1. Ralf Averbeck Department of Physics & Astronomy Seminar at January 24, 2007 The Charm of RHIC Electrons - Light Messengers from Heavy Quarks

2. R. Averbeck, 2, 1/24/2007 Outline l introduction l strongly interacting matter l relativistic heavy-ion collisions l probing the hot and dense medium l electrons from heavy flavor at RHIC l reference: p+p collisions l cold nuclear matter: d+Au collisions l hot matter: Au+Au collisions l going beyond single electrons: correlations l summary & outlook

3. R. Averbeck, 3, 1/24/2007 Nuclear matter as QCD laboratory l “ordinary” nuclear matter is made from nucleons l 3 (light) constituent quarks, carrying color charge l quarks interact via the exchange of gluons l gluons carry color charge (“charged photons”)! l key observations l isolated quarks are NEVER observed (“confinement“) l quark masses account for ~1% of the nucleon mass l properties of QCD (Quantum Chromo Dynamics), the theory of strong interaction l learn more → “extraordinary” nuclear matter

4. R. Averbeck, 4, 1/24/2007 The QCD phase diagram l study fundamental properties of matter by l excitation to extreme temperature and/or density l phase transition from nuclear to “quark-gluon“ matter l unique approach: relativistic nuclear collisions l center-of-mass energy: where do you want to go today? l highest temperature at lowest baryon density  colliders: RHIC @ BNL and LHC @ CERN l moderate temperature at highest baryon density  fixed-target: FAIR @ GSI

5. R. Averbeck, 5, 1/24/2007 RHIC and its experiments l highest CMS energy currently available at l RHIC (Relativistic Heavy-Ion Collider) located at Brookhaven National Laboratory l p+p: √s ≤ 500 GeV (polarized beams!) l A+A: √s NN ≤ 200 GeV (per nucleon-nucleon pair) STAR l experiments with specific focus l BRAHMS (until Run-6) l PHOBOS (until Run-5) l multi purpose experiments l PHENIX l STAR

6. R. Averbeck, 6, 1/24/2007 The experimental challenge STAR l ONE central Au+Au collision at max. energy l MANY secondary particles l how to look into the heart of matter? PHENIX

7. R. Averbeck, 7, 1/24/2007 g g medium A view behind the curtain l “tomography” with scattering experiments Rutherford:  → atom  discovery of the nucleus l SLAC: electron → proton  discovery of quarks l calibration of hard probes l theoretically –perturbative QCD (pQCD) l experimentally –measurement in p+p –in-situ control: direct photons l “tomography“ at RHIC l probe has to be “auto generated” in the collision l hard parton (quark, gluon) scattering, leading to –direct photons from quark- gluon Compton scattering –high p T jets –heavy quark-antiquark pairs l once calibrated for p+p collisions modifications observed in p(d)+A & A+A tell about the “medium”

8. R. Averbeck, 8, 1/24/2007 Direct photons at √s NN = 200 GeV l photons from quark-gluon Compton scattering p-p Au-Au l direct photons are a calibrated probe N binary : number of “binary” collisions, determined from the collision geometry (Glauber) l no strong final state interaction Medium produced in Au+Au collisions is transparent for direct photons!

9. R. Averbeck, 9, 1/24/2007 (Light) hadrons at √s NN = 200 GeV l pQCD in reasonable agreement with p+p data l medium modifications in cold (hot) matter: d+Au (Au+Au)? l nuclear modification factor: l limiting factor, preventing R AA to drop even further: surface (“Corona”) emission Medium produced in Au+Au collisions is opaque for light quark and gluon jets! Quantitative assessment of medium parameters requires in-between (“grey”) probe

10. R. Averbeck, 10, 1/24/2007 l heavy quarks (cc, bb): m u,d ~ MeV, m c ~1.25 GeV, m b ~4.5 GeV hard process (m q >>  QCD ) –production at leading order (LO) –mainly gluon fusion l naïve expectation l large mass → small energy loss l confirmed in (most) models l (quantitative) details depend on energy loss mechanism –example: energy loss via gluon radiation –larger parton mass implies less energy loss in forward direction (“dead cone” effect) ( Dokshitzer, Kharzeev: PLB 519(2001)199 ) –partially compensated by medium induced gluon radiation ( Armesto, Salgado, Wiedemann: PRD 69(2003)114003 ) l systematic experimental study l heavy quarks from p+p, p(d)+A, and A+A collisions in addition: bound states, quarkonia (J ,  Heavy quarks to the rescue? D mesons,  ’, 

