1. Experimental Review of high pt phenomena at mid-rapidity at RHIC Workshop on Crititical Examination of RHIC Paradigms UT-Austin, Texas 14-17th April, 2010 Ahmed Hamed (Texas A&M University) ‏ 1

2. 2 Machine and Experiments As in typical high energy physics experiments, detectors design is driven by the delivered luminosity and DAQ. At high pt Spectra Correlations Wealth of beautiful data

3. Contents:  The Road Behind “Spectra and Correlations”  Consulting the theory Table of Contents and Disclaimer Disclaimer: The road behind is personal view, so biases are expected. 3  The Road Ahead  Baseline  Initial-State Effects  The RHIC Paradigm of Jet Suppression  Challenges to the Paradigm  Summary

4. 4 How many bodies are required before we have a problem? In eighteenth-century Newtonian mechanics, the three-body problem was insoluble. With the birth of relativity and QED, the two- and one-body problems became insoluble. And within modern QFT, the problem of zero bodies (vacuum) is insoluble. So, if we are out after exact solutions, no bodies at all is already too many! R. D. Mattuck G. E. Brown: History carries the answer

5. e -6pT Power law At mid-rapidity 5 What are high-pt particles? Power law spectra p T > 2 GeV/c Rare processes p+p P P High-pt particle “biased event” What is the functional form of pt and s? High pt particles:

6. P P 6 Hard Scattering: o High-p T particles are produced from the hard scattering processes. xP High momentum transfer Why high-pt particles are interesting? Direct access for the underlying scale Rates are calculable via pQCD “Factorization” p+p x T ~2P t /  s

7. 7 Methods of high pt particles? Full jet reconstruction Associated yield w/ high-pt particle, Fragmentation Function, I AA Detector Particle P P xP Direct access for the underlying scale Inclusive single high-pt particle spectra- Leading particle, R AA “Golden channel” direct 

8. 8 Baseline p+p “Spectra” show good agreement with NLO pQCD predictions  0 and direct  spectra Phys. Rev. Lett. 98, (2007) 012002

9. 9 Baseline p+p “full jet”STAR Jet cross section at mid-rapidity is Consistent with world data Well described by NLO pQCD

10. 10 Control experiment “d+Au”-SpectraSTAR STAR show good agreement with NLO pQCD predictions  0 and direct  spectra arXiv:0912.3838

11. 11 Au+Au-Spectra PRL. 96, 202301 (2006) Direct photons follow binary scaling  0 and  yields in central collisions are suppressed by factor of  5 compared to their yields in p+p No suppression for charged hadrons in d+Au relative to p+p PRL. 91 072304 (2003) ‏

12. 12 Au+Au-Correlations ? Suppression of the back-to-back correlation in central Au+Au STAR, PRL 91 (2003) 072304 4 2GeV trg assoc Surface-bias Central Au+Au

13. 13 Theory Support Experiment LPM-effect based approaches: BDMPS & AMY Opacity expansion: GLV & ASW Medium enhanced higher twist effects Medium modified MLLA  assumes that factorization holds and extract medium parameters. Extracted parameters indicate a medium formation with much higher energy density than that of CNM. Phenomenological studies for RHIC data

14. 14 RHIC Paradigm of jet suppression From qualitative to quantitative description, the challenges start! Hadron suppression in central Au+Au is final-state effect Suppressions took place at the partonic level Hadron emission is surface-biased Conjecture Perform similar measurements for different particles, and at different kinematics regime.

15. 15 Challenges to the Paradigm-I Heavy quarks suppress as much as light quark PRL 98 (2007)192301 STAR

16. 16 Challenges to the Paradigm-II No sign for the color factor effect. arXiv: 0804.4760 STAR preliminary STAR

17. 17 Challenges to the Paradigm-III Tangential emission ? 8 6GeV trg assoc Non interacting jets. h h STAR 8 < p T (trig) < 15 GeV/c

18. 18 Picture Evolution Surface emission h h Tangential emission or Punch-through h No color factor effect Re-appearance of away-side jet with no broadening Suppression at the hadronic level Different hadrons (  0,  ) suppress to similar level Suppression at the partonic level Direct  follows binary scaling Away-side suppressed in central AuAu but not in vacuum Final state effect and surface bias Different partons (light, heavy) suppress to similar level Final state effect and tangential emission Do we really probe the medium?

