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

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

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

4. 4 Questions about energy loss What is the dominant mechanism: radiative or elastic? –Heavy/light, quark/gluon difference, L 2 vs L dependence How important is the LPM effect? –L 2 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  E? How many radiations? Virtuality evolution/interplay with fragmentation?

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

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

7. 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/q 4 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(  E)

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

9. 9 Two extreme scenarios p+p Au+Au pTpT 1/N bin d 2 N/d 2 p T Scenario I P(  E) =  (  E 0 ) ‘Energy loss’ Shifts spectrum to left Scenario II P(  E) = a  (0) + b  (E) ‘Absorption’ Downward shift (or how P(  E) says it all) P(  E) encodes the full energy loss process R AA not sensitive to details of mechanism Would need  E/E ~ 0.2 to get R AA ~ 0.2 Would need a = 0.2, b = 0.8 to get R AA ~ 0.2

10. 10 Energy loss spectrum Brick L = 2 fm,  E/E = 0.2 E = 10 GeV Typical examples with fixed L  E/E> = 0.2 R 8 ~ R AA = 0.2 Different theoretical approximation (ASW, WHDG) give different results – significant? Significant probability to lose no energy (P(0)) Broad distribution, large E-loss (several GeV, up to  E/E = 1) Theory expectation: mix of partial transmission+continuous energy loss – Can we see this in experiment?

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

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

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

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

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

16. 16 R AA and I AA in a single model Armesto, Cacciari, Salgado et al. R AA and I AA give similar density Model has L 2 built in – But how sensitive is it? Hydro + ASW-BDMPS

17. 17 L scaling: elastic vs radiative T. Renk, PRC76, 064905 R AA : input to fix densityRadiative scenario fits data; elastic scenarios underestimate suppression Indirect measure of path-length dependence: single hadrons and di-hadrons probe different path length distributions Confirms L 2 dependence  radiative loss dominates

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

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

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

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

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

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

24. 24 Summary so far Large suppression of light hadrons  parton energy loss Rough estimates:  E/E ~ 0.2, or 80% absorption QCD predicts broad distribution P(  E) Geometry: many small path lengths –Effectively black/white NB: means no sensitivity to dynamics! I AA vs R AA : L 2 preferred – radiative dominates R AA vs reaction plane: modulation in data larger than expected? Heavy quarks: suppression larger than expected

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

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

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

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

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

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

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

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

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

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

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

36. 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:  -hadron, jets, charmonia (  )

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

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

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

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

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

42. 42 From RHIC to LHC -Larger p T -reach: typical parton energy > typical  E -Energy dependenc of E-loss with high- energy jets Larger initial density  = 10-15 GeV/fm 3 at RHIC  ~ 100 GeV/fm 3 at LHC 10k/year Large cross sections for hard processes Including heavy flavours Validate understanding of RHIC data Direct access to energy loss dynamics, P(  E)

43. 43 Energy loss distribution Brick L = 2 fm,  E/E = 0.2 E = 10 GeV Typical examples with fixed L  E/E> = 0.2 R 8 ~ R AA = 0.2 Significant probability to lose no energy (P(0)) Broad distribution, large E-loss (several GeV, up to  E/E = 1) Broad distribution; typical energy loss ~5 GeV RHIC:  E ~ E jet, LHC: E jet >  E  sensitivity to P(  E)

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

45. 45 Heavy-to-Light ratios at LHC Heavy-to-light ratios: mass effect For p T > 10 GeV charm is ‘light’ R D/h probes colour-charge dep. of E loss R B/h probes mass dep. of E loss Armesto, Dainese, Salgado, Wiedemann, PRD71 (2005) 054027 Colour-charge and mass dep. of E loss

46. 46 ALICE heavy flavour performance m b = 4.8 GeV D 0  K  B  e + X 1 year at nominal luminosity (10 7 central Pb-Pb events, 10 9 pp events) Alice can measure charm from 0 < p T < 20 GeV and bottom (semi-leptonic decays) 3 < p T < 20 GeV

