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Parton Energy Loss at RHIC: Experimental Viewpoint

This article explores the understanding of parton energy loss at RHIC from an experimental perspective, including the mechanisms of energy loss, density of scattering centers, and the role of fields and medium properties. It also discusses different theoretical approaches and their implications for modeling parton energy loss and the measurements of RAA.

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Parton Energy Loss at RHIC: Experimental Viewpoint

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  1. What do we understand about parton energy loss at RHIC? An experimentalists viewpoint Marco van Leeuwen, Utrecht University

  2. Hard probes of QCD matter Use ‘quasi-free’ partons from hard scatterings Calculable with pQCD to probe ‘quasi-thermal’ QCD matter Interactions between parton and medium: • Radiative energy loss • Collisional energy loss • Hadronisation: fragmentation and coalescence Quasi-thermal matter: dominated by soft (few 100 MeV) partons Use the strength of pQCD to explore QCD matter Sensitive to medium density, transport properties

  3. Energy loss in QCD matter radiated gluon propagating parton kT QCD bremsstrahlung(+ LPM coherence effects) Transport coefficient l Energy loss Energy loss probes: Density of scattering centers: Nature of scattering centers, e.g. mass: radiative vs elastic loss Or no scattering centers, but fields  synchrotron radiation?

  4. Questions about energy loss • What is the dominant mechanism: radiative or elastic? • Heavy/light, quark/gluon difference, L2 vs L dependence • How important is the LPM effect? • L2 vs L dependence • Can we use this to learn about the medium? • Density of scattering centers? • Temperature? • Or ‘strongly coupled’, fields are dominant? Phenomenological questions: Large vs small angle radiation Mean DE? How many radiations? Virtuality evolution/interplay with fragmentation?

  5. p0 RAA – high-pT suppression : no interactions RAA = 1 Hadrons: energy loss RAA < 1 : RAA = 1 0: RAA≈ 0.2 Hard partons lose energy in the hot matter Factor 4-5 suppression – A large effect!

  6. Parton energy loss and RAA modeling Qualitatively: Parton spectrum Energy loss distribution Fragmentation (function) known pQCDxPDF extract `known’ from e+e- Vacuum fragmentation (?) ‘Same’ in p+pand Au+Au Contains medium properties e.g. density profile

  7. Four theory approaches • Multiple-soft scattering (ASW-BDMPS) • Full interference (vacuum-medium + LPM) • Approximate scattering potential • Opacity expansion (GLV/WHDG) • Interference terms order-by-order (first order default) • Dipole scattering potential 1/q4 • Higher Twist • Like GLV, but with fragmentation function evolution • Hard Thermal Loop (AMY) • Most realistic medium • LPM interference fully treated • No interference between vacuum frag and medium Allow definition of ‘quenching weights’ P(DE)

  8. Extracting , T Bass et al, PRC79, 024901 Large differences between formalisms Not all approaches can be ‘correct’ Can we decide which formalism(s) is/are correct? All can be fit to RAA – RAA is not decisive

  9. Two extreme scenarios (or how P(DE) says it all) Scenario I P(DE) = d(DE0) Scenario II P(DE) = a d(0) + b d(E) 1/Nbin d2N/d2pT ‘Energy loss’ ‘Absorption’ p+p Downward shift Au+Au Shifts spectrum to left pT Would need DE/E ~ 0.2 to get RAA ~ 0.2 Would need a = 0.2, b = 0.8to get RAA ~ 0.2 P(DE) encodes the full energy loss process RAA not sensitive to details of mechanism

  10. Energy loss spectrum Typical examples with fixed L <DE/E> = 0.2 R8 ~ RAA = 0.2 Brick L = 2 fm, DE/E = 0.2 E = 10 GeV Significant probability to lose no energy (P(0)) Broad distribution, large E-loss (several GeV, up to DE/E = 1) Theory expectation: mix of partial transmission+continuous energy loss – Can we see this in experiment? Different theoretical approximation (ASW, WHDG) give different results – significant?

