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  1. High pT jets: quenching, Eloss, shape modification We got some good answers but what is the question??? Hot and dense matter in the RHIC-LHC era Tata Institute for Fundamental Research Feb. 12, 2008 G. David, BNL PHENIX Coll. Credits: Andrew Adare, Terry Awes, Mike McCumber, Hua Pei, Matt Nguyen, Klaus Reygers, … The PHENIX Collaboration Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  2. Why use high pT jets to get medium properties? It all starts with this picture: - if a medium is formed (and fast, O(1)fm/c) and its size is O(10)fm/c, hard-scattered partons will travel in it before fragmenting - they will interact with the medium, and lose energy, therefore, their yield at high pT will be depleted w.r.t. p+p yields (and the loss goes somewhere!) - photons will not lose energy, so in g-jet measurements they calibrate the original parton energy - such jet suppression will characterize the medium, you just have to decode it  It is as simple as that, with minor  complications - hard scattering can occur anywhere, including close to the surface - PDFs may be different in protons and ions - jets are hard to reconstruct, so we often need a proxy (leading fragment) - the lost energy flows into the vast sea of other soft particles - the calibration is tainted since hard scattering is not the only source of energetic photons - … Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  3. Trigger p0 Medium “Conditional” charged hadron at high-pt Assoc h Problems and possible ways out High pT partons fragment into jets, which are hard to reconstruct in HIC – have to rely on leading particle(s) Bulk suppression (f-integrated) Establishing the original parton energy  g-jet • In the medium initial geometry • and evolution influences DE • Bulk suppression w.r.t. reaction plane • Multiparticle correlations …and even more complex measurements Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  4. The starting point: nuclear modification factor • Hadrons are suppressed, direct photons are not • No suppression in d+Au • Evidence for parton energy loss • Static medium Run 2: (PRL 94, 232301 (2005)). • 1D expansion, e.g., GLV model This is a f-integrated, inclusive observable (“bulk suppression”). Of course it can be redefined into double, triple… differentials • RAA constrains medium properties Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  5. Improved p+p Reference Data RAA relates A+A yields to p+p yields. Where does the reference come from?  p+p 62 GeV (Run 6) J.Phys.G31:S491 (2005) PHENIX 62 GeV p+p  cross section approx. 2 times higher than ISR average. Mantra: same experiment, same systematics buys you more precision! Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  6. World data vs data from the same experiment The point: Same accelerator, same experiment, similar systematic errors  more precise mapping of the evolution (even if individual errors are relatively large) p0 RAA, 62GeV Au+Au: p0 points are the same, but the reference changed from fit to world data to our own p+p measurement Newp0 RAA, 62GeV Au+Au compared to suppression in 200GeV Au+Au If the new result survives, the physics message changes quite a bit! Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  7. pT- and centrality dependence:New p0 RAA in Au+Au and Cu+Cu at sNN = 200 GeV Cu+Cu, 200 GeV, 0-10% Cu+Cu, 200 GeV, 60-94% Spectra are similar at all centralities and p+p  RAA shapes similar (~constant)  integration makes sense Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  8. Npart dependence of p0 RAA in Au+Au at sNN = 200 GeV Parton energy loss models suggest: PHENIX, arXiv:0801.4020 [nucl-ex] Relation to RAA: Fit Npart dependence of RAA with: Centrality Dependence of RAA consistent with parton energy loss There is no end in sight: U+U will show even more suppression Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  9. System size dependence:Npart dependence of p0 RAA in Au+Au and Cu+Cu Npart scaling of RAA expected at the same sNN Indeed observed: RAA in Au+Au and Cu+Cu similar at same Npart Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  10. Energy scan / I:pT dependence of p0RAA in central Cu+Cu • 62.4, 200 GeV: • Suppression consistent with parton energy loss for pT > 3 GeV/c • 22.4 GeV: • No suppression • Enhancement consistent with calculation that describes Cronin enhancement in p+A • Parton energy loss starts to prevail over Cronin enhancement between 22.4 and 62.4 GeV PHENIX, arXiv:0801.4555 [nucl-ex] Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  11. Energy scan / II: centrality dependence of p0RAA in Cu+Cu • 62.4, 200 GeV: • Npart Dependence of RAA consistent with parton energy loss • 22.4 GeV • Enhancement independent of centrality • Possible explanations • Weak centrality dependence of Cronin enhancement • Cronin enhancement offset by parton energy loss PHENIX, arXiv:0801.4555 [nucl-ex] Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  12. Sloss: a measure of the fractional parton energy loss DE/E Centrality dependence, all energies PHENIX preliminary • RAA depends on energy loss and steepness of parton spectrum • Thus, define “fractional energy loss”: • Relation to RAA for a pion spectrum described by power law with power n Energy dependence, same Npart • RAA 0.5 – 0.6 in Pb+Pb at 17.3 GeV (0-1%, p+C reference, WA98) • However, Sloss at 17.3 GeV is much smaller than at RHIC • Au+Au, 200 GeV: Sloss = 0.2 • Pb+Pb, 17.3 GeV: Sloss = 0.05 PHENIX preliminary Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  13. Suppression: comparison of particle