47. 47 ALICE performance: heavy-to-light 1 year at nominal luminosity (10 7 central Pb-Pb events, 10 9 pp events)

48. 48  = ln( E Jet / p hadron ) p T hadron ~2 GeV for E jet =100 GeV Borghini and Wiedemann, hep-ph/0506218 Medium modification of fragmentation MLLA calculation: good approximation for soft fragmentation extended with ad-hoc implementation medium modifications Recent progress: showering with medium-modified Sudakov factors, see Carlos’s talk and arXiv:0710.3073 Trends intuitive: suppression at high z, enhancement at low z z0.370.140.050.020.007

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

50. 50 Jet shapes

51. 51 Jet shapes in ATLAS

52. 52 Conclusion Parton energy loss at RHIC is large  E ~ E  Limited sensitivity to E-loss dynamics Di-hadron suppression indicates L 2 dependence  radiative dominates Heavy flavour more suppressed than expected Jet broadening significant Fragmentation function modification difficult to measure – expected ? Future at RHIC –Direct measurements of charm, heavy/light ratios –Increase  -jet statistics –Improve understanding of jet results Future at LHC –Verify/test our understanding in a new regime –May reach into E >  E regime –Abundant jets E ~ 100 GeV Sensitivity to E-loss dynamics We still have 4+ formalisms – Need to devise tests of validity

53. 53 Thanks for your attention!

54. 54 Heavy quark fragmentation Light quarks Heavy quarks Heavy quark fragmentation: leading heavy meson carries large momentum fraction More handle to extract P(  E)?

55. 55 Direct photons: no interactions PHENIX Direct  spectra Scaled by N coll PHENIX, PRL 94, 232301 Direct  in A+A scales with N coll Centrality A+A initial state is incoherent superposition of p+p for hard probes

56. 56 Geometry III Brick isolines from left to right: R7 = 0.45, 0.25, 0.15, 0.05

57. 57 Determining the medium density PQM (Loizides, Dainese, Paic), Multiple soft-scattering approx (Armesto, Salgado, Wiedemann) Realistic geometry GLV (Gyulassy, Levai, Vitev), Opacity expansion (L/ ), Average path length WHDG (Wicks, Horowitz, Djordjevic, Gyulassy) GLV + realistic geometry ZOWW (Zhang, Owens, Wang, Wang) Medium-enhanced power corrections (higher twist) Hard sphere geometry AMY (Arnold, Moore, Yaffe) Finite temperature effective field theory (Hard Thermal Loops) For each model: 1.Vary parameter and predict R AA 2.Minimize  2 wrt data Models have different but ~equivalent parameters: Transport coeff. Gluon density dN g /dy Typical energy loss per L:  0 Coupling constant  S PHENIX, arXiv:0801.1665, J. Nagle WWND08

58. 58 Medium density from R AA PQM = 13.2 GeV 2 /fm +2.1 - 3.2 ^ GLV dN g /dy = 1400 +270 - 150 WHDG dN g /dy = 1400 +200 - 375 ZOWW  0 = 1.9 GeV/fm +0.2 - 0.5 AMY  s = 0.280 +0.016 - 0.012 Data constrain model parameters to 10-20% Method extracts medium density given the model/calculation Theory uncertainties need to be further evaluated e.g. comparing different formalisms, varying geometry But models use different medium parameters – How to compare the results?

