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Lecture II: parton energy loss at high p T

Lecture II: parton energy loss at high p T. Marco van Leeuwen Utrecht University. Jyv ä skyl ä Summer School 2008. 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:

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Lecture II: parton energy loss at high p T

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  1. Lecture II: parton energy loss at high pT Marco van Leeuwen Utrecht University Jyväskylä Summer School 2008

  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 m2 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. Relativistic Heavy Ion Collider Au+Au sNN= 200 GeV PHENIX STAR RHIC: variety of beams: p+p, d+Au, Au+Au, Cu+Cu Two large experiments: STAR and PHENIX Smaller experiments: PHOBOS, BRAHMS decomissioned Recent years: Large data samples, reach to high pT

  5. STAR and PHENIX at RHIC STAR PHENIX PHENIX STAR 2p coverage, -1 < h < 1 for tracking + (coarse) EMCal Partial coverage 2 x 0.5p, -0.35 < h < 0.35 Finely segmented calorimeter + forward muon arm PID by TOF, dE/dx (STAR), RICH (PHENIX) Optimised for acceptance (correlations, jet-finding) Optimised for high-pt p0, g, e, J/y (EMCal, high trigger rates) (PHOBOS, BRAHMS more specialised)

  6. Hadron production in p+p and pQCD PRL 91, 241803 Star, PRL 91, 172302 Brahms, nucl-ex/0403005 NLO calculations: W. Vogelsang p0 and charged hadrons at RHIC in good agreement with NLO pQCD Perturbative QCD ‘works’ at RHIC energies

  7. Nuclear geometry: Npart, Nbin, L, e b y L Npart: nA + nB (ex: 4 + 5 = 9 + …) Nbin: nA x nB (ex: 4 x 5 = 20 + …) • Two limits: • - Complete shadowing, each nucleon only interacts once, s Npart • No shadowing, each nucleon interact with all nucleons it encounters, s  Nbin • Soft processes: long timescale, large s,stot Npart • Hard processes: short timescale, small s, stot Nbin Transverse view Density profile r: rpart or rcoll Eccentricity x Path length L, mean <L>

  8. Centrality examples ... and this is what you see in a presentation central peripheral mid-central This is what you really measure

  9. Centrality dependence of hard processes Total multiplicity: soft processes Binary collisions weight towards small impact parameter ds/dNch 200 GeV Au+Au • Rule of thumb for A+A collisions (A>40) • 40% of the hard cross section is contained in the 10% most central collisions

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

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

  12. 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

  13. 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 P(DE) encodes the full energy loss process RAA not sensitive to details of mechanism

  14. 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?

  15. Parton energy loss and RAA modeling Qualitatively: Parton spectrum Energy loss distribution Fragmentation (function) known pQCDxPDF extract `known’ from e+e- This is what we are after Need deconvolution to extract P(DE) Parton spectrum and fragmentation function are steep  non-trivial relation between RAA and P(DE)

  16. Determining the medium density • For each model: • Vary parameter and predict RAA • Minimize 2 wrt data • Models have different but ~equivalent parameters: • Transport coeff. • Gluon density dNg/dy • Typical energy loss per L: e0 • Coupling constant aS PHENIX, arXiv:0801.1665,J. Nagle WWND08 PQM (Loizides, Dainese, Paic),Multiple soft-scattering approx (Armesto, Salgado, Wiedemann)Realistic geometry GLV (Gyulassy, Levai, Vitev), Opacity expansion (L/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)

  17. Medium density from RAA +2.1 - 3.2 ^ PQM <q> = 13.2 GeV2/fm +0.2 - 0.5 +270 - 150 ZOWW e0 = 1.9 GeV/fm GLV dNg/dy = 1400 +0.016 - 0.012 +200 - 375 AMY as = 0.280 WHDG dNg/dy = 1400 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?

  18. Some pocket formula results GLV/WHDG: dNg/dy = 1400 T(t0) = 366 MeV PQM: (parton average) T = 1016 MeV AMY: T fixed by hydro (~400 MeV), as = 0.297 Large differences between models

  19. 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

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

  21. 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 Near side: No modification  Fragmentation outside medium? Away-side: Suppressed by factor 4-5  large energy loss Note: per-trigger yields can be same with energy-loss

  22. Medium density from di-hadron measurement associated  trigger J. Nagle, WWND2008 8 < pT,trig < 15 GeV d-Au IAA constraint DAA constraint DAA + scale uncertainty Au-Au Medium density fromaway-side suppression and single hadron suppression agree Theory: ZOWW, PRL98, 212301 e0=1.9 GeV/fm single hadrons (Experiment and theory updates in the works) Data: STAR PRL 95, 152301 zT=pT,assoc/pT,trig

  23. Conclusion so far • Hard probes experimentally accessible at RHIC • Luminosity still increasing, so more to come? • Ncoll scaling seen for g, total charm xsec • Large suppression of light hadrons  parton energy loss We have a dense, strongly interacting system in Heavy Ion collisions at RHIC But how dense? All models say: T > 300 MeV, but large spread

  24. Path length dependence I Collision geometry Au+Au Centrality Cu+Cu Out of plane <L>, density increase with centrality Vary L and density independently by changing Au+Au  Cu+Cu In-plane Change L in single system in-plane vs out of plane