11. R. Averbeck, 11, 1/24/2007 l total cross section measurements at lower √s l recent review: C. Lourenco & H. Woehri: Phys. Rep. 433 (2006) 127 -charm and bottom cross sections measured at the same √s can differ by more than a factor 10! Heavy quark data pre-RHIC l CDF: PRL 91, 241804 (2003) –reconstruction of charmed mesons for p T > 5 GeV/c only! l differential cross section at higher √s (1.8 TeV) (PRL 91, 241804 (2003))

12. R. Averbeck, 12, 1/24/2007 l ideal (but very challenging in HI environment) direct reconstruction of charm decays (e.g. ) l STAR (for p T < 3 GeV/c) l helpful to constrain charm cross section Charm measurements at RHIC D0  K+ -D0  K+ - l alternative (but indirect, and still challenging) l contribution of semileptonic decays to lepton spectra l PHENIX & STAR l only systematic study: electron spectra at y~0 K+K+ -- PRL 94, 062301 (2005)

13. R. Averbeck, 13, 1/24/2007 e ± from heavy flavor: problem I l electrons are RARE! charged pions: (  + +  - )/2 neutral pions:  0 electrons: (e + + e - )/2 (e + + e - )/2 from heavy flavor l how to measure a clean spectrum of inclusive e ± ?

14. R. Averbeck, 14, 1/24/2007 3 detectors for event characterization: vertex, centrality, reaction plane PHENIX & STAR at RHIC l muons 1.2 < |  | < 2.4 l p > 2 GeV/c l tracking l muon ID: l “absorber” l electrons |  | < 0.35 l p T > 0.2 GeV/c l tracking l electron ID: l RICH + EMC 2 central electron/photon/hadron spectrometer arms:  0.35 p  0.2 GeV/c l charged particles |  | < 1 l p T > 0.15 GeV/c l charged particle ID: l TPC (dE/dx) l Time-of-Flight detector l additional electron ID: EMC 2 forward muon spectrometers: 1.2 < |  | < 2.4 p  2 GeV/c PHENIX optimized for leptons, but can do hadrons STAR optimized for hadrons, but can do leptons

15. R. Averbeck, 15, 1/24/2007 e ± from heavy flavor: problem II l there are MANY electrons sources l Dalitz decay of light neutral mesons –most important    →  e + e - –but also:  ’  l conversion of photons in material –main photon source:    →  –in material:  → e + e - l weak kaon decays –K e3, e.g.: K ± →   e ± e l dielectron decays of vector mesons –  → e + e - l direct radiation –conversion of direct photons in material –virtual photons:  * → e + e - l thermal radiation l heavy flavor decays l how to extract e ± from heavy flavor decays from the inclusive spectrum? PHOTONIC NON PHOTONIC

16. R. Averbeck, 16, 1/24/2007 Extracting e ± from heavy flavor l PHENIX l cocktail subtraction method –ALL relevant background sources are measured –background subtraction  e ± from semileptonic heavy quark decays l converter subtraction method –converter of known thickness added for part of the run –converter multiplies photonic background by KNOWN factor PRL 96(2006)032001 p+p @ √s = 200 GeV l STAR l large acceptance –direct measurement of ~60% of photonic background –rest: extrapolation + cocktail

17. R. Averbeck, 17, 1/24/2007 l test case: p+p at √s = 200 GeV (PRL 97, 252002 (2006)) l how well is the e ± background determined? l comparison of two methods –converter measurement –cocktail calculation l excellent agreement How well does this work for PHENIX? l how large is the ratio of signal to background? l S/B > 1 for p T > 2.5 GeV/c l only Dalitz decays and photon conversions are important l PHENIX: conversion ~ 0.5 x Dalitz l STAR: conversion ~ 5 x Dalitz

18. R. Averbeck, 18, 1/24/2007 l e ± from heavy flavor decays l comparison with FONLL calculation: Fixed Order Next-to-Leading Log pQCD (M. Cacciari, P. Nason, R. Vogt PRL95,122001 (2005)) l theory has uncertainties / parameters too l data are at “upper edge” of theory band p+p @ √s = 200 GeV: the reference l total cross section l  cc = 567  57(stat)±224(sys)  b PRL 97, 252002 (2006) l does this look familiar?

