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The Charm and Beauty of an (almost) Perfect Liquid Open Heavy Flavor Production at RHIC

The Charm and Beauty of an (almost) Perfect Liquid Open Heavy Flavor Production at RHIC. Thomas S. Ullrich NSCL Seminar February 7,  2007. Theory of Strong Interactions: QCD. Quarks (u,d,s,c,b,t) and gluons (gauge bosons) Forces between quarks: exchange of gluons “Emergent” Phenomena

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The Charm and Beauty of an (almost) Perfect Liquid Open Heavy Flavor Production at RHIC

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  1. The Charm and Beauty of an (almost) Perfect LiquidOpen Heavy Flavor Production at RHIC Thomas S. Ullrich NSCL Seminar February 7,  2007

  2. Theory of Strong Interactions: QCD • Quarks (u,d,s,c,b,t) and gluons (gauge bosons) • Forces between quarks: exchange of gluons • “Emergent” Phenomena • Asymptotic Freedom • at short distance the coupling is weak • at large distances it is strong • quarks only in bound states • Confinement • Free quarks not observed in nature Hadronic matter  QuantumChromoDynamics as(Q2) ~ 1 / log(Q2/L2)

  3. QCD Phase Diagram of Hadronic Matter Can we explore the phase diagram of nuclear matter ? • We think so ! • by colliding nuclei in the lab • by varying the nuclei size (A) and colliding energy (s) • by studying spectra and correlation of the produced particles Plasma ≡ ionized gas which is macroscopically neutral & exhibits collective effects Usually plasmas are e.m., here color forces

  4. The Phase Transition in the Laboratory Chemical freezeout (Tch  Tc): inelastic scattering ceases Kinetic freeze-out (Tfo Tch): elastic scattering ceases

  5. 2 concentric rings of 1740 superconducting magnets 3.8 km circumference counter-rotating beams of ions from p to Au Long Island Relativistic Heavy Ion Collider - RHIC • RHIC’s unprecedented capabilities: • Large √s  • Access to reliable pQCD probes • Clear separation of valence baryon number and glue • Polarized p+p collisions • Accelerator complex • Routine operation at 2-4 x design luminosity (Au+Au) • Extraordinary variety of operational modes • Species: Au+Au, d+Au, Cu+Cu, p+p • Energies √s : • 22 GeV (Au+Au, Cu+Cu, p) • 56 GeV (Au+Au) • 62 GeV (Au+Au,Cu+Cu, p+p) • 130 GeV (Au+Au), • 200 GeV (Au+Au, Cu+Cu, d+Au, p+p) • 410 GeV & 500 GeV (p) BRAHMS PHOBOS PHENIX STAR

  6. Experiments at RHIC STAR Solenoidal Field Large- Tracking TPC’s, Si-Vertex Tracking RICH, EM Cal, TOF ~500 Collaborators PHENIX Axial Field High Resolution & Rates 2 Central Arms, 2 Forward Arms TEC, RICH, EM Cal, Si, TOF, -ID ~500 Collaborators Silicon Vertex             Tracker Coils Magnet E-M Calorimeter Time Projection           Chamber Time of    Flight Electronics Platforms Forward Time Projection Chamber • Leptons, Photons, and Hadrons in Selected • Solid Angles • Simultaneous Detection of Various Phase • Transition Phenomena • Measurements of Hadronic Observables • using a Large Acceptance • Event-by-Event Analyses of Hadrons • Jets

  7. What Did We Find at RHIC and How? • Will present sample of results from various points of the collision process: • Final State • Yields of produced particles Thermalization, Hadrochemistry 3. Probes of dense matter Tomography: jets traversing the hot and dense matter 2. Early State Hydrodynamic flow from initial spatial asymmetries

  8. “Peripheral” “Central” Reaction Plane Final State • Does the huge abundance of final state particles reflect athermaldistribution? Central: 200 GeV Au+Au: ~4800 charged particles in final state • In these complicated events, we have (a posteriori ) control over the event geometry: • Degree of overlap • Orientation with respect to overlap

  9. A. Adronic et al., NPA772:167 Origin of the (Hadronic) Species • Ansatz: • Assume all distributions described by one temperature T and • one ( baryon) chemical potential m • One ratio (e.g., p / p ) determines m / T : • A second ratio (e.g., K / p ) provides T →m • Then predict all other hadronic ratios and yields  Temperature of hadron gas at chemical freeze-out T ~ 160 MeV, mb ~ 20 MeV NOTE: Truly thermal implies No memory (!)

