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Parton energy loss in heavy-ion collisions from RHIC to LHC

Parton energy loss in heavy-ion collisions from RHIC to LHC. Andrea Dainese INFN Legnaro. Layout. The discovery of “jet quenching” at RHIC High- p T particle production in nucleus-nucleus collisions, according to perturbative QCD Calculating parton energy loss (BDMPS framework)

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Parton energy loss in heavy-ion collisions from RHIC to LHC

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  1. Parton energy lossin heavy-ion collisionsfrom RHIC to LHC Andrea Dainese INFN Legnaro Padova, 09.05.2006 Andrea Dainese

  2. Layout • The discovery of “jet quenching” at RHIC • High-pT particle production in nucleus-nucleus collisions, according to perturbative QCD • Calculating parton energy loss (BDMPS framework) • Light-flavour hadron suppression in the Parton Quenching Model at RHIC andLHC • Energy loss for heavy quarks • RHIC: electrons RAA vs data • LHC: testing E loss with heavy-to-light ratios • Conclusions Padova, 09.05.2006 Andrea Dainese

  3. PHENIX p0 (0-10%) Discovery at RHIC:high-pT suppression • Nuclear modification factor of pT distributions: factor 5 suppression! compilation by D.d’Enterria Padova, 09.05.2006 Andrea Dainese

  4. What about jets? pp  2 back-to-back jets Jets via di-hadron correlations: • trigger: highest-pT track, (4--6 GeV) • Df distribution: 2 GeV < pT < pTtrigger near side away side STAR Coll., PRL 90 (2003) 082302. slide courtesy of P.Jacobs Padova, 09.05.2006 Andrea Dainese

  5. Discovery at RHIC:no away-side jet in central Au-Au • Quantified via ratio of integrals: ~1 in peripheral AA ~0 in central AA STAR Coll., PRL 90 (2003) 082302. Padova, 09.05.2006 Andrea Dainese

  6. Final-state effect Is it final-state energy loss? • … or saturation of the parton densities in the initial nuclei? Control experiment, without medium: d-Au no high-pT suppression the away-side jet is there PHENIX Coll., PRL 91 (2003) 072303. STAR Coll., PRL 91 (2003) 072304. Padova, 09.05.2006 Andrea Dainese

  7. figure by D. d’Enterria Final-state effect Is it final-state energy loss? • … or saturation of the parton densities in the initial nuclei? “Control experiment”, with medium-blind probe: g’s in Au-Au no suppression, but Ncoll scaling Padova, 09.05.2006 Andrea Dainese

  8. h RAB Au 1 medium formed in the collision q q Au q h q ~2-4 GeV/c pT kLkT fgPb / fgp Q2 = 5 GeV2 Cronin enhancement h p pp  h+X LHCRHICSPS RAA<1~1>1 q p h x High-pT particle prod. in AA collisions • A schematic view: • Ingredients: • pp baseline (pQCD) • initial-state effects: • PDF (anti)shadowing • kT broadening (Cronin) • final-state effects: • energy loss • in-medium hadronization (coalescence) Padova, 09.05.2006 Andrea Dainese

  9. … Parton Energy Loss • Partons travel ~4 fm in the high-density medium • Bjorken (`82): energy loss due to elastic scattering Bjorken, FERMILAB-Pub-82/59-THY (1982). Padova, 09.05.2006 Andrea Dainese

  10. path length L hard parton Parton QCD Energy Loss • Partons travel ~4 fm in the high colour-density medium • Successive calculations (`92 ): a QCD mechanism dominates, medium-inducedgluon radiation • Coherent wave-function gluon acummulates kT due to multiple inelastic scatterings in the medium  it decoheres and is radiated kT Gyulassy, Pluemer, Wang, Baier, Dokshitzer, Mueller, Peigne’, Schiff, Levai, Vitev, Zhakarov, Salgado, Wiedemann, … Padova, 09.05.2006 Andrea Dainese

  11. path length L w kT l Calculating Parton Energy Loss BDMPS-Z formalism STATIC MEDIUM transport coefficient Radiated-gluon energy distrib.: (BDMPS case) Casimir coupling factor: 4/3 for q, 3 for g sets the scale of the radiated energy related to constraint kT <w, controls shape at w << wc Baier, Dokshitzer, Mueller, Peigne‘, Schiff, NPB 483 (1997) 291. Zakharov, JTEPL 63 (1996) 952. Salgado, Wiedemann, PRD 68(2003) 014008. Padova, 09.05.2006 Andrea Dainese

