fusions. • (M. Gyulassy & Z. Lin, PRC 51 (1995) 2177) • Sensitive to initial gluon distribution • Nuclear and medium effects in the initial state

10. Baseline – production in p+p collisions • Heavy Quark production is a “hard” process pQCD Calculation on NLO • depends on: • Quark mass mc, mb • Factorization scale mF (typically mF = mT or 2mT) • Renormalization scale mR (typically mR = mF) • Parton density functions (PDF) • Fragmentation functions (FF) – plays important role • Fixed-Order plus Next-to-Leading-Log (FONLL) • designed to cure large logs for pT >> mq where mass is not relevant M. Cacciariet al., PRL 95:122001,2005

11. p m Charm cross section from STAR • Use all possible signals • D mesons • Electrons • Muons • Charm cross section is well constrained • 95% of the total cross section • Direct measurement • D-mesons and muons constrain the low-pT region Y. Zhang (STAR), Hard Probes 2006

12. hep-ex/0609010 Charm cross section from PHENIX • Many different datasets • Non-photonic electron spectra • Improving statistics over time • Reducing pT cut • Reduces extrapolation uncertainties

13. Charm production at RHIC: total cross section • FONLL as baseline • Large uncertainties due to quark masses, factorization and renormalization scale • Phenix about a factor of 2 higher but consistent within errors • Only electrons but less background • STAR data about a factor of 5 higher • More material but it is the only direct measurement of D-mesons • 95% of the total cross section is measured

14. Charm production at RHIC: total cross section • Data from both experiments independently indicate total cross section follow Nbin scaling • Charm is produced by initial collisions • No room for thermal production in the sQGP

15. Charm production at RHIC: spectra shape • Does FONLL describe the spectral shape despite of any normalization discrepancy? • Both STAR and PHENIX recently submitted electron spectra up to about 10 GeV/c • How do they compare to FONLL?

16. Charm production at RHIC: spectra shape • FONLL describes the shape well • Experiments do not agree to each other • Low material in Phenix • Less electron background to subtract • Direct measurement of D-mesons at STAR and low-pTm • Is this shown only at high-pT?

17. Charm cross section: the issue • STAR and PHENIX reported charm cross section in different collision configurations • Data are self-consistent within experiments • Both cross section and spectral shapes • Both suggest Nbin scaling in the cross section • But experiments do not agree to each other • PHENIX is a factor of ~2 lower than STAR • D-mesons/muons/electrons measurement vs. Lower electron background • Very important issue to be addressed in the next months • Low material run at STAR and more precise D-mesons measurements are needed Are the discrepancies show stoppers on the understanding of the interaction between heavy quarks and the medium created at RHIC?

18. Pedestal&flow subtracted Energy loss in the medium • Light quarks • High pT suppression / quenching of away-side jet for light quark hadrons • Can we learn something about the medium?

19. K.J. Eskola, H. Honkanken, C.A. Salgado, U.A. Wiedemann, Nucl. Phys. A747 (2005) 511 Increasing density Energy loss in the medium • Strong suppression observed for light quarks creates bias towards surface emission • Medium is extremely opaque for light quarks • What about heavy quarks?

20. Q light (M.DjordjevicPRL 94 (2004)) Open Heavy Flavors – Energy Loss in Medium • In vacuum, gluon radiation suppressed at q < mQ/EQ • “dead cone” effect implies lower energy loss (Dokshitzer-Kharzeev, ‘01) • energy distribution w dI/dw of radiated gluons suppressed by angle-dependent factor • Smaller energy loss would probe inside the medium • Collisional E-loss: qg  qg, qq  qq • dE/dx  ln p - small?

21. Collisional EL for heavy quarks • Collisional and radiative energy losses are comparable! • M.G.Mustafa,Phys.Rev.C72:014905 • A. K. Dutt-Mazumder et al.,Phys.Rev.D71:094016,2005 • Should strongly affect heavy quark RAA M. Djordjevic, nucl-th/0603066

22. STAR High-pT electrons and energy loss PHENIX nucl-ex/0611018 STAR nucl-ex/0607012 (*) (*) updated data

23. Electron RAA from d+Au to central Au+Au PHENIX nucl-ex/0611018 STAR nucl-ex/0607012 • Use of non-photonic electron spectra as proxy for energy loss study • RAA show increasing suppression from peripheral to central Au+Au • First evidence of heavy quark EL • Differences between STAR and PHENIX disappear in RAA • Is it smaller than for light-quark hadrons?

24. Understanding NPE suppression PHENIX nucl-ex/0611018 STAR nucl-ex/0607012 • Radiative EL with reasonable gluon densities do not explain the observed suppression • Djordjevic, Phys. Lett. B632 81 (2006) • Even extreme conditions with high transport coefficient do not account for the observed suppression • Armesto, Phys. Lett. B637 362 (2006) • Other EL mechanisms?

