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

2. 2 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

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

4. 4 Lowering p T : gluon fragments/bulk response 3 < p t,trigger < 4 GeV p t,assoc. > 2 GeV Au+Au 0-10% STAR preliminary associated  trigger Jet-like peak `Ridge’: associated yield at large  dN/d  approx. independent of  Strong  -  asymmetry suggests coupling to longitudinal flow Long. flow J. Putschke, M. van Leeuwen, et al d+Au, 200 GeV

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

6. 6  broadening at lower p T p t,assoc. > 2 GeV L. Gaillard Gradual increase of  width over  for p T,trig < 3  width Correlation peak widths  width At low p T, ridge and jet merge  broadened peak Are these still ‘jets’?

7. 7 Separating jet and ridge: p T -spectra Jet spectra Yield (p t,assoc > p t,assoc,cut )‏ Ridge spectra Yield (p t,assoc > p t,assoc,cut )‏ p t,assoc,cut Jet (peak) spectra harden with p T,trig Peak dominated by jet fragmentationRadiated gluons ‘thermalise’ in the medium? Jet and ridge  separate dynamics inclusive Ridge yield and spectra independent of p T,trig Slope of spectra similar to inclusives J. Putschke, M. van Leeuwen, et al inclusive

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

9. 9 Hadronisation through coalescence fragmenting parton: p h = z p, z<1 recombining partons: p 1 +p 2 =p h Fries, Muller et al Hwa, Yang et al Meson p T =2p T,parton Recombination of thermal (‘bulk’) partons produces baryons at larger p T Recombination enhances baryon/meson ratios Hot matter Baryon p T =3p T,parton

10. 10 Associated yields from coalescence Baryon p T =3p T,parton Meson p T =2p T,parton Expect large baryon/meson ratio associated with high-p T trigger No associated yield with baryons from coalescence: Expect reduced assoc yield with baryon triggers 3<p T <4 GeV (Hwa, Yang) Hard parton Hot matter Baryon p T =3p T,parton Meson p T =2p T,parton Hard parton Hot matter Recombination of thermal (‘bulk’) partons ‘Shower-thermal’ recombination

11. 11 Jet-like peak: ( Λ+Λ) /2K 0 S ≈0.5 STAR Preliminary Associated baryon/meson ratios STAR Preliminary Ridge: ( Λ+Λ) /2K 0 S ≈ 1 Note: systematic error due to v 2 not shown Similar to p+p inclusive ratio Baryon/meson enhancement in the ridge? L. Gaillard, J. Bielcikova, C. Nattras et al. No shower-thermal contribution?

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

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

14. 14 0-12% 4.0 < p T trig < 6.0 GeV/c 6.0 < p T trig < 10.0 GeV/c 3.0 < p T trig < 4.0 GeV/c Preliminary Au+Au 0-12% 1.3 < p T assoc < 1.8 GeV/c Low p T trig : broad shape, two peaksHigh p T trig : broad shape, single peak Away-side shapes Fragmentation becomes ‘cleaner’ as p T trig goes up Suggests kinematic effect? M. Horner, M. van Leeuwen, et al

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

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

17. 17 Preliminary Near side yield |  |>0.9 Away side yield |  |<0.9 8 < p T trig < 15 GeV 8 < p T < 15 GeV z T =p T assoc /p T trig Energy loss in action Near- and away-side show yield enhancement at low p T Possible interpretation: di-jet → di-jet (lower Q 2 ) + gluon fragments ‘primordial process’ High-p T fragments as in vacuum Near side: ridge Away-side: broadening M. Horner, M. van Leeuwen, et al Au+Au / d+Au 8 < p T < 15 GeV Near side yield ratio z T =p T assoc /p T trig 0.2 1.0 Lower p T trig Preliminary Away side yield ratio z T =p T assoc /p T trig Au+Au / d+Au M. Horner, M. van Leeuwen, et al Lower p T trig Away-side: gradual transition to suppression at higher p T

18. 18 Intermediate p T summary Three unexpected phenomena: –Large baryon/meson ratio –Near-side ‘ridge’, peak broadening –Away-side: double-hump Is there a connection? Many ideas proposed, but difficult to model accurately Low-p T yields enhanced

19. 19 Part IV: Quantitative interpretation Again P(  E) –Sensitivity of R AA, I AA Fragmentation bias –Case study: di-trigger correlations (3-hadron)  -jet and jet measurements What can we learn about energy loss from experiment?

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

21. 21 P(  E) in a collision ~15 GeV Renk, Eskola, hep-ph/0610059 Hydro profile Di-hadron emission points Box density Radiation spectrum In a nuclear collision model, P(  E) integrates over geometry P(  E) 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. 22 RAA insensitive to P(DE) T. Renk, PRC 74, 034906 Input energy loss distributionResulting R AA Use very different (hypothetical) P(  E) distributions All ‘fit’ R AA, except  E/E = const Need more differential probes to constrain energy loss distribution R AA folds geometry, energy loss and fragmentation

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

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

25. 25 Summary so far Best achievable goal: determine P(  E) experimentally (Or at least some features of it) Difficult in practice: R AA (at RHIC) not sensitive I AA limited sensitivity (fragmentation bias)

26. 26 Comparison to Model(s) Including Systematic errors Many models explain R AA. 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) 22 11 The derived transport coefficient, the mean-4-momentum transfer 2 /mean free path, is strongly model dependent and under intense theoretical debate, e.g. see Baier,Schiff JHEP09(2006)059. also consistent with: Fit by PHENIX including systematic errors arXiv:0801.1665

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

28. 28 A very interesting new formula for the x E distribution was derived by PHENIX in PRD74 measured Ratio of jet transverse momenta 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) Relates ratio of particle p T Can be determined

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

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