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High-p T probes of QCD matter

High-p T probes of QCD matter. Marco van Leeuwen, Utrecht University. Part II: in-medium energy loss. Nuclear geometry, N bin scaling photons charm High-p T measure of energy loss Single, di-hadrons – first quantitative steps Differential tests of energy loss Path-length dependence

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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 II: in-medium energy loss • Nuclear geometry, Nbin scaling • photons • charm • High-pT measure of energy loss • Single, di-hadrons – first quantitative steps • Differential tests of energy loss • Path-length dependence • Heavy vs light (charm/beauty) • Quark vs gluon (baryon/meson)

  3. Nuclear geometry: Npart, Nbin, L, e b y L Npart: nA + nB (ex: 4 + 5 = 9 + …) Nbin: nA x nB (ex: 4 x 5 = 20 + …) • Two limits: • - Complete shadowing, each nucleon only interacts once, s Npart • No shadowing, each nucleon interact with all nucleons it encounters, s  Nbin • Soft processes: long timescale, large s,stot Npart • Hard processes: short timescale, small s, stot Nbin Transverse view Density profile r: rpart or rcoll Eccentricity x Path length L, mean <L>

  4. Centrality dependence in A+A Binary collisions weight towards small impact parameter ds/dNch 200 GeV Au+Au • Rule of thumb for A+A collisions (A>40) • 40% of the hard cross section is contained in the 10% most central collisions

  5. Testing Nbin scaling I: direct photons q + q  g + g Production through PHENIX, PRL 94, 232301 q + g  q + g Centrality RAA=1 (Ncoll scaling) for incoherent superposition of p+p collisions Direct g in p+p agree with pQCD Direct g in A+A scales with Ncoll

  6. Testing Nbin scaling II: Charm PRL 94 (2005) NLO prediction: m ≈ 1.3 GeV, reasonably hard scale at pT=0 Total charm cross section scales with Nbin in A+A Scaling observed in PHENIX and STAR – scaling error in one experiment?

  7. Energy loss in QCD matter Au+Au 200 GeV, 0-5% central g: no interactions RAA = 1 Hadrons: energy loss RAA < 1 g: RAA = 1 p0, h±: RAA≈ 0.2 D. d’Enterria ‘nuclear modification factor’ High-pT hadron production suppressed in Au+Au collisions Hard partons lose energy in the hot matter

  8. Two extreme scenarios (or how P(DE) says it all) Scenario I P(DE) = d(DE0) Scenario II P(DE) = a d(0) + b d(E) 1/Nbin d2N/d2pT ‘typical energy loss’ ‘partial transmission’ p+p Downward shift Au+Au Shifts spectrum to left pT P(DE) encodes the full energy loss process RAA not sensitive to details of mechanism … need more differential probes

  9. Radiative energy loss in QCD Energy loss process characterized by a single constant kT~m Transport coefficient l Energy loss Transport coefficient sets medium properties pQCD expectation (Baier et al) Non-perturbative: is a Wilson loop (Wiedemann) (Liu, Rajagopal, Wiedemann) From AdS/CFT e.g. N=4 SUSY: Transport coefficient is a fundamental parameter of QCD matter

  10. from inclusive hadron suppression Eskola et al. ‘04 RAA: ~ 5-15 GeV2/fm Large uncertainty – inclusive hadrons insensitive

  11. Intermediate pT – not only fragmentation M. Konno, QM06 High pT: Au+Au similar to p+p  Fragmentation dominates Large baryon/meson ratio in Au+Au ‘intermediate pT‘ Baryon/meson = 0.2-0.5 p/ ~ 1, /K ~ 2

  12. Di­hadron correlations Combinatorialbackground 8 < pTtrig < 15 GeV associated pTassoc > 3 GeV Dj trigger Near side Away side Use di-hadron correlations to probe the jet-structure in p+p, d+Au and Au+Au

  13. Di-jet kinematics Pout PTh2 PL,h PT,jet1 PTh1 PT,jet2 JT kT,xy kT measures di-jet acoplanarity JT distribution measures transverse jet profile PL,h distribution measures longitudinal jet profile Use z=pL,h/Ejet or x = ln(Ejet/pL,h)  approx indep of Ejet Di-hadron correlations: naïvely assume PTh1~PTjet1: zT = pT,h2/pTh1 Pout ~ JT Not a good approximation!

  14. Generic expectations from energy loss • Longitudinal modication: • out-of-cone  energy lost, suppression of yield, di-jet energy imbalance • in-cone  softening of fragmentation • Transverse modification • out-of-cone  increase acoplanarity kT • in-cone  broadening of jet-profile Ejet kT~m l fragmentation after energy loss?