  11. How geometry complicates matters L ~ R, large q Energy loss distribution summed over all partons Static medium For ‘typical partons’ L and q are correlated M. Verweij, UU L < R, small q Resulting P(DE/E) peaked at ~ 0 and 1 Black-white scenario No sensitivity to continuous E-loss? L > R, smaller q

  12. Path length dependence I Out of plane In-plane Change L in single system in-plane vs out of plane Collision geometry Au+Au Centrality Cu+Cu <L>, density increase with centrality Vary L and density independently by changing Au+Au  Cu+Cu Turns out to be not very precise

  13. RAA vs reaction plane angle C. Vale, PHENIX, QM09 Azimuthal modulation, path length dependence largest in ASW-BDMPS But why? – No clear answer yet Data prefer ASW-BDMPS

  14. Di­hadron correlations Combinatorialbackground 8 < pTtrig < 15 GeV associated pTassoc > 3 GeV  trigger Near side Away side Use di-hadron correlations to probe the jet-structure in p+p, d+Au and Au+Au

  15. Di­hadron yield suppression trigger Near side associated Away side 8 < pT,trig < 15 GeV Near side Yield in balancing jet, after energy loss Yield of additional particles in the jet trigger STAR PRL 95, 152301 Away side associated Suppression byfactor 4-5 in central Au+Au No suppression Away-side: Suppressed by factor 4-5  large energy loss Near side: No modification  Fragmentation outside medium? Note: per-trigger yields can be same with energy-loss

  16. RAA and IAA in a single model Hydro + ASW-BDMPS Armesto, Cacciari, Salgado et al. RAA and IAA give similar density Model has L2 built in – But how sensitive is it?

  17. L scaling: elastic vs radiative T. Renk, PRC76, 064905 RAA: input to fix density Radiative scenario fits data; elastic scenarios underestimate suppression Indirect measure of path-length dependence: single hadrons and di-hadrons probe different path length distributions Confirms L2 dependence  radiative loss dominates

  18. L-dependence from surface bias Near side trigger, biases to small E-loss Away-side large L Away-side suppression IAA samples different path-length distribution than inclusives RAA

  19. Dead cone effect light Radiated wave front cannot out-run source quark Expected energy loss Heavy quark: b < 1 M.DjordjevicPRL 94 Wicks, Horowitz et al, NPA 784, 426 Dead cone effect reduces E-loss Result: minimum angle for radiation Most pronounced for bottom

  20. Heavy quark suppression Using non-photonic electrons Expect: heavy quarks lose less energy due to dead-cone effect PHENIX nucl-ex/0611018, STAR nucl-ex/0607012 Measured suppression of non-photonic electrons larger than expected Radiative (+collisional) energy loss not dominant? E.g.: in-medium hadronisation/dissociation (van Hees, et al) • Djordjevic, Phys. Lett. B632, 81 • Armesto, Phys. Lett. B637, 362

  21. Heavy Quark comparison Armesto, Cacciari, Salgado et al. No minimum – Heavy Quark suppression too large for ‘normal’ medium density

  22. Charm/bottom separation arXiv:0903.4851 hep-ex Bottom/charm ratio in p+p agrees with theory expectations (FONLL) Combine rB and RAA to extract RAA for charm and bottom

  23. RAA for c  e and b  e pT > 5 GeV/c Combined data show: electrons from both B and D suppressed B.Biritz QM09 Large suppression suggestsadditional energy loss mechanism (resonant scattering, dissociative E-loss) I: Djordjevic,Gyulassy, Vogt and Wicks, Phys. Lett. B 632 (2006) 81; dNg/dy = 1000 II: Adil and Vitev, Phys. Lett. B 649 (2007) 139 III: Hees, Mannarelli, Greco and Rapp, Phys. Rev. Lett. 100 (2008) 192301

