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High-p T probes of QCD matter

High-p T probes of QCD matter. Marco van Leeuwen, Utrecht University. Part III: intermediate p T. Di-hadron correlations at intermediate p T Near-side: the ridge Away-side: double-bump Coalescence and identified associated hadron yields. Energy loss in a QCD medium.

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High-p T probes of QCD matter

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  1. High-pT probes of QCD matter Marco van Leeuwen, Utrecht University

  2. Part III: intermediate pT • Di-hadron correlations at intermediate pT • Near-side: the ridge • Away-side: double-bump • Coalescence and identified associated hadron yields

  3. Energy loss in a QCD medium A more complete picture Energy loss and fragmentation Or in-medium fragmentation Unmodified fragmentation after energy loss Or Fragmentation in the medium completely modified Time-scales matter In-medium energy loss Fragmentation Hadron formation time Lower pTassoc: measure radiation fragments Lower pTtrig: explore timescale

  4. Lowering pT: gluon fragments/bulk response  trigger Long. flow Long. flow d+Au, 200 GeV Au+Au 0-10% STAR preliminary Jet-like peak associated 3 < pt,trigger < 4 GeV pt,assoc. > 2 GeV J. Putschke, M. van Leeuwen, et al `Ridge’: associated yield at large  dN/d approx. independent of  Strong - asymmetry suggests coupling to longitudinal flow

  5. Near-side/ridge shape in more detail pt,assoc. > 2 GeV STAR Preliminary STAR preliminary yield(in  window  window center Ridge yield approx. independent of deta for 1< Dh < 2 Ridge also visible for larger pTtrig ( 4-6 GeV and 6-12 GeV )

  6. Dh broadening at lower pT Correlation peak widths pt,assoc. > 2 GeV Dh width Dj width L. Gaillard Gradual increase of Dh width over Dj for pT,trig < 3 At low pT, ridge and jet merge  broadened peak Are these still ‘jets’?

  7. Separating jet and ridge: pT-spectra Jet spectra Ridge spectra Yield (pt,assoc > pt,assoc,cut)‏ Yield (pt,assoc > pt,assoc,cut)‏ J. Putschke, M. van Leeuwen, et al inclusive inclusive pt,assoc,cut pt,assoc,cut Ridge yield and spectra independent of pT,trig Slope of spectra similar to inclusives Jet (peak) spectra harden with pT,trig Peak dominated by jet fragmentation Radiated gluons ‘thermalise’ in the medium? Jet and ridge  separate dynamics

  8. Baryon enhancement M. Konno, QM06 High pT: Au+Au similar to p+p  Fragmentation dominates Large baryon/meson ratio in Au+Au ‘intermediate pT‘ p/ ~ 1, /K ~ 2 Hadronisation by coalescence?3-quark pT-sum wins over fragmentation

  9. Hadronisation through coalescence Fries, Muller et al Hwa, Yang et al fragmenting parton: ph = z p, z<1 recombining partons: p1+p2=ph Recombination of thermal (‘bulk’) partonsproduces baryons at larger pT Baryon pT=3pT,parton MesonpT=2pT,parton Recombination enhancesbaryon/meson ratios Hot matter

  10. Associated yields from coalescence Recombination of thermal (‘bulk’) partons ‘Shower-thermal’ recombination Baryon pT=3pT,parton MesonpT=2pT,parton Baryon pT=3pT,parton MesonpT=2pT,parton Hot matter Hot matter Hard parton (Hwa, Yang) Hard parton No associated yield with baryons from coalescence: Expect reduced assoc yield with baryon triggers 3<pT<4 GeV Expect large baryon/meson ratio associated with high-pT trigger

  11. Associated baryon/meson ratios STAR Preliminary L. Gaillard, J. Bielcikova, C. Nattras et al. STAR Preliminary Jet-like peak:(Λ+Λ)/2K0S≈0.5 Ridge:(Λ+Λ)/2K0S≈ 1 Note: systematic error due to v2 not shown Similar to p+p inclusive ratio No shower-thermal contribution? Baryon/meson enhancement in the ridge?

  12. Associated baryon/meson ratios STAR Preliminary STAR Preliminary pTtrig > 4.0 GeV/c 2.0 < pTAssoc< pTtrig Jet-peak Ridge region p/p ratio in jet-peak < inclusive p/p ratio in ridge > inclusive Ridge and jet-peak have different hadro-chemistry, different production mechanism

  13. More medium effects: away-side Mach Cone/Shock wave T. Renk, J. Ruppert Gluon radiation+Sudakov Stöcker, Casseldery-Solana et al A. Polosa, C. Salgado Medium response (shock wave)or gluon radiation with kinematic constraints? (other proposals exist as well: kT-type effect or Cherenkov radiation) 3.0 < pTtrig < 4.0 GeV/c 1.3 < pTassoc < 1.8 GeV/c Trigger particle Au+Au 0-10% d+Au M. Horner, M. van Leeuwen, et al Near side: Enhanced yield in Au+Au consistent with ridge-effect Away-side: Strong broadening in central Au+Au ‘Dip’ at  = 

  14. Away-side shapes Preliminary 3.0 < pTtrig < 4.0 GeV/c 4.0 < pTtrig < 6.0 GeV/c 6.0 < pTtrig < 10.0 GeV/c 1.3 < pTassoc < 1.8 GeV/c Au+Au 0-12% 0-12% M. Horner, M. van Leeuwen, et al Low pTtrig: broad shape, two peaks High pTtrig: broad shape, single peak Fragmentation becomes ‘cleaner’ as pTtrig goes up Suggests kinematic effect?