  25. Path length I: centrality dependence Comparing Cu+Cu and Au+Au RAA: inclusive suppression Away-side suppression B. Sahlmüller, QM08 6 < pT trig < 10 GeV O. Catu, QM2008 Modified frag: nucl-th/0701045 - H.Zhang, J.F. Owens, E. Wang, X.N. Wang Inclusive and di-hadron suppression seem to scale with Npart Some models expect scaling, others (PQM) do not

  26. Npart scaling? PQM: no scaling of with Npart PQM - Loizides – private comunication Geometry (thickness, area) of central Cu+Cu similar to peripheral Au+Au

  27. Path length II: RAA vs L Le RAA as function of angle with reaction plane PHENIX, PRC 76, 034904 Out of Plane In Plane 3<pT<5 GeV/c Suppression depends on angle, path length

  28. RAA Le Dependence 50-60% 0-10% Au+Au collisions at 200GeV PHENIX, PRC 76, 034904 Phenomenology: RAA scales best with Le Little/no energy loss for Le< 2 fm ?

  29. Modelling azimuthal dependence A. Majumder, PRC75, 021901 RAA RAA pT (GeV) pT (GeV) RAA vs reaction plane sensitive to geometry model

  30. 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

  31. Path length III: ‘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

  32. 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

  33. Summary of L-dependence • Centrality, system size dependence as expected (r Npart) • Angle-dependence under studymore subtle, needs work • RAA vs IAA indicates L2 dependence  radiative E-loss

  34. Heavy quark suppression light Using non-photonic electrons Expected energy loss M.DjordjevicPRL 94 Wicks, Horowitz et al, NPA 784, 426 PHENIX nucl-ex/0611018, STAR nucl-ex/0607012 Expect: heavy quarks lose less energy due to dead-cone effect Most pronounced for bottom Measured suppression of non-photonic electrons larger than expected • Djordjevic, Phys. Lett. B632, 81 • Armesto, Phys. Lett. B637, 362 Radiative (+collisional) energy loss not dominant? E.g.: in-medium hadronisation/dissociation (van Hees, et al)

  35. Light flavour reference Armesto, Cacciari, Salgado et al. Note again: RAA and IAA fit same density

  36. Heavy Quark comparison No minimum – Heavy Quark suppression too large for ‘normal’ medium density

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

  38. Charm/bottom separation Combine rB and RAA to extract RAA for charm and bottom

  39. 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

  40. D/B from e-K correlations • 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 B → e + D D → e + K D → e + K arXiv:0903.4851 hep-ex

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

  42. Equalibration of rare probes • Rare probes: not chemically equilibrated in the jet spectrum. • Example 1: flavor not contained in the medium, but can be produced off the medium (e.g. photons) • Need enough yield to outshine other sources of Nrare. • Example 2: flavor chemically equilibrated in the medium • E.g. strangeness at RHIC • Coupling of jets (flavor not equilibrated) to the equilibrated medium should drive jets towards chemical equilibrium. R. Fries, QM09

  43. Equilibration process: jet conversion path length L hard parton gluon Quark Flavour of leading parton changes through interactions with medium W. Liu, R.J. Fries, Phys. Rev. C77 (2008) 054902

  44. RAA forp, K and p RAA(K) ~ 0.4 at high pT > 5.0 GeV Consistent with jet conversion calculations STAR preliminary pT (GeV)

  45. Summary • Large suppression of high-pT hadron production  partons lose energy • 4 different theoretical frameworks (radiative E-loss) • Can all describe single hadron suppression (and often di-hadron suppression) • T = 300 - 1000 MeV • Path length dependence • RAA vs reaction plane not fully understood? • RAA, IAA simultaneous fit: Strong indication of L2 dependence  radiative dominates • Heavy quarks • Expected to lose less energy (dead cone effect)Not observed ‘A lot of ins, a lot of outs’ – The Dude

  46. Extra slides

  47. Transport and medium properties Transport coefficient 2.8 ± 0.3 GeV2/fm (model dependent) e 23 ± 4 GeV/fm3 pQCD: T  400 MeV (Baier) (Majumder, Muller, Wang)  ~5 - 15 GeV/fm3T ~ 250 - 350 MeV Viscosity Total ET t0 = 0.3-1fm/c From v2 (see previous talk: Steinberg) (Bjorken) Lattice QCD: h/s < 0.1 (Meyer) Broad agreement between different observables, and with theory A quantitative understanding of hot QCD matter is emerging

  48. Kaons in p+p STAR preliminary STAR preliminary Charged and neutral kaons are extended up to 15 GeV/c in p+p collisions. Charged and neutral kaons are consistent. Phys. Rev. C 75 (2007) 64901

  49. Quark vs gluon energy loss arXiv: 0804.4760 QM08 2 AB 1 d N / dp d h path length L Eg T R hard parton = AB N 2 pp d / dp d STAR preliminary  h ~ 9/4 In pQCD: bin T Quark Eq Quark Color charge effect of parton energy loss in heavy ion collisions. Energy Loss when jet pass the medium, which is characterized by m2/l Suppression for proton > p

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