  12. Finite parton energy (qualitatively) • If E < wc (e.g. small pT parton with large L): • dependence on parton energy • : smaller sensitivity to density Calculating Parton Energy Loss gluons volume-density and interaction cross section Probe the medium Padova, 09.05.2006 Andrea Dainese

  13. pT dN/dpT PYTHIA Fragmentation pT – DE pT Application: Parton Quenching Model • BDMPS + Glauber-based medium geometry and density profile + PYTHIA for parton generation and fragmentation • The procedure in short: • generate parton (q or g) with PYTHIA (or back-to-back pair) • calculate its L and average along the path • use quenching weights (from wdI/dw) to get energy loss • quench parton and then hadronize it (independent fragm.) Dainese, Loizides, Paic, EPJC 38 (2005) 461. Padova, 09.05.2006 Andrea Dainese

  14. (Quark Matter 05) matches pT-indepence of suppression at high pT Model vs RHIC “light” data • Density ( ) “tuned” to match RAA in central Au-Au at 200 GeV Similar results by Eskola, Honkanen, Salgado, Wiedemann Dainese, Loizides, Paic, EPJC 38 (2005) 461. Padova, 09.05.2006 Andrea Dainese

  15. pT > 4.5 GeV Model vs RHIC dataCentrality dependence of RAA • Centrality evolution according to Glauber-model collision geometry Dainese, Loizides, Paic, EPJC 38 (2005) 461. Padova, 09.05.2006 Andrea Dainese

  16. STAR preliminary Charged hadrons Testing L-dependence? RAA for 63Cu-63Cu RAA Padova, 09.05.2006 Andrea Dainese

  17. PHENIX p0 v2 v2 from E loss: Dainese, Loizides, Paic, EPJC 38 (2005) 461 Testing L-dependence? v2 at high pT from E loss • The azimuthal asymmetry --v2 or RAA(f)-- of high-pT particle yields in non-central collisions tests the path-length dependence of E loss (almond-shaped medium) Padova, 09.05.2006 Andrea Dainese

  18. Extrapolation in c.m.s. energyIntermediate RHIC energy s = 62 GeV • Extrapolation in s: assuming  Ngluons/volume  (s)0.6 (EKRT saturation model) • First test: energy extrapolation works reasonably well EKRT: Eskola, Kajantie, Ruuskanen, Tuominen, NPB 570 (2000) 379. PHENIX Coll., JPG 31 (2005) S473. Padova, 09.05.2006 Andrea Dainese

  19. Model prediction for LHC • Extrapolation to LHC according to saturation model gives: • Most partons are absorbed • Already at RHIC, only those from the surface can escape … surface emission dominates Padova, 09.05.2006 Andrea Dainese

  20. Model vs RHIC dataDisappearence of the away-side jet near side away side STAR Coll., nucl-ex/0501016. Padova, 09.05.2006 Andrea Dainese

  21. Observation of true di-jets in Au-Au STAR Padova, 09.05.2006 Andrea Dainese

  22. Di-jets associated yields (IAA) Padova, 09.05.2006 Andrea Dainese

  23. Tangential di-jets? Padova, 09.05.2006 Andrea Dainese

  24. Putting the pieces together Padova, 09.05.2006 Andrea Dainese

  25. ? large  RAA indep. of Limited sensitivity of RAA • Strong suppression requires very large density • Surface emission scenario • RAA determined by geometry rather than by density itself • Limited sensitivity to • Need more differential observables: • massive partons • study of jet shapes • … Eskola, Honkanen, Salgado, Wiedemann, NPA 747 (2005) 511. Padova, 09.05.2006 Andrea Dainese

  26. Heavy Quarks Padova, 09.05.2006 Andrea Dainese

  27. Gluonsstrahlung probability Dokshitzer Q Lower E loss for heavy quarks ? • In vacuum, gluon radiation suppressed at q < mQ/EQ  “dead cone” effect • Dead cone implies lower energy loss(Dokshitzer-Kharzeev, 2001): • energy distribution wdI/dw of radiated gluons suppressed by angle-dependent factor • suppress high-w tail Dokshitzer, Khoze, Troyan, JPG 17 (1991) 1602. Dokshitzer and Kharzeev, PLB 519 (2001) 199. Padova, 09.05.2006 Andrea Dainese