19. 19 The Theory! Medium q   E L  form <<  form >> Static Medium Dynamic Medium Scattering power of the medium ^ q   q  2  =  2 / Independent successive scattering centers 1/  << Relative phase: dE rad /dz  C R  s E dE rad /dz  C R  s ln(E) dE rad /dz  L 2 dE rad /dz  L P(  E)  F( E, L, C R, f )‏ Conjecture of energy loss functional form “independent variables” The parton propagation is “time-ordered” and time-oredered perturbation theory is the natural framework to calculate the radiation amplitude. Different models successfully describe the data with very different medium parameters ( q~3-19 GeV 2 /fm). 

20. 20 Hard Scattering in nuclear medium D vac c/h (z) ‏ p+p or peripheral Au+Au Hard Scattering in vacuum-QCD . ab h c Central Au+Au Gluon radiation is induced by multiple scattering Hard Scattering in the medium med c/h D (z) ‏ Hadron productions in nuclear medium can differ significantly from vacuum. Phys. Rev. Lett. 40, 1624 (1978). o nPDFs do not spoil the requirements for factorization up to NLO according to DIS off nuclei and Drell-Yan process on nuclear target, nPDF is universal. o What about the FFs in vacuum and in medium? Seeking fragmentation free measurements!

21. 21 LO channels of  dir The  dir -jet coincidence measurement is “golden probe” to study the parton energy loss in the medium created at RHIC. The Road Ahead Full jet reconstruction in minimum bias data Uniform spatial distribution of the hard scattering vertex  hh

22. 22 Parton Initial Energy Dependence of Energy LossSTAR STAR Different theoretical calculations describe the data within the current uncertainty No significant dependence on the parton initial energy 0-10%

23. 23 0-10% STAR Path-length Dependence of Energy Loss STAR measurement does not show path-length dependence. PHENIX measurement show path-length dependence.

24. 24  0 v 2 (pt) and R AA (  ) show statistically significant dependence on the path length particularly at pt < 6GeV Path-length Dependence of Energy Loss

25. 25 Full Jet Reconstructions “Spectra” First results of full jet reconstruction in heavy ion collisions

26. 26 Full Jet Reconstructions “Spectra” Full energy NOT recovered, jet broadened

27. 27 Full Jet Reconstructions “Correlation” No centrality dependent broadening observed within sensitivity Significant suppression of recoil Jets indicates broadening: Energy shifts to larger cone radii (>0.4) or some Jets “absorbed” in the limit

28. 28 Summary 1. Baseline and initial state effects seek improvements, eRHIC 3. More reliable theoretical calculations are required. 2. Measure the energy loss dependence on the independent variables separately  PHENIX, STAR: direct photons v2 at high-pt  PHENIX, STAR: direct photons-h correlations in- and out-of-plane  STAR,  0 -h in- and out-of-plane. 5. Extend the kinematics range to lower zt 6. Full jet reconstruction is another promising tool for the parton energy loss 4. More differential observables are needed

29. 29 “No amount of experimentation can ever prove me right; a single experiment can prove me wrong” If a model fit data, it is really great! Albert Einstein RHIC enjoys a plenty of beautiful data for very difficult problem in which vacuum means too many bodies, so careful and very critical interpretations are required.

30. 30 Backup slides

31. 31 Challenges to the Paradigm-IV

32. 32  v 2 of electromagnetically interacting particles.  Path length dependence of parton energy loss. Why v 2 of direct photons? V 2 <0 V 2 >0 V 2 =0 x y Phys.Rev. Lett. 90,132301 (2003)‏ JETP Lett. 80, 1 (2004)‏ Jet-medium photons v 2 < 0 v 2 = 0 v 2 > 0 Compton scattering Frag. photons  Production mechanisms of photons v 2 = 0No preferred direction w.r.t. reaction plane v 2 < 0 Particles preferred to traverse through the longer path “out-of-plane” v 2 > 0 Particles preferred to traverse through the shorter path “in-plane”

33. 33 Previous measurements of direct photons v 2 at RHIC Phys. Rev. Lett. 96.032302 (2006)‏ This measurements implied that v 2 of direct photons is ~ 0 PHENIX BBC: 3.1 < |  | < 3.9 Measured  0 and inclusive  : |  | < 0.35

34. 34

35. 35 New Results : Direct  -Triggered I AA vs. centrality Ahmed M. Hamed QM2009 Knoxville, Tennessee Theory describes the centrality dependence of the direct photon-triggered I AA within the current uncertainties. The p T dependence (also in other centralities) does not support the volume vs. surface emission picture within the measured range of p T associated.