  15. Note I: Large backrgounds 214 213 212 211 STAR, Phys Rev Lett 95, 152301 STAR, Phys Rev Lett 95, 152301 Not quite so bad for the “Double hump” region: S/B~1/20

  16. Note II: background also has a shape After subtraction Assoc hadron distribution C. Pruneau, QM06 Flow background Δ12 ‘Ad hoc’ approach: Zero (jet) Yield at Minimum (ZYAM) Is it a good approximation? Could background (flow) be modified by jet?

  17. Energy loss in action Near side yield Away side yield Away side yield ratio Near side yield ratio |Dj|<0.9 |Dj|>0.9 8 < pT < 15 GeV 8 < pTtrig < 15 GeV Au+Au / d+Au Au+Au / d+Au 8 < pT < 15 GeV Preliminary Preliminary M. Horner, M. van Leeuwen, et al zT=pTassoc/pTtrig zT=pTassoc/pTtrig zT=pTassoc/pTtrig zT=pTassoc/pTtrig Preliminary Lower pTtrig Lower pTtrig 1.0 M. Horner, M. van Leeuwen, et al 0.2 Near- and away-side show yield enhancement at low pT Away-side: gradual transition to suppression at higher pT Possible interpretation: di-jet → di-jet (lower Q2) + gluon fragments ‘primordial process’ Near side: ridge Away-side: broadening High-pT fragmentsas in vacuum

  18. Intermediate pT summary • Three unexpected phenomena: • Large baryon/meson ratio • Near-side ‘ridge’, peak broadening • Away-side: double-hump Low-pT yields enhanced Is there a connection? Many ideas proposed, but difficult to model accurately

  19. Part IV: Quantitative interpretation What can we learn about energy loss from experiment? • Again P(DE) • Sensitivity of RAA, IAA • Fragmentation bias • Case study: di-trigger correlations (3-hadron) • g-jet and jet measurements

  20. Radiation spectrum P(DE) Salgado and Wiedemann, RD68, 014008 Radiation spectrum calculated in pQCD Subject to approximations, uncertainties Can we measure this in experiment? Broad distribution, expect large fluctuations in energy loss

  21. P(DE) in a collision ~15 GeV Renk, Eskola, hep-ph/0610059 Radiation spectrum Di-hadron emission points Hydro profile Box density In a nuclear collision model, P(DE) integrates over geometry • P(DE) is a very broad distribution: • Need large kinematic reach to measure the distribution • Width dominated by intrinsic process ‘surface bias’ not such a useful concept

  22. RAA insensitive to P(DE) Input energy loss distribution Resulting RAA T. Renk, PRC 74, 034906 Use very different (hypothetical)P(DE) distributions All ‘fit’ RAA, except DE/E = const RAA folds geometry, energy loss and fragmentation Need more differential probes to constrain energy loss distribution

  23. IAA insensitive to P(DE) T.Renk, PRC Away-side slope: some sensitivity to medium density model (black core model deviates) Still limited sensitivity to P(DE)

  24. Fragmentation bias PHENIX PRD74: 072002 LEP: Quarks: D(z) ~ exp(-8.2 z) Gluons: D(z) ~ exp(-11.4 z) For exp(-b z) fragmentation: Small difference in dN/dxE or dN/dzT from large difference in D(z) slopes Shape determined by power-law exponent n Explains similarity of zT-slopes in d+Au and Au+Au In other words: di-hadron correlations do not constrain the parton energy  Limited sensitivity to P(DE) For exponential fragmentation

  25. Summary so far Best achievable goal: determine P(DE) experimentally (Or at least some features of it) Difficult in practice: RAA (at RHIC) not sensitive IAA limited sensitivity (fragmentation bias)

  26. Comparison to Model(s) Including Systematic errors 2 1 also consistent with: Many models explain RAA. All have different assumptions about nuclear overlap geometry, medium expansion, parton propagation, etc, and use a parameter to characterize the medium. For example, we give a fit to the PQM model, Dainese, Loizides,Paic, EPJC38, 461 (2005) The derived transport coefficient , the mean-4-momentum transfer2/mean free path, is strongly model dependent and under intense theoretical debate, e.g. see Baier,Schiff JHEP09(2006)059. Fit by PHENIX including systematic errors arXiv:0801.1665

  27. Zhang, Owens Wang, Wang Model Zhang, Owens, Wang and Wang, PRL 98 (2007) 212301 found in their model, 0=1.6-2.1 GeV/fm 2 1 Fit by PHENIX including systematic errors arXiv:0801.1665 Again a precision of 20-25% (1)

  28. A very interesting new formula for the xE distribution was derived by PHENIX in PRD74 Relates ratio of particle pT Ratio of jet transverse momenta measured Can be determined If formula works, we can also use it in Au+Au to determine the relative energy loss of the away jet to the trigger jet (surface biased by large n)

  29. Exponential Frag. Fn. and power law crucial Fragment spectrum given pTt is weighted to high zt by ztn-2 Incomplete gamma function Bjorken parent-child relation: parton and particle invariant pT spectra have same power n, etc.

  30. Shape of xE distribution depends on and n but not on b 1.0 0.8 0.6 0.4 0.2

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