  28. m/E = 0.1 massive massless dead cone Lower E loss for heavy quarks ? • Detailed massive calculation shows that: Medium-induced gluon radiation fills the dead cone Still, kT-integrated radiation is suppressed, but only for quite large m/E (i.e. large m, low pT)* kT R = 105 *Warning: large uncertainties Armesto, Salgado, Wiedemann, PRD 69 (2004) 114003. Padova, 09.05.2006 Andrea Dainese

  29. Model for heavy quarks • Initially only charm • Baseline at RHIC: PYTHIA, with EKS98 shadowing, tuned to match pT-shape of D cross section measured in d-Au by STAR (QM04) • Heavy-quark E loss using (m/E)-dependent QW • extracted from p0,hRAA (central) • Thermalize charms that lose all energy dN/dmTmT exp(-mT/T), T = 300 MeV baseline dN/dpT baseline RAA Armesto, Dainese, Salgado, Wiedemann, PRD 71 (2005) 054027. EKS98: Eskola, Kolhinen, Salgado, EPJC 9 (1999) 61. Padova, 09.05.2006 Andrea Dainese

  30. other contributions become important larger uncertainties Charm RAA at RHIC (extracted from light-hadron data) effect of the mass thermalized component Small effect of mass for charm (~50% for D, ~30% for e) at low pT [large uncertainties!] Basically no effect in “safe” pT-region Armesto, Dainese, Salgado, Wiedemann, PRD 71 (2005) 054027. Padova, 09.05.2006 Andrea Dainese

  31. Large uncertainty on b/c crossing point in pT: from scales/masses variation it changes from 3 to 9 GeV PHENIX hep-ex/0508034 Note: still, electron cross section in pp at 200 GeV underestimated at high pT Charm + beauty + DY electrons at RHIC(pQCD baseline revised, and its uncertainties) FONLL: electron spectrum may be ~50% c + ~50% b for 3 < pT < 8 GeV Drell-Yan component investigated as well: < 10% up to 10 GeV FONLL calculation: Cacciari, Nason, Vogt, PRL95 (2005) 122001 Drell-Yan from: Gavin et al., hep-ph/9502372 Comparison: Armesto, Cacciari, Dainese, Salgado, Wiedemann, hep-ph/0511257 Padova, 09.05.2006 Andrea Dainese

  32. Theory predicts e RAA (~0.4)larger than p RAA (~0.2) e spectra in principle sensitive to mass hierarchy Perturbative uncertainty on the pp baseline comparable to model-intrinsic uncertainty in determination of Preliminary STAR data not conclusive for comparison with theory RAA of c and b electrons at RHIC • Due to larger mass of b quark electron RAA increased to ~0.4 • Mass uncertainty (in E loss) and scales uncertainty (in pQCD  b/c crossing) were studied b c+b c Armesto, Cacciari, Dainese, Salgado, Wiedemann, hep-ph/0511257 Padova, 09.05.2006 Andrea Dainese

  33. Heavy Quark Energy Loss at LHC • ~100 cc pairs and ~5 bb pairs per central Pb-Pb collision • Experiments will measure with good precision RAA for D and B, and for their decay leptons What can we learn from a comparative quenching study of massive and massless probes at the LHC? Padova, 09.05.2006 Andrea Dainese

  34. Heavy Flavour RAA at LHC • Baseline: PYTHIA, with EKS98 shadowing, tuned to reproduce c and b pT distributions from NLO pQCD (MNR) • (m/E)-dep. E loss with • * EKRT Saturation model: Eskola, Kajantie, Ruuskanen, Tuominen, NPB 570 (2000) 379. Armesto, Dainese, Salgado, Wiedemann, PRD 71 (2005) 054027. MNR: Mangano, Nason, Ridolfi, NPB 373 (1992) 295. Padova, 09.05.2006 Andrea Dainese