  10. out-of-plane y number of particles  1 + 2v2cos(2) + 2v4cos(4) + … Au nucleus in-plane x Au nucleus z Non-central Collisions Elliptic Flow – Indicator for Early Thermalization Use a Fourier expansion to describe the angular dependence of the particle density • v2 provides information about the interactions while the system was still oblong Au+Au at b=7 fm • shape washes out as it expands • v2 sensitive to early interactions and pressure gradients

  11. The “Flow” is ~Perfect • Huge asymmetry found at RHIC • massive effect in azimuthal distribution w.r.t reaction plane • The “fine structure” v2(pT) for different mass particles shows good agreement with ideal (zero viscosity) hydrodynamics  “perfect liquid” • Hydro favors soft equation of state liquid is not a hadron gas

  12. baryons mesons The Constituents “Flow” • Scaling flow parameters by quark content nq (baryons=3, mesons=2) resolves meson-baryon separation of final state hadrons  liquid of constituents (partons)

  13. Simplest way to establish the properties of a system Calibrated probe (electrons, X-Rays) Calibrated interaction (beam of known energy and direction) Suppression pattern tells about density profile Probes of Dense Matter – Jet Tomography Tomography: • Simplest way to establish the properties of a system • Calibrated probe (jets -> leading hadrons – pT distributions) • Calibrated interaction (energy loss in the hot, dense medium) • Suppression pattern tells about density profile Au+Au Collision p+p Collision

  14. How to Measure ? Compare Au+Au with p+p Collisions  RAA Nuclear Modification Factor: Average number of NN collision in an AA collision No “Effect”: R < 1 at small momenta R = 1 at higher momenta where hard processes dominate Suppression: R < 1

  15. High-pT Suppression – Matter is Opaque • Observations at RHIC: • Photons are not suppressed • Good! g don’t interact with medium • Ncoll scaling works • Hadrons are not suppressed in peripheral collisions • Good! medium not dense • Hadrons are suppressed in central collisions • Huge: factor 5 • Azimuthal correlation function shows ~complete absence of “away-side” jet • Partner in hard scatter is absorbed in the dense medium

  16. Cause of Jet Quenching ? • Elastic scattering (Bjorken 1982) • Gluon Bremsstrahlung (factor ~ 10 larger) • Multiple final-state gluon radiation of the produced hard parton induced by the traversed dense colored medium • DEloss ~ gluon (gluon density) • DEloss ~ DL2 (medium length) • Deduced initial density at t0 = 0.2 fm/c: • e≈ 15-25 GeV/fm3 • Other “causes” were ruled out

  17. To Summarize (so far) • At RHIC we see the • hottest • densest • matter • ever studied in the laboratory • flows • as a (nearly) perfect fluid • with systematic patterns consistent withquark degrees of freedom • and a viscosity to entropy density ratio • lower • than any other known fluid It appears to be a strongly couple plasma  sQGP not the asymptotically plasma of “free” quarks and gluons as expected Problem is we know very little about it’s properties! T=200-400 MeV ~ 3.5·1012 K e=30-60 enuclear matter large “elliptic” flow valence quark scaling

  18. Towards the Quantitative: Limitations of RAA • Surface bias effectively leads to saturation of RAA 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 Use RAA to extract medium density:

  19. Q Dead cone effect implies lower heavy quark energy lossin matter: High-pT Heavy Quarks are Gray Probes Dokshitzer and Kharzeev, PLB 519 (2001) 199. Origin of surviving jets pT= 15 GeV (radial propagation only) Wicks, Horowitz, Djordjevic and Gyulassy, nucl-th/0512076 jets go that way 