25. Understanding NPE suppression PHENIX nucl-ex/0611018 STAR nucl-ex/0607012 • Collisional EL may be significant for heavy quarks • Wicks, nucl-th/0512076 • van Hess, Phys. Rev. C73 034913 (2006) • Still marginal at high-pT

26. Understanding NPE suppression PHENIX nucl-ex/0611018 STAR nucl-ex/0607012 • Charm alone seems to describe better the suppression at high-pT • Dead cone more significant for bottom quark  larger collisional (relative) EL

27. Understanding NPE suppression PHENIX nucl-ex/0611018 STAR nucl-ex/0607012 • Other effects may contribute to the observed suppression • What if heavy quarks fragment inside the medium and are suppressed by dissociation? • Adil and Vitev, hep-ph/0611109 • Similar suppression for B and D at high-pT

28. V.Greco, C.M. Ko nucl-th/0405040 solid: STAR open: PHENIX PRL91(03) Van Hees & Rapp, PRC 71, 034907: resonant heavy-light quark scattering via scalar, pseudoscalar, vector, and axial vector D-like-mesons Open Heavy Flavors – Elliptic Flow • Observed large elliptic flow of light/s quark mesons at RHIC • Strong evidence for thermalization • What about charm? • Naïve kinematical argument: need mq/T ~ 7 times more collisions to thermalize • v2 of charm closely related to RAA

29. Do heavy quarks flow? PHENIX nucl-ex/0611018 • Study of non-photonic single electrons (from semileptonic D decays) • First hint of strong charm v2 for pT<2 GeV/c • Compatible with v2charm = v2light-q • Seems to decrease at higher-pT (????) • Does the suppression of charm makes bottom evident in this region in Au+Au? Increase statistics

30. Many questions… • The NPE RAA and v2 shows interesting results • Suppression is very large when compared to the expectation from radiative energy loss that seems to work well for light quark hadrons • Other possible mechanisms? • Collisional EL, resonances, in medium fragmentation… • Need to investigate in detail different aspects of the suppression • Centrality dependence, system size, … • But, very important, need to disentangle charm from bottom! PHENIX nucl-ex/0611018 STAR nucl-ex/0607012

31. e-h correlations in p+p: bottom vs. charm See Xiaoyan Lin’s talk for STAR • Understand charm and bottom production is a key point to understand suppression and flow • Direct measurement is very complicated • One possible idea: electron-hadron correlations • Near side peak dominated by decay kinematics • Preliminary e-h correlations from p+p collisions in STAR • Extract relative bottom contribution for different electrons pT

32. e-h correlations in p+p: bottom vs. charm See Xiaoyan Lin’s talk for STAR • FONLL has large uncertainties in the b/(c+b) ratio • Could the data nail it down? • First measurement of open-bottom at RHIC • Non-zero contribution of bottom • Very close to FONLL predictions

33. Some considerations… • Heavy flavor is an important tool to understand HI physics at RHIC • First RHIC results are interesting and challenging • Large differences in cross section between Phenix and STAR • Why so much suppression at high-pT? • Do heavy flavors flow? • Charm and bottom relative production. Where bottom starts dominating? • First attempts from STAR indicates a non-zero contribution of bottom to the NPE spectra • Very first step on the understanding of heavy quark EL

34. We are just in the beginning… • Heavy flavor is challenging • Measurements are complicated and hungry for statistics • The future is promising… • STAR and PHENIX upgrades visioning heavy flavor measurements • RHIC II upgrades will provide more luminosity

35. Extras

36. Open Heavy Flavor – Goals and Requirements

37. How to do it? • RHIC-II: increased luminosity (RHIC-II ≈ 40 × RHIC) • collision diamond s = 20 cm at RHIC and s = 10 cm at RHIC II • gain in usable luminosity is larger than “nominal” increase • PHENIX & STAR: more powerful upgraded detectors crucial to the Heavy Flavor physics program - completed in mid/near term ~5 years. • STAR: • DAQ upgrade increases rate to 1 KHz, triggered data has ~ 0 dead time. • Silicon tracking upgrade for heavy flavor, jet physics, spin physics. • Barrel TOF for hadron PID, heavy flavor decay electron PID. • EMCAL + TOF J/y trigger useful in Au+Au collisions. • Forward Meson Detector • PHENIX: • Silicon tracker for heavy flavor, jet physics, spin physics. • Forward muon trigger for high rate pp + improved pattern recognition. • Nose cone calorimeter for heavy flavor measurements. • Aerogel + new MRP TOF detectors for hadron PID. • Hadron-blind detector for light vector meson e+e- measurements.

38. Charm production at RHIC: spectra shape FONLL describe the shape well, despite normalization

39. pp pp Systematic of charm cross section data • Exp. discrepancy is not a new event • Discrepancy with theory has also a long history • Only recently data and theory touched the bases Theory has to deal with many choices of parameters Experiments need to deal with many corrections on data if measuring NPE Knowledge evolves in both sides with time!

40. Where bottom become significant? • Large uncertainties in FONLL prediction on the relative b/c yield • It is important to reduce the uncertainties by measuring the relative contribution