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Lecture II: spectra and di-hadrons

Lecture II: spectra and di-hadrons. Marco van Leeuwen, Utrecht University. Lectures for Helmholtz School Feb/March 2011. Hard probes of QCD matter. Heavy-ion collisions produce ‘quasi-thermal’ QCD matter Dominated by soft partons p ~ T ~ 100-300 MeV.

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Lecture II: spectra and di-hadrons

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  1. Lecture II: spectra and di-hadrons Marco van Leeuwen, Utrecht University Lectures for Helmholtz School Feb/March 2011

  2. Hard probes of QCD matter Heavy-ion collisions produce‘quasi-thermal’ QCD matter Dominated by soft partons p ~ T ~ 100-300 MeV Hard-scatterings produce ‘quasi-free’ partons  Initial-state production known from pQCD  Probe medium through energy loss Use the strength of pQCD to explore QCD matter Sensitive to medium density, transport properties

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

  4. 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>

  5. 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

  6. Testing volume (Ncoll) scaling in Au+Au 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

  7. 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 Sizable disagreement between STAR and PHENIX – scaling error in one experiment?

  8. Generic expectations from energy loss • Longitudinal modification: • out-of-cone  energy lost, suppression of yield, di-jet energy imbalance • in-cone  softening of fragmentation • Transverse modification • out-of-cone  increase acoplanarity kT • in-cone  broadening of jet-profile Ejet kT~m l fragmentation after energy loss?

  9. Energy loss in QCD matter l radiated gluon QCD bremsstrahlung(+ LPM coherence effects) propagating parton Energy loss probes: Density of scattering centers: Nature of scattering centers, e.g. mass: radiative vs elastic loss Transport coefficient

  10. LPM interference Landau-Pomeranchuk-Migdal effect Formation time important l Zapp, QM09 radiated gluon propagating parton Radiation senses region with extent tf Lc = tf,max If l < tf, multiple scatterings add coherently: Energy loss w = 3 GeV, kT = 0.2 GeV, tf =20 fm/c Typical length in nucleus 2-5 fm

  11. 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?

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

  14. Parton energy loss and RAA modeling Qualitatively: Parton spectrum Energy loss distribution Fragmentation (function) known pQCDxPDF extract `known’ from e+e- medium effect Medium effect P(DE) is only part of the story Parton spectrum and fragmentation function are steep  non-trivial relation between RAA and P(DE)

  15. 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

  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. Geometry Density along parton path Density profile Profile at t ~ tform known Longitudinal expansion dilutes medium  Important effect Space-time evolution is taken into account in modeling

  20. Determining ASW: HT: AMY: Bass et al, PRC79, 024901 Large density: AMY: T ~ 400 MeV Transverse kick: qL ~ 10-20 GeV All formalisms give RAA ~ constant, but large differences in medium density Need more differential measurements to test formalisms At RHIC: DE large compared to E, differential measurements difficult

  21. 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?

  22. Need to ask critical questions! Brian Cole, QM08 summary: Scrutinise theory, experiment and interpretation What do we know? What do we not know? How can we improve? Good understanding needed to communicate our resultswith other scientists (e.g. particle physicists)

  23. 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

  24. 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

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

  26. 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

  27. 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 ?

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

  29. RAA vs reaction plane angle Majumder, van Leeuwen, arXiv:1002.2206 Azimuthal modulation, path length dependence largest in ASW-BDMPS But why? – No clear answer yet Data prefer ASW-BDMPS

  30. Path length dependence and v2 PHENIX PRL105, 142301 v2 at high pt due to energy loss Most calculations give too small effect

  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. 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

  33. Di-hadrons at high-pT: recoil suppression d+Au Au+Au 20-40% Au+Au 0-5% pTassoc > 3 GeV pTassoc > 6 GeV High-pT hadron production in Au+Au dominated by (di-)jet fragmentation Suppression of away-side yield in Au+Au collisions: energy loss

  34. 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 Near side: No modification  Fragmentation outside medium? Away-side: Suppressed by factor 4-5  large energy loss

  35. Interpreting di-hadron measurements Scenario I: Some lose all, Some lose nothing Scenario II: All lose something Di-hadron measurement: Away-side yield is (semi-)inclusive, sodoes not measure fluctuations of energy loss Multi-hadron measurements potentially more sensitive All is encoded in energy loss distribution P(DE)

  36. A closer look at azimuthal peak shapes 8 < pT(trig) < 15 GeV/c pT(assoc)>6 GeV Vitev, hep-ph/0501225 p Df Broadening due to fragments of induced radiation Induced acoplanarity (BDMPS): No away-side broadening: • No induced radiation • No acoplanrity (‘multiple-scattering’)

  37. Comparing single- and di-hadron results Armesto, Cacciari, Salgado et al. RAA and IAA fit with similar density Calculation uses LPM-effect, L2 dependence

  38. Surface bias Near side trigger, biases to small E-loss (No suppression seen) Away-side large L Away-side suppression IAA samples longer path lengths than inclusives RAA

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

  40. RAA at LHC ALICE PHENIX ALICE, PLB 696, 30 RAA at LHC has much stronger pT-dependence ?

  41. RAA RHIC and LHC II Overlaying the two results: PHENIX p0 and ALICE h± pT-dependence not too different… N.B.: Large uncertainties in RHIC result at high pT

  42. LHC results vs models ASW N=1 opacity DGLV N=1 opacity ASW MS Calculations: M. Verweij Radiative energy loss calculations do not reproduce the rise with pT

  43. 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)

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

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

  46. 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:

  47. Charm/bottom separation STAR, PRL 105, 202301 Combine rB and RAA to determine RAA for charm and bottom

  48. RAA for c  e and b  e pT > 5 GeV/c Combined data show: electrons from both B and D suppressed STAR, PRL 105, 202301 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

  49. 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

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

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