species:p0, h, f Mesons and Direct g in Au+Au at 200 GeV Same suppression pattern for p0 and h: Consistent with parton energy loss and fragmentation in the vacuum Larger RAA for f (and likely also w) Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  14. Getting quantitative: statistical analysis Final results (Run-4) on p0 RAA (PHENIX) Does this bulk (f-integrated) quantity really tell you something? Would it tell you something if the errors on the last points were reduced? Important: often increase in statistics not only reduces your statistical error, but opens up new ways to reduce systematic errors as well! arXiv 0801.1665 Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  15. Quantitative constraints on opacity (PQM) Experimental uncertainties only! PQM predictions (one specific implementation) for various <q> (red curve: best fit) Note: <q> is not cast in stone, it’s implementation dependent; theoretical uncertainties (much) bigger than experimental ones (Rajagopal: 4-14) PQM: radiative loss, static medium, no IS mult. scat., no mod. PDF. arXiv 0801.1665 Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  16. Quantitative constraints on gluon density (GLV) Experimental uncertainties only! GLV predictions for various dNg/dy (red curve: best fit) GLV: <L>, opacity exp., Bj. exp. medium, radiative only, IS mult. scat., mod. PDF. arXiv 0801.1665 Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  17. Quantitative constraints on gluon density (WHDG) Experimental uncertainties only! WHDG predictions for various dNg/dy (red curve: best fit) WHDG: <L>, opacity exp., Bj. exp. medium, radiative and collisional, no IS mult. scat., no mod. PDF. arXiv 0801.1665 Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  18. p0 RAA fitted with a simple straight line Slope consistent with zero: m = 0.0017 +/-0.0035 (+/- 0.0070) c/GeV (1 and 2s) 1, 2, 3s uncertainty contours With present experimental uncertainties the statement that single high pTp0 is “fragile” to opacity is not supported (more uncertainty in theories). This of course doesn’t mean that multi-differential observables should not be pursued. But they also come at a price! arXiv 0801.1665 Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  19. A case for higher statistics Higher statistics helps improve on systematic errors as well! Five highest points contribute 70% of the total c2. If the fits are limited to 5-10GeV/c, p-values increase to 55% (PQM), 36% (GLV) 17% (WHDG), 75% (linear fit) Theoretical uncertainties are much larger! Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  20. A step forward: p0 RAA vs reaction plane Double-differential RAA reveals strong pTand reaction plane (geometry) dependence  stronger constraint on energy loss models But requires more statistics (RXPN  better detector resolution is equivalent to higher statistics) Does this mean the era of bulk RAA is over? Not quite! Unbiased Still hard to interpret PRC 76 (2007) 034904 Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  21. Pathlength dependence of suppression • Approximate scaling in rLxyexpected for parton energy loss • Experimental evidence weak • Path length dependence of parton energy loss remains an open question PHENIX, PRC 76, 034904 Density time path length averaged over jet productions points in transverse (x,y) plane Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  22. RAA est mort – vive l’RAA “Theory shoot-out” at HP2006: - confronting Eloss models (mostly with PHENIX preliminary p0 RAA data)  f-integrated RAA doesn’t have enough discriminating power - theorist’s plea: give us double-differential quantities (control pathlength!) repeated several times at Jaipur (QM’08) That is a very reasonable request and we are working on it But there is a catch: - at any given moment (Run-?, RHIC-II) we have some fixed amount of data - from these, RAA can be analyzed better than RAA(f) (stats, reaction plane syst.) - the issue is not only statistics: better statistics usually brings syst. errors down Therefore, the question becomes quantitative: - what is the incremental gain in discriminating power on the theory side? - what is the incremental loss in precision on the experimental side? - which way to get maximum physics insight? Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  23. Calibrated probe – how well calibrated? The “holy grail” of jet tomography: g-jet correlations • Leading Order picture • (almost) exact momentum balance • w/ away-side jet • Compton dominance • p+p: measure gluon distribution function • A+A: • calibrated probe of energy loss • more sensitive probe than single particle spectra, di-hadron correlations • the golden channel for jet tomography? • the fine print • fragmentation photons • initial state effects (shadowing , kT) • still sensitive to geometry / space-time evolution • quark vs. gluon energy loss Very low rates: aemas Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  24. -h correlations – fragmentation photons ~D(z) •  “measures” recoil parton momentum • Measure fragmentation function D(Z) Use near side peak to determine direct  associated with hadron, i.e. fragmentation photons Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  25. 