  35. mass effect For 10 < pT < 20 GeV, charm behaves like a m=0 quark, light-flv hadrons come mainly from gluons RD/h enhancement probes colour-charge dep. of E loss RB/h enhancement probes mass dep. of E loss Colour-charge and mass dep. of E loss with Heavy-to-Light ratios at LHC Heavy-to-light ratios: Compare g  h , c  D and b  B Armesto, Dainese, Salgado, Wiedemann, PRD71 (2005) 054027 Padova, 09.05.2006 Andrea Dainese

  36. Beauty-to-Charm ratio: RB/D • Compare c and b  same colour charge • Mass effect  Enhancement of factor ~2, independent of (for ) • Challenging from experimental point of view… Padova, 09.05.2006 Andrea Dainese

  37. Summary • One of the most exciting discoveries at RHIC ! • We are dealing with energy loss in an extremely opaque medium, but … • theoretically, we are just starting to learn how to probe the medium; still large uncertainties (see also next talk) • experimentally, going to more differential observables • vs system size / azimuthal angle (path-length dependence) • vs quark mass -- c (b) SUPPRESSED at RHIC, but need to disentangle c and b to clarify picture • The LHC will be a `hard / heavy probes machine’ and quenching studies will play a central role • Example: Heavy-to-light ratios as probes of E loss… • … colour-charge dependence (RD/h) • … parton-mass dependence (RB/h, RB/D?) Padova, 09.05.2006 Andrea Dainese

  38. EXTRA SLIDES Padova, 09.05.2006 Andrea Dainese

  39. NA57 PLB623 (2005) 17 NA49, QM2005 E loss (Wang) Parton energy loss at SPS (s  17 GeV)?RCP(K) vs E loss calculations • Wang calculation: pQCD + Cronin + (anti-)shadowing + energy loss (gluon density scaled by dNch/dh) • Good agreement between data and both Wang and PQM calculations ( ) Padova, 09.05.2006 Andrea Dainese

  40. Open points (2): the opacity problem • Can we really probe the medium? • Need to relate extracted to an energy density e • QCD estimate for ideal QGP: • A recent analysis* of RHIC data, similar to that presented, extracts energy density 5 larger than that estimated from produced transverse energy dET/dy (Bjorken estimate) • Opacity problem: the interaction of the hard parton with the medium is much stronger than expected (Baier) Baier, NPA 715 (2003) 209. * Eskola, Honkanen, Salgado, Wiedemann, NPA 747 (2005) 511. Padova, 09.05.2006 Andrea Dainese

  41. Collisional energy loss revisited Mustafa and Thoma, Acta Phys.Hung. A22, 93 (2005) Earlier estimates: collisional (elastic) energy loss is negligible relative to radiative effects New estimate: Boltzmann transport + 1-D Bjorken expansion Q(p) = RAA RAA = 0.25-0.4 for reasonable pathlength, similar to radiative E-loss Can we really ignore collisional energy loss? Padova, 09.05.2006 Andrea Dainese

  42. 8 < pT(trig) < 15 GeV/c STAR Preliminary pT(assoc) > 5 GeV/c 1/Ntrig dN/dj Reappearance of di-jets:surface emission of tangential partons? Padova, 09.05.2006 Andrea Dainese

  43. Medium Expansion • The density of scattering centres is time-dependent: a = 1.5, 1.0, 0.5, 0 • Dynamical scaling law: same spectrum obtained for equivalent static trasport coefficient • Calculations for a static medium apply to expanding systems Salgado and Wiedemann, PRL 89 (2002) 092303 Padova, 09.05.2006 Andrea Dainese

  44. Static vs Bjorken Expansion Padova, 09.05.2006 Andrea Dainese

  45. Static vs Bjorken: numbers Padova, 09.05.2006 Andrea Dainese

  46. f = p/2 f = 0 Armesto, Cacciari, Dainese, Salgado, Wiedemann, hep-ph/0511257, PLB to appear v2 at high pT from E loss • The azimuthal asymmetry --v2 or RAA(f)-- of D and B mesons in non-central collisions tests: • at low/moderate pT: recombination scenario, v2 of c/b quarks, hence degree of thermalization of medium • at higher pT: path-length dependence of E loss (almond-shaped medium => v2~ 0.05--0.10) STAR results withdrawn; new analysis in progress PHENIX p0 v2 Cole @ Quark Matter 05 v2 from E loss: Dainese, Loizides, Paic, EPJC 38 (2005) 461 Padova, 09.05.2006 Andrea Dainese

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