19. R. Averbeck, 19, 1/24/2007 STAR’s e ± from p+p collisions l ratio of e ± from heavy flavor decays to FONLL pQCD expectation l STAR (scaled down by 25% compared to original preprint) l earlier STAR publication l PHENIX: PRL 97, 252002 (2006) l PHENIX & STAR e ± spectra exhibit the SAME shape as predicted by FONLL!! l scale difference: factor ~2 l PHENIX: superior electron measurement l STAR: D meson measurement should help constrain  cc

20. R. Averbeck, 20, 1/24/2007 STAR data STAR: D mesons versus e ± l D meson and electron measurements at “low” p T l consistent within (large) uncertainties l e ± from heavy flavor decays l who is right/wrong? l pro PHENIX: e ± data l pro STAR: D data l how to resolve this issue? l PHENIX: D measurement –difficulty: K identification l STAR: reduce material –difficulty: Silicon Vertex Tracker l is this a “show stopper”?

21. R. Averbeck, 21, 1/24/2007 PHENIX PRELIMINARY 1/T AB EdN/dp 3 [mb GeV -2 ] Cold nuclear matter: PHENIX l e ± spectrum from heavy flavor decays in d+Au at 200 GeV l d+Au data scaled down assuming binary collision scaling l scaled d+Au data are consistent with fit to p+p reference l agreement holds for various d+Au centrality classes l no indication for large cold effects on heavy flavor production at y = 0.

22. R. Averbeck, 22, 1/24/2007 STAR e ± data Cold nuclear matter: STAR l nuclear modification factor R dA for e ± from heavy quark decay l R dA is consistent with binary scaling l indication for “Cronin” enhancement (initial state scattering, p T broadening) l consistent with PHENIX l PHENIX & STAR l conclude the SAME regarding cold nuclear matter effects on e ± from heavy flavor decays! l comparison of PHENIX/STAR d+Au and p+p data l normalization discrepancy cancels in ratio (d+Au)/(p+p)!

23. R. Averbeck, 23, 1/24/2007 PHENIX: PRL 94, 082301 (2005) Hot matter: e ± yield in Au+Au l spectra of e ± from heavy flavor decays for different centralities l total yield in Au+Au follows binary collision scaling (as expected for hard probe)! total yield for p T > 0.8 GeV/c charm cross section per NN collision: 622 ± 57 ± 160  b l STAR: 1.4 ± 0.2 ± 0.4 mb (d+Au) è central Au+Au collision: ~20 cc pairs! extrapolation to full phase space

24. R. Averbeck, 24, 1/24/2007 Binary scaling of “charm” yield at RHIC l PHENIX and STAR measure heavy quark production in various systems determine  cc per binary collision l experiments are self consistent but not consistent with each other l spectral shapes measured by PHENIX & STAR agree in p+p and d+Au → what about Au+Au?

25. R. Averbeck, 25, 1/24/2007 PRL 96, 032301 (2006) Discovery of heavy quark energy loss l cocktail analysis of PHENIX Run-2 Au+Au data set l strong modification of heavy quark e ± spectra at high p T (similar to   ) l uncertainties too large for stronger conclusions!