  20. Hadronic decay channels D0  K(B.R.: 3.8%) D K p(B.R.: 9.1%) D*±D0π (B.R.: 68%  3.8% (D0  K) = 2.6%) Lc p K p (B.R.: 5%) Pro: Direct clean identification (peak) Cons: No trigger Large combinatorial background Need handle on decay vertex charm ct~100-200 mm bottom ct~400-500 mm  requires high resolution silicon vertex detectors Semileptonic decay channels c  ℓ+ + anything (B.R.: 9.6%) D0  ℓ+ + anything(B.R.: 6.87%) D ℓ + anything(B.R.: 17.2%) b  ℓ+ + anything (B.R.: 10.9%) B ℓ + anything(B.R.: 10.2%) Pro: Can deploy (simple) trigger Cons: Continuum: cannot disentangle bottom and charm contributions? “Photonic” Electron Background: g conversions (p0 gg) p0, h, h’ Dalitz decays r, f, … decays (small) Ke3 decays (small) (or m) How to Measure Open Heavy Flavor ? STAR only PHENIX & STAR

  21. D0 at RHIC (STAR) • Lack of high precision m-vertex detectors brute force method • select K and π tracks from dE/dx in TPC • combine all pairs from same events  signal+background • combine all pairs from different events  background(event-mixing technique) • subtract background from signal+background  signal PRC 71 (2005) 064902; PRL 94 (2005) 062301

  22. D0 signal at STAR: d+Au and Au+Au • Low S/B ratio (~1:600) compensated by large yield • First identified D mesons in Heavy Ions • A first step – but current method has its limits (only moderate pT reach) PRL 94 (2005) 062301 QM2005 nucl-ex/0510063

  23. PHENIX RICH Electromagnetic calorimeter STAR pT < 4 GeV/c TPC + ToF pT>1.5 GeV/c  TPC + EMC PHENIX Mass (GeV/c2) Not All Electrons are from Heavy Flavor • The Problem: not all electrons are from Heavy Flavor • Actually most electrons are from sources other than heavy flavors • Conversion Electrons: g e+ + e- (Note: p0, h→ e+ + e- is huge) • Dalitz Decays: p0  g + e+ + e- • h, w, f, decays (small) • PHENIX • very little material in acceptance •  very few conversion •  background estimated from known p0 and h spectra (cocktail method) • STAR • lots of material in acceptance •  use large acceptance TPC to measure background •  make use of fact that Conversion and Dalitz electrons come in pairs with low mass

  24. Collins, Soper, Sterman, Nucl. Phys. B263 (1986) 37 corrections hard scattering parton distribution function Heavy Flavor Production – QCD Calculations in pp A generic heavy quark production process: This part is QCD How accurately can we predict it? Baseline prediction:  Restrict to collinear factorization and calculate as many perturbative orders as possible State of the art for fixed order perturbative calculations for c & b  Next-to-Leading-Order: NLO Fixed-Order plus Next-to-Leading-Log:FONLL Uncertainty in several input parameters →

  25. 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 STAR: scc ~ 1.3 ± 0.3 mb

  26. hep-ex/0609010 Charm Cross-Section from PHENIX Cross-section from non-photonic electron spectra • Improving statistics over time • New: low pT cut • Reduces extrapolation uncertainties PHENIX: scc ~ 0.570 ± 0.23 mb

  27. Charm Production at RHIC: Total Cross Section The Problem: • Experiments do not agree! • Phenix about a factor of 2 higher than NLO/FONLL • electrons only but less background • STAR data about a factor of 5 higher than NLO/FONLL • More material but it is the only direct measurement of D-mesons • 95% of the total cross section is measured

  28. Charm Production in p+p: Spectral Shapes • Spectra shapes appear to agree (within errors) • Discrepancy almost constant over pT-range • Normalization issues only?

