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

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 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 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 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  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 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 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 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 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 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 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 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

% 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 Note I: Large backrgounds STAR, Phys Rev Lett 95, Not quite so bad for the “Double hump” region: S/B~1/20

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

21 P(  E) in a collision ~15 GeV Renk, Eskola, hep-ph/ 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 RAA insensitive to P(DE) T. Renk, PRC 74, 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 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 Fragmentation bias PHENIX PRD74: 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 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 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:

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

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 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 Shape of x E distribution depends on and n but not on b