26. R. Averbeck, 26, 1/24/2007 Dramatic progress: Run-2 → Run-4 l Run-4 Au+Au data sample: ~10 9 MB events (~40 x Run-2) l PHENIX: nucl-ex/0611018 l electron measurement extended beyond RICH Cerenkov threshold for pions (p T > 5 GeV/c) l stringent Cerenkov ring selection l “shower shape” cuts in the electromagnetic calorimeter

27. R. Averbeck, 27, 1/24/2007 l Run-4 Au+Au data sample: ~10 9 MB events (~40 x Run-2) l stronger high p T suppression in central collisions l strikingly similar to suppression of light hadrons except for l intermediate p T l highest p T ? l careful: decay kinematics! l bottom??? l indication for light vs. heavy quark mass hierarchy in energy loss at intermediate p T Dramatic progress: Run-2 → Run-4 nucl-ex/0611018

28. R. Averbeck, 28, 1/24/2007 Heavy flavor e ± R AA : PHENIX vs. STAR l is the disagreement between PHENIX & STAR a normalization issue “only”? l use R AA of e ± from heavy flavor decays as test case l for d+Au collisions PHENIX & STAR agree in R dA l the same is true for the Au+Au system in –peripheral –mid-central –central collisions l differences between PHENIX & STAR “disappear” in R AA !

29. R. Averbeck, 29, 1/24/2007 l calculations invoking heavy quark energy loss by gluon radiation Heavy flavor e ± R AA : data vs. theory l describing the measured suppression is difficult –radiative energy loss of charm and bottom quarks is not enough with typical gluon densities of the produced medium in Au+Au collisions (Djordjevic et al., PLB 632(2006)81) –models involving extreme conditions, implemented via a large transport coefficient q (Armesto et al., PLB 637(2006)362) –agree better with e ± data –very “opaque” medium –problems with entropy conservation l there must be something else

30. R. Averbeck, 30, 1/24/2007 l the return of collisional energy loss Heavy flavor e ± R AA : data vs. theory l collisional energy loss can be important for heavy quarks –the original idea is old (1982): –J.D. Bjorken (Fermilab-Pub-82/59-THY) l implement collisional energy loss into models –agreement with data gets better, but isn’t perfect yet –collisional + radiative energy loss: Wicks et al., nucl-th/0512076 –additional resonant elastic scattering: van Hees & Rapp, PRC73 (2006) 034913

31. R. Averbeck, 31, 1/24/2007 l and now for something completely different: collisional dissociation Heavy flavor e ± R AA : data vs. theory l let’s take heavy quark dynamics serious l what if heavy quarks –fragment inside the medium –form D/B mesons, which then dissociate –Adil & Vitev, hep-ph/0611109 l strong suppression for charm AND bottom at high p T l open questions –how do heavy quarks interact in detail with the medium produced in Au+Au collisions at RHIC –where does bottom decay become important? l need more information

32. R. Averbeck, 32, 1/24/2007 e ± (  ± ) from semileptonic heavy quark decays are correlated with products from the original cc pair l hadrons originating from the same parent D/B meson decay – eh correlations (“near side”) → bottom/charm l hadrons originating from the decay of the associated D/B meson – eh correlations (“away side”): too insensitive l leptons from the decay of the associated D/B meson – ee correlations: → energy loss / thermalization – e  correlations: → “intermediate” rapidity –  correlations: → “forward” rapidity Electrons are not “born alone” K+K+ --

33. R. Averbeck, 33, 1/24/2007 eh correlations in p+p: b vs. c l azimuthal angle correlation of e ± from heavy flavor decay with hadrons l “near side” correlation is dominated by decays kinematics –bottom is “wider” than charm due to the larger parent meson mass l assumptions –decays are described properly in PYTHIA –background of (jet) correlations of photonic electrons with hadrons is subtracted properly l ratio of bottom/charm can be determined from line shape analysis –preliminary STAR result agrees with FONLL within large (model dependent) uncertainties l alternative (more direct) approach l invariant mass of eh pairs –pairs with m eh >m D ARE from B decays

34. R. Averbeck, 34, 1/24/2007 l invariant mass analysis of e + e - pairs (~870x10 6 MB events) l problem: HUGE combinatorial background –subtracted via event mixing (sys. error in BG normalization ~0.25 %) Dielectrons in Au+Au (I) PHENIX Preliminary Systematic and Normalization Error l finally l a spectrum with familiar features (J/  ) l what else? l where are correlated charm decays? l how does the interaction of charm with the medium manifest itself?