  15. Highest pT: focus on fragmentation d+Au Au+Au 20-40% Au+Au 0-5% pTassoc > 3 GeV pTassoc > 6 GeV High-pT hadron production in Au+Au dominated by (di-)jet fragmentation Suppression of away-side yield in Au+Au collisions: energy loss

  16. Di­hadron yield suppression Away side 8 < pT,trig < 15 GeV Near side Yield in balancing jet, after energy loss Yield of additional particles in the jet STAR PRL 95, 152301 Suppression byfactor 4-5 in central Au+Au No suppression Near side: No modification  Fragmentation outside medium? Away-side: Suppressed by factor 4-5  large energy loss Note: per-trigger yields can be same with energy-loss

  17. Extracting the transport coefficient Di-hadron suppression Di-hadrons Inclusive hadrons Zhang, H et al, nucl-th/0701045 Inclusive hadron suppression Di-hadrons provide stronger constrain on density 2-minimum narrower for di-hadrons Extracted transport coefficient from singles and di-hadrons consistent 2.8 ± 0.3 GeV2/fm

  18. Model dependence of C. Loizides hep-ph/0608133v2 Di-hadrons Inclusive hadrons Zhang, H et al, nucl-th/0701045 2.8 ± 0.3 GeV2/fm Twist expansion (Wang, Wang,…)‏ Multiple soft scattering(BDMPS, Wiedemann, Salgado,…)‏ Different calculational frameworks Different approximations to the theory give significantly different results Uncertainties: • Formalism for QCD radiation • Geometry (density profile)

  19. Need to ask critical questions! Brian Cole, QM08 summary: Scrutinise theory, experiment and interpretation What do we know? What do we not know? How can we improve? Good understanding needed to communicate our resultswith other scientists (e.g. particle physicists)

  20. Interpreting di-hadron measurements Scenario I: Some lose all, Some lose nothing Scenario II: All lose something Di-hadron measurement: Away-side yield is (semi-)inclusive, so does not measure fluctuations of energy loss Multi-hadron measurements potentially more sensitive All is encoded in energy loss distribution P(DE)

  21. A closer look at azimuthal peak shapes 8 < pT(trig) < 15 GeV/c pT(assoc)>6 GeV Vitev, hep-ph/0501225 p Df Broadening due to fragments of induced radiation Induced acoplanarity (BDMPS): No away-side broadening: • No induced radiation • No acoplanrity (‘multiple-scattering’)

  22. More detailed tests of energy loss • Path length dependence • Elastic (L) vs radiative (L2) • Measurements wrt centrality, reaction plane, v2 • Mass dependence • Heavy quarks  dead cone effect • Quark/gluon differences • Color factors

  23. L scaling: elastic vs radiative T. Renk, PRC76, 064905 RAA: input to fix density Expect small away-side suppression in elastic scenario Indirect measure of path-length dependence: single hadrons and di-hadrons probe different path length distributions

  24. Path length I: centrality dependence Comparing Cu+Cu and Au+Au RAA: inclusive suppression Away-side suppression B. Sahlmüller, QM08 6 < pT trig < 10 GeV O. Catu, QM2008 Modified frag: nucl-th/0701045 - H.Zhang, J.F. Owens, E. Wang, X.N. Wang Inclusive and di-hadron suppression seem to scale with Npart Some models expect scaling, others (PQM) do not

  25. Npart scaling? PQM: no scaling of with Npart PQM - Loizides – private comunication Geometry (thickness, area) of central Cu+Cu similar to peripheral Au+Au

  26. Path length II: RAA vs L Le RAA as function of angle with reaction plane PHENIX, PRC 76, 034904 Out of Plane In Plane 3<pT<5 GeV/c Suppression depends on angle, path length

  27. RAA Le Dependence 50-60% 0-10% PHENIX, PRC 76, 034904 Au+Au collisions at 200GeV Phenomenology: RAA scales best with Le Little/no energy loss for Le< 2 fm ?

  28. Path Length III: v2 at high-pT v2 for p0 PQM: Dainese, Loizides, Paic, Eur Phys J C38, 461 Models tend to predict low v2 Agreement improves for high-pT Promising measurements, not much new in recent years Main issue: how do jets influence the event-plane (non-flow)

  29. Modelling azimuthal dependence A. Majumder, PRC75, 021901 RAA RAA pT (GeV) pT (GeV) RAA vs reaction plane sensitive to geometry model Awaiting more detailed model-data comparison

  30. Heavy-light difference light Expect: dead-cone effect M.DjordjevicPRL 94 (2004) Armesto et al, PRD71, 054027 Wicks, Horowitz et al, NPA 784, 426 Armesto plot Below 10 GeV: charm loses 20-30% less energy than u,d Bottom loses ~80% less Expected suppression of D mesons ~0.5 times light hadrons

  31. Charm RAA (using non-phot electrons) PHENIX nucl-ex/0611018 STAR nucl-ex/0607012 Measured suppression of non-photonic electrons larger than expected Larger medium density? Note: c/b ratio important • Djordjevic, Phys. Lett. B632 81 (2006) • Armesto, Phys. Lett. B637 362 (2006)