  24. Summary so far • Large suppression of light hadrons  parton energy loss • Rough estimates: DE/E ~ 0.2, or 80% absorption • QCD predicts broad distribution P(DE) • Geometry: many small path lengths • Effectively black/white NB: means no sensitivity to dynamics! • IAA vs RAA: L2 preferred – radiative dominates • RAA vs reaction plane: modulation in data larger than expected? • Heavy quarks: suppression larger than expected

  25. Jets Basic idea: recover radiated energy – parton energy before E-loss Alternative: use recoil photon in g-jet events (low statistics) Out-of cone radiation Suppression of jet yieldRAAjets < 1 In-cone radiation: Softening of fragmentation function and/or broadening of jet structure Salgado, Wiedemann, PRL93, 042301 Early prediction: out-of-cone radiation small effect

  26. Fragmentation functions Qualitatively: Dashed lines: include gluon fragments (assuming 1 gluon emitted) Fragmentation functions sensitive to P(DE) Distinguish GLV from BDMPS?

  27. Are FF sensitive to P(DE) ? Toy model curves P(DE) toy model Fragmentation function ratio Fragmentation functions are sensitive to P(DE) – Somewhat

  28. g-hadron results 8 < ET,g < 16 GeV A. Hamed, STAR, QM09 PHENIX, PRC80, 024908 STAR Preliminary Large suppression for away-side: factor 3-5 Results agree with model predictions Would like to see z-dependence, uncertainties still large

  29. Jet finding in heavy ion events STAR preliminary pt per grid cell [GeV] η j ~ 21 GeV Jets clearly visible in heavy ion events at RHIC Combinatorial background Needs to be subtracted • Use different algorithms to estimate systematic uncertainties: • Cone-type algorithms simple cone, iterative cone, infrared safe SISCone • Sequential recombination algorithmskT, Cambridge, inverse kT http://rhig.physics.yale.edu/~putschke/Ahijf/A_Heavy_Ion_Jet-Finder.html FastJet:Cacciari, Salam and Soyez; arXiv: 0802.1188

  30. Jet spectra p+p Au+Au central Note kinematic reach out to 50 GeV • Jet energy depends on R, affects spectra • kT, anti-kT give similar results Take ratios to compare p+p, Au+Au

  31. Jet RAA at RHIC M. Ploskon, STAR, QM09 Jet RAA >> 0.2 (hadron RAA) Jet finding recovers most of the energy loss  measure of initial parton energy Some dependence on jet-algorithm? Under study…

  32. Radius dependence M. Ploskon, STAR, QM09 RAA depends on jet radius: Small R jet is single hadron Jet broadening due to E-loss ?

  33. Fragmentation functions Use recoil jet to avoid biases 20<pt,rec(AuAu)<25 GeV E. Bruna, STAR, QM09 pt,rec(AuAu)>25 GeV STAR Preliminary Recoil suppression in reconstructed jets small

  34. Di-jet suppression Jet IAA Away-side jet yield suppressed  partons absorbed E. Bruna, STAR, QM09 STAR Preliminary ... due to large path length (trigger bias) Elena Bruna for the STAR Collaboration - QM09 STAR Preliminary 34

  35. Emerging picture from jet results • Jet RAA ~ 1 for sufficiently large R – unbiased parton selection • Away side jet fragmentation unmodified – away-side jet emerges without E-loss • Jet IAA ~ 0.2 – Many jets are absorded (large E-loss) Study vs R, E to quantify P(DE) and broadening Ongoing developments of event generators for modified fragmentation important for measurement, interpretation JEWEL, q-PYTHIA, YaJEM, PYQUEN, MARTINI

  36. RHIC future • Machine upgrades • Stochastic cooling to increase luminosity • Several polarisation upgrades • Experiment upgrades • Vertex detectors for charm, bottom • Forward calorimeters • STAR: TOF for PID • DAQ upgrades to deal with rates Increased luminosity: g-hadron, jets, charmonia (U)

  37. Delivered Integrated Luminosity Heavy ion runs Polarized proton runs Integrated nucleon-pair luminosity LNN [pb-1] Integrated nucleon-pair luminosity LNN [pb-1] Nucleon-pair luminosity allows comparison of luminosities of different species Collider luminosity increases as experience grows – Often beyond original design!