35. R. Averbeck, 35, 1/24/2007 l intermediate mass region of e + e - continuum (from  to J/  ) l expected to be dominated by charm decays l contribution from thermal radiation (not shown) is possible Dielectrons in Au+Au (II) PHENIX Preliminary l charm interaction with medium l energy loss l loss of angular correlation l p+p reference is unavailable l R CP : the poor man’s R AA l charm quarks interact strongly with the medium: thermalization?

36. R. Averbeck, 36, 1/24/2007 l collective motion of the medium produced in Au+Au collisions at RHIC l elliptic flow l spatial anisotropy in initial stage l momentum anisotropy in final stage l elliptic flow strength Does charm thermalize? l R AA /R CP << 1 → strong interaction with the medium l large charm mass implies long thermalization time scale l unless interaction with the medium is very strong pYpY pXpX Y X Z Reaction plane: Z-X plane High pressure Low pressure asymmetric pressure gradients (early, self quenching)

37. R. Averbeck, 37, 1/24/2007 G. Moore and D. Teaney: PRC 71, 064904 (2005) Interaction of charm with the medium l do charm quarks participate in collective motion? l elliptic flow parameter v 2 l momentum aniso- tropy w.r.t. reaction plane orientation l viscous 3-d hydrodynamics calculation l R AA and v 2 go hand in hand! l decreasing diffusion coefficient D of charm quarks in the medium –R AA of charm quarks gets smaller at high p T –v 2 of charm quarks gets larger l this should still be visible in the e ± from semi leptonic decays l where there is energy loss there should be elliptic flow!

38. R. Averbeck, 38, 1/24/2007 χ 2 minimum result D->e 2σ 4σ 1σ Does charm flow? l strong elliptic flow of electrons from D meson decays → v 2 D > 0 l v 2 c of charm quarks? l recombination Ansatz: (Lin & Molnar, PRC 68 (2003) 044901) l universal v 2 (p T ) for all quarks l simultaneous fit to , K, e v 2 (p T ) a = 1 b = 0.96  2 /ndf: 21.85/27 l within recombination model: charm flows as light quarks!

39. R. Averbeck, 39, 1/24/2007 Combining R AA and v 2 l large suppression and v 2 of electrons → charm thermalization l transport models suggest l small heavy quark relaxation time small diffusion coefficient D HQ x (2  T) ~ 4-6 l this value constrains the ratio viscosity/entropy –  /s ~ (1.5 – 3) / 4  –within a factor 2-3 of conjectured lower quantum bound –consistent with –light hadron v 2 analysis (R. Lacey et al., nucl-ex/0609025) –p T fluctuation analysis (S. Gavin & M. Abdel-Aziz, nucl-th/0606061) l while this conclusion is MODEL DEPENDENT it motivates the term “perfect fluid” for the medium produced in Au+Au collisions at RHIC nucl-ex/0611018

40. R. Averbeck, 40, 1/24/2007 Summary: heavy quarks at RHIC l first systematic and comprehensive “heavy quark” measurements in hadronic collisions l heavy quarks are a COMPLEMENTARY hard probe l unique and powerful observables l agreement between PHENIX & STAR is not perfect l many surprising results l challenges for the current theoretical understanding l much more to expect with increasing luminosity and detector upgrades available at RHIC

41. R. Averbeck, 41, 1/24/2007 The future is bright for heavy quark physics l at RHIC l detector upgrades –helpful for electron measurements (in particular for low to intermediate-mass e + e - pairs) –Dalitz and conversion rejection for single e ± and e + e - pairs –hadron blind detector (“HBD”) available in PHENIX by Fall 2006! –helpful for improved reaction plane measurements –PHENIX reaction plane detector –needed for optimum heavy quark measurements –measurement of displaced heavy quark decay vertices –silicon vertex trackers are THE cornerstones in the upgrade programs of both PHENIX and STAR l RHIC-II (40 x design luminosity of RHIC) –luminosity matters: – J/  and  spectroscopy and high statistics c & b data l and elsewhere l ALICE / CMS / ATLAS @ LHC: √s LHC ~ 30 x √s RHIC l CBM @ FAIR: “terra incognita” RHIC is CHARMING and the future looks BEAUTIFUL!