  29. scc / Ncoll Binary Scaling of Charm Production • If • Charm is produced by initial collisions  scaling with number of NN collisions (Ncoll) in different systems (pp, dA, AA) Data from both experiments independently indicate total cross section follows Ncoll scaling  No room for thermal production in the sQGP

  30. pp pp What’s About the Experimental Discrepancy ? • Charm measurement through non-photonic electrons are difficult • Background from photonic electrons is a huge problem • PHENIX has less material (less photon conversions) in it’s acceptance but uncertainties related to background are reflected by STARs systematic errors • Large discrepancies in charm cross-section has a long history • STAR: repeat measurement in 2008 with substantially less material • Final answer can only come from a measurement of D mesons While the current difference is an issue it does not affect the main message relevant for Heavy Ion Collisions in what follows!

  31. Big Surprise RAA: from d+Au to Central Au+Au Measurement of non-photonic electrons from semileptonic D decays showsubstantial suppressionin central Au+Au collisions comparable to that from light mesons N.B.: STAR and PHENIX in agreement! (normalization off in all systems ?!) d+Au: no suppression expected  slight enhancement expected (Cronin effect)  Central Au+Au: little suppression expected ?! Semi-Central Au+Au: very little suppression expected  Peripheral Au+Au: no suppression expected 

  32. Agreement in RAA Across Experiments centrality STAR and PHENIX in agreement! (normalization off in all systems ?!)

  33. Theory Behind RAA of Heavy Flavor ?! • Describing the suppression is difficult for models • radiative energy loss with typical gluon densities is not enough(Djordjevic et al., PLB 632(2006)81) • models involving a very opaque medium agree better(Armesto et al., PLB 637(2006)362) • collisional energy loss / resonant elastic scattering(Wicks et al., nucl-th/0512076, van Hees & Rapp, PRC 73(2006)034913) • heavy quark fragmentation and dissociation in the medium → strong suppression for charm and bottom(Adil & Vitev, hep-ph/0611109)

  34. N. Armesto et al, nucl-ex/0511257 b c Before quenching After quenching Or is the Solution Much Simpler? • Charm/bottom composition in all E-loss calculations based on FONLL with ‘average’ parameter set • Charm/bottom composition is an assumption … • Even FONLL leaves lots of room • Shifting bottom dominance to higher pT could change the picture !

  35. How to Disentangle Charm and Bottom? • e-h correlations in pp: • Significant difference between D decays and B decays in the near-side • Difference is largely due to decay kinematics, not the production dynamics e Measure/fit:

  36. A first hint from p+p: bottom vs. charm • First measurement of open-bottom at RHIC • Non-zero contribution of bottom • Very close to NLO/ FONLL predictions • Caveats • subtraction of large background • slight model dependence Final answer has to come from direct D & B measurements !

  37. Hendrik, Greco, Rapp nucl-th/0508055 c flow c,b flow c,b don’t flow Elliptic Flow of Non-Photonic Electrons • Naïve kinematical argument: need Mc/T ~ 7 times more collisions to thermalize Recall: dN/d w.r.t. the reaction plane: dN/df ~ 1+2 v2(pT)cos(2f) + …. v2 measures Elliptic Flow • Large v2 is an indication of early thermalization Observed strong elliptic flow at RHIC for light quark hadrons and baryons What’s about charm? Observe large elliptic flow of non-photonic electrons for pT < 2 GeV/c Is charm thermalized ? Need better data …

  38. Mid-Term Upgrades at RHIC • Detector Upgrades relevant for heavy flavor physics Machine Upgrade (RHIC-II): Increased luminosity (x 40) via electron cooling

  39. Summary (sorry, too early for conclusions …) • Thecharm cross-sectionappears to followbinary scaling  no thermal production  created at initial first impact • The charm productioncross-section is largerthan expected fromNLO  not understood so far • Measurement of non-photonic electrons from semileptonic D/B decays showsubstantial suppression in central Au+Au collisions • Models invoking radiative + collisional energy loss get closest to data • Uncertainties in bottom vs. charm contributions • Large elliptic flowof non-photonic electrons for pT < 3 GeV/c  charm quarks thermalize in medium? • Heavy Flavor: rare, difficult, beam-time and luminosity hungry •  will take some time, upgrades essential (RHIC-II) •  everything we observed so far was unexpected

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