1/Ntrig dN/dDf(A. U.) 1/Ntrig dN/dDf(A. U.) 0 Direct photon – hadron Df correlations in Run-7 Au+Au 1/Ntrig dN/dDf(A. U.) Au+Au analysis is challenging: Additional sources of uncertainty from ZYAM normalization, flow subtraction and p0 combinatorial background Little or no near-side production associated with direct photon triggers Away-side yields indicate large jet suppression in g+jet channel Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  26. Dihadron correlations: system, energy, centrality dependence Away-side structure vs. beam species, beam energies, and centrality All cases: ● Peripheral similar to p-p ● Central shows development of “lobe”-like structure Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  27. Two-particle correlations – head, shoulder IAA is defined as the modification of per-trigger yield Yjet_ind, of AA relative to p+p. Strong dependence on associated pT Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  28. IAA for head and shoulder regions IAA for head and head/shoulder regions Strong partner pT dependence Jet energy redistributed via medium-jet interaction: high pT suppression, low pT enhancement SR more enhanced than HR One possibility: widening of head component: incoherent radiation, Eloss coherent radiation (Mach, Cherenkov) arXiv:0801.4545 [nucl-ex] Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  29. Shape vs centrality (Npart) nucl-ex/0611019 ● Shape saturates above 100 Npart Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  30. High pTp0-h correlations – near-side, away-side widths Near side RMS Away side RMS No significant dependence on centrality, although broadening has been predicted! (And it is in the same ballpark as p+p.) Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  31. PRL 98 212301, 2007 tangential emmision reaction plane punch- through Why the discrepancy? • Some possibilities: • Theorists are overpredicting E-loss • High pT dijets don’t probe the medium • Sizable P(DE) fluctuations  we observe mainly punch-thru • Geometric bias  we observe primarily surface emission Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  32. Change the surface-bias of near-side? Trigger p0 Trigger p0 Medium “Conditional” charged hadron at high-pt Assoc h Assoc h Select events that have botha high-pt p0and a back-to-back hadron (back-hemisphere of p0 ) Removes some events where hard-scattering occurs near surface but not tangential (large difference between path lengths) Path lengths comparable in dense medium. A.k.a., 2+1 correlations Shift distribution of hard scattering towards center of medium. Near-side parton travels through more medium Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  33. 2+1 changes near-side jets of both p+p and Cu+Cu • Per-trigger yield of p+p on near side increase with conditional particle pT. • Expected in p+p! HigherQ2comes withhigher pT away-side particle. • In Cu+Cu the yield also increases but not same slope as in p+p. Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  34. Centrality dependence of near-side yields Cu+Cu yield increases from central (left) to peripheral (right) in each bin and approaches p+p (most right point in each bin) The fact that Cu+Cu yield is reduced at central is possibly due to 1) weaker surface-bias, 2) more “+1” particles from underlying event Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  35. Summary First constraints on free parameters (gluon density, transport coefficient)  right now limited by uncertainties in the theory Jet tomography emerging, but be careful:  exclusive processes may prefer special regions of phase space RAA dominated by Cronin at SPS energies, suppression dominates at 62GeV (new Cu+Cu results) First promising results on photon-jet and fragmentation photons  the “wise’s stone”, but starving for statistics, challenge in Au+Au Measuring “excitation functions” in the same experiment (energy/species scan) is extremely important Comprehensive theoretical description is needed within one framework and theoretical uncertainties have to be estimated We already got quite a few good answers – so, what are the right questions? Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  36. Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  37. Direct photon – hadron Df correlations in p+p Df [rad] Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  38. Jets are affected by medium, on both near and far side. Medium effect on jets vary on centrality and pT. Thus, we quantify the medium effects as the suppression of jet, using per-trigger-yield, I_AA, J_AA. This suppression shows strong indication of jet particle sources at different kinetic region. 2+1 correlation brings another method of controlling jet source via the surface-bias, especially on exploring the near side jet suppression. PHENIX has the brand new 2007 Au+Au data and we are showing many more results in this QM08 and near future. Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  39. - PHENIX is measuring both Ridge and Shoulder - Shoulder & Head variation consistent with contributions of both medium response and suppressed in-vacuum jet fragmentation - Ridge and Shoulder measurements consistent with medium response, inconsistent with in-vacuum jet fragmentation - Ridge & Shoulder share much of the same behavior - appear at similar pT - similar centrality dependence - softer than p-p counterparts - baryon-meson ratios larger than jet fragmentation - balance pT - At low enough pT, some triggers must come from medium response Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  40. Medium response p+p, peripheral Au+Au central Au+Au New: - Near-side Modification – “Ridge” - Away-side Modification – “Shoulder” Typical: - Near-side Jet - Away-side Jet – “Head” Near-side Ridge theories: Boosted Excess, Backsplash, Local Heating,… Away-side Shoulder theories: Mach, Jet Survival + Recom, Scattering,… Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL

  41. Post-QM, Feb. 12-14, 2008, TIFR, Mumbai, India -- G. David, BNL