  38. g-hadron luminosity projection Gradual increase in p+p and Au+Au luminosity reduces measurement uncertainties

  39. Inner tracking upgrades at RHIC PHENIX STAR Goals: Charm flow, spectra, RAA b-tagging for bottom RAA

  40. Heavy-light ratios light M.DjordjevicPRL 94 (2004) Armesto et al, PRD71, 054027 Wicks, Horowitz et al, NPA 784, 426 Armesto plot Heavy-light RAA ratios directly sensitive to dead-cone effect Effect sizeable, should be measurable with vertex detectors

  41. Large Hadron Collider at CERN CMS 2010: p+p collisions @ 7-10 TeV 2010: Pb+Pb collisions @ ? TeV ALICE ATLAS 3 Large general purpose detectors ALICE dedicated to Heavy Ion Physics, PID p,K, out to pT > 10 GeV ATLAS, CMS: large acceptance, EM+hadronic calorimetry

  42. From RHIC to LHC Larger initial density = 10-15 GeV/fm3 at RHIC ~ 100 GeV/fm3 at LHC 10k/year Validate understanding of RHIC data • Larger pT-reach:typical parton energy > typical E • Energy dependenc of E-loss with high-energy jets Large cross sections for hard processes Direct access to energy loss dynamics, P(E) Including heavy flavours

  43. Energy loss distribution Typical examples with fixed L <DE/E> = 0.2 R8 ~ RAA = 0.2 Brick L = 2 fm, DE/E = 0.2 E = 10 GeV Significant probability to lose no energy (P(0)) Broad distribution, large E-loss (several GeV, up to DE/E = 1) Broad distribution; typical energy loss ~5 GeV RHIC: DE ~ Ejet, LHC: Ejet > DE  sensitivity to P(DE)

  44. RAA at LHC GLV BDMPS T. Renk, QM2006 RHIC RHIC S. Wicks, W. Horowitz, QM2006 LHC: typical parton energy > typical E Expected rise of RAA with pT depends on energy loss formalism Nuclear modification factor RAA at LHC sensitive to radiation spectrum P(E)

  45. Heavy-to-Light ratios at LHC Colour-charge and mass dep. of E loss Heavy-to-light ratios: mass effect For pT > 10 GeV charm is ‘light’ RD/h probes colour-charge dep. of E loss RB/h probes mass dep. of E loss Armesto, Dainese, Salgado, Wiedemann, PRD71 (2005) 054027

  46. ALICE heavy flavour performance mb = 4.8 GeV D0 Kp B  e + X 1 year at nominal luminosity (107 central Pb-Pb events, 109 pp events) Alice can measure charm from 0 < pT < 20 GeV and bottom (semi-leptonic decays) 3 < pT < 20 GeV

  47. ALICE performance: heavy-to-light 1 year at nominal luminosity (107 central Pb-Pb events, 109 pp events)

  48. Medium modification of fragmentation MLLA calculation: good approximation for soft fragmentation extended with ad-hoc implementation medium modifications Borghini and Wiedemann, hep-ph/0506218 pThadron ~2 GeV for Ejet=100 GeV =ln(EJet/phadron) z 0.37 0.14 0.05 0.02 0.007 Trends intuitive: suppression at high z, enhancement at low z Recent progress: showering with medium-modified Sudakov factors, see Carlos’s talk and arXiv:0710.3073

  49. Full jet reconstruction performance Simulation input reference Simulated result Medium modified (APQ)‏ Full jet reco in ALICE is sensitive to modification of fragmentation function E > E, explore dynamics of energy loss process

  50. Jet shapes

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