  32. Heavy flavour (charm) II PHENIX, nucl-ex/0611018 Next step: charm v2 New PHENIX data plot? Radiative energy loss underestimates v2HF Note: most HF measurements at RHIC indirect (electrons), upgrades under way for direct charm (beauty) measurements

  33. Color factors Color factors measured at LEP QCD : For SU(3) : Nc = 3 CA = 3, CF = 4/3 CA/CF=9/4 CF ~ strength of a gluon coupling to a quark CA ~ strength of the gluon self coupling TF ~ strength of gluon splitting into a quark pair Expect gluons radiate ~ twice more energy than quarks

  34. Subprocesses and quark vs gluon PYTHIA (by Adam Kocoloski) gq qq gg p+pbar dominantly from gluon fragmentation

  35. Comparing quark and gluon suppression Baryon & meson NMF PRL 97, 152301 (2006) STAR Preliminary, QM08 STAR Preliminary Curves: X-N. Wang et al PRC70(2004) 031901 Protons less suppressed than pions, not more No sign of large gluon energy loss

  36. Quark vs gluon suppression GLV formalism BDMPS formalism WHDG + renk plot Renk and Eskola, PRC76,027901 Quark/gluon difference larger in GLV than BDMPS (because of cut-off effects DE < Ejet?) ~10% baryons from quarks, so baryon/meson effect smaller than gluon/quark Are baryon fragmentation functions under control? Conclusion for now: some homework to do...

  37. So, where are we? • Single hadron and di-hadron suppression give similar medium density • Favour L2 dependence – radiative • Path length dependence • Centrality dependence and A-dependence constrain medium density models • Measurements wrt reaction plane pursued – some open issues • Heavy quark suppression similar to light quarks • Baryon suppression less or similar to mesons • No sign of quark/gluon difference Lots of data available. Time for a ‘global fit’ of the medium properties at RHIC? Specific predictions lead to specific questions That’s how we make progress in science!

  38. Properties of medium at RHIC Transport coefficient 2.8 ± 0.3 GeV2/fm (model dependent)  23 ± 4 GeV/fm3 pQCD: T  400 MeV (Baier) (Majumder, Muller, Wang)  ~5 - 15 GeV/fm3T ~ 250 - 350 MeV Viscosity Total ET  = 0.3-1fm/c (Bjorken) From v2 Lattice QCD:/s < 0.1 (Meyer) Broad agreement between different observables, and with theory A quantitative understanding of hot QCD matter is emerging

  39. Extra slides

  40. Direct photons are emitted at all stages then surviving unscathed strongly ( a << as, almost transparent to medium) Direct photon history hard scatt Cartoon only: sources of g, mean pT vs time (dashed: hadrons) pT (GeV) jet Brems. jet-thermal jet fragmentation sQGP hadron gas hadron decays log t 1 10 107 Good Write-up to read: nucl-ex/0611009 (fm/c) Cartoon from G. David, Hard Probe 2006

  41. Sources of Radiation in A+A (interaction of jet and medium) Hard scattered partons interact with thermal partons in matter • Compton scattering of hard scattered and thermal partons (Jet-photon conversion) • A recent calculation predicted yields for radiative and collisional E-loss case • This itself probes the matter on similar way as jets do. New way to look at photons? • Bremsstrahlung from hard scattered partons in medium Turbide et al., PRC72, 014906 (2005) R. Fries et al., PRC72, 041902 (2005) Turbide et al., arXiv:0712.0732 Liu et al., arXiv:0712.3619, etc.. C. Gale, NPA774(2006)335

  42. Direct photons in 200GeV Au+Au • Remember the extended “highlight plots” from PHENIX • Consistent with old published result up to ~ 12GeV/c • Direct photons suppressed at very high pT? • A theory: F. Arleo (JHEP 0609 (2006) 015) • Isospin effect, in addition to jet-quenching(BDMPS) and shadowing. • Jet-photon conversion is not taken into account • Low pT region is underestimated because of lack of jet-photon conversion?

  43. Nuclear geometry: Glauber theory for p+A Normalized nuclear density r(b,z): Nuclear thickness function Inelastic cross section for p+A: • Hard processes with large momentum transfer: • short coherence length  successive NN collisions independent • p+A is incoherent superposition of N+N collisions

  44. Glauber scaling of hard process cross sections in p(m)+A sinel for 7 GeV muons on nuclei M.May et al, Phys Rev Lett 35, 407 (1975) A1.00 NA50 Phys Lett B553, 167 A Experimental tests of Glauber scaling: sDrell-Yan/A in p+A at SPS Small/hard cross sections in p+A scale as A1.0

  45. Glauber Scaling for A+A b is suitably averaged over impact parameter distribution of events b-dependent nuclear overlap function: Hard process rate for restricted impact parameter range: Number of binary nucleon-nucleon collisions in A+B:

  46. Jet-medium Enhancement tested… • …allowing a quantative test of Jet-medium enhancement predictions • At high pT data favors standard suppression predictions, not enhancement

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