59. 59 Some pocket formula results Large differences between models GLV/WHDG: dN g /dy = 1400 T(  0 ) = 366 MeV PQM: (parton average) T = 1016 MeV AMY: T fixed by hydro (~400 MeV),  s = 0.297

60. 60 Naive picture for di-hadron measurements P T,jet,1 P T,jet,2 Fragment distribution (fragmentation fuction) Out-of-cone radiation: P T,jet2 < P T,jet1 Ref: no Eloss In-cone radiation: P T,jet2 = p T,jet1 Softer fragmentation Naive assumption for di-hadrons: p T,trig measures P T,jet So, z T =p T,assoc /p T,trig measures z

61. 61 d-Au Au-Au Medium density from di-hadron measurement I AA constraint D AA constraint D AA + scale uncertainty J. Nagle, WWND2008 associated  trigger  0 =1.9 GeV/fm single hadrons Medium density from away-side suppression and single hadron suppression agree Theory: ZOWW, PRL98, 212301 Data: STAR PRL 95, 152301 8 < p T,trig < 15 GeV z T =p T,assoc /p T,trig (Experiment and theory updates in the works)

62. 62 Heavy Quark Fragmentation II Significant non-perturbative effects seen even in heavy quark fragmentation

63. 63 Use e-K invariant mass to separate charm and bottom Signal: unlike-sign near-side correlations Subtract like-sign pairs to remove background Use Pythia to extract D, B yields arXiv:0903.4851 hep-ex D/B from e-K correlations B → e + D D → e + K

64. 64 Luminosity projections Also:  -hadron correlations Projection of uncertainties in Upsilon(1S) R AA for two sets of integrated luminosity.  Heavy Flavor signals study color screening with quarkonia J/Ψ

65. 65 Testing N coll scaling II: Charm PRL 94 (2005) NLO prediction: m ≈ 1.3 GeV, reasonably hard scale at p T =0 Total charm cross section scales with N bin in A+A Scaling observed in PHENIX and STAR – scaling error in one experiment?

66. 66 B D X.Y. Lin, hep-ph/0602067 Charm/bottom separation Idea: use e-h angular correlations to tag semi-leptonic D vs B decay D → e + hadrons B peak broader due to larger mass Extract B contribution by fitting:

67. 67 Charm-to-Bottom Ratio PHENIX p+p measuments agree with pQCD (FONLL) calculation arXiv:0903.4851 hep-ex

68. 68 STAR TOF – now fully installed TOF 1/β cut rejects hadrons providing nearly complete and accurate electron identification for di-lepton program. Large statistics for di-hadron, fragmentation studies Identification of decay products from charm, bottom

69. 69 STAR inner tracker upgrade (HFT) 2 30.5 ~ 30 microns pointing resolution at 0.7 GeV/c ~ 30 microns secondary vertex resolution (large p) 3 Layers: SSD: existing double sided strip detector IST: intermediate strip layer PIXEL: 2 inner layers of high resolution Pixel (MAPS) (18*18  m) and thin 0.4% X o per layer Main goal: heavy flavour spectra and v 2 at low and high p T

70. 70 PHENIX Silicon Vertex Detectors VTX: silicon VerTeX barrel tracker –Fine granularity, low occupancy 50  m×425  m pixels for L1 and L2 R1=2.5cm and R2=5cm –Stripixel detector for L3 and L4 80  m×1000  m pixel pitch R3=10cm and R4=14cm –Large acceptance |  |<1.2, almost 2  in  plane –Standalone tracking FVTX: Forward silicon VerTeX tracker –2 endcaps with 4 disks each –pixel pad structure (75  m x 2.8 to 11.2 mm) FVTX endcaps 1.2<|  |<2.7 mini strips VTX barrel |  |<1.2

71. 71 Physics Projections with HFT+TOF Charm collectivity  Medium properties, light flavor thermalization Charm energy loss  Energy loss mechanisms, Medium properties

72. 72 PHENIX VTX Performance Large suppression of heavy flavour Current result mix of b and c VTX can separate b and c Expected with VTX (0.4/nb ~3 weeks in RUN11)

73. 73 ALICE charm performance pp,  s = 14 TeV charm (D 0  K  )beauty (B  e+X) 1 year at nominal luminosity (10 9 pp events) A. Dainese Alice can measure charm from 0 < p T < 20 GeV and bottom (semi-leptonic decays) 3 < p T < 30 GeV in p+p events