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Jets, high-p T hadrons and prompt photons

Jets, high-p T hadrons and prompt photons. Marco van Leeuwen, Utrecht University. QM2011 student lecture 22 May 2011. Hard processes in QCD. Hard process: scale Q >> L QCD Hard scattering High-p T parton(photon) Q~p T Heavy flavour production m >> L QCD. Factorization.

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Jets, high-p T hadrons and prompt photons

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  1. Jets, high-pT hadrons and prompt photons Marco van Leeuwen, Utrecht University QM2011 student lecture 22 May 2011

  2. Hard processes in QCD • Hard process: scale Q >> LQCD • Hard scattering High-pT parton(photon) Q~pT • Heavy flavour production m >> LQCD Factorization • Cross section calculation can be split into • Hard part: perturbative matrix element • Soft part: parton density (PDF), fragmentation (FF) parton density matrix element FF QM interference between hard and soft suppressed (by Q2/L2 ‘Higher Twist’) Soft parts, PDF, FF are universal: independent of hard process

  3. Seeing quarks and gluons In high-energy collisions, observe traces of quarks, gluons (‘jets’)

  4. Fragmentation and parton showers mF MC event generators implement ‘parton showers’ Longitudinal and transverse dynamics High-energy parton (from hard scattering) Hadrons Q ~ mH ~ LQCD large Q2 Analytical calculations: Fragmentation Function D(z, m) z=ph/Ejet Only longitudinal dynamics

  5. Jet Quenching High-energy parton (from hard scattering) Hadrons • How is does the medium modify parton fragmentation? • Energy-loss: reduced energy of leading hadron – enhancement of yield at low pT? • Broadening of shower? • Path-length dependence • Quark-gluon differences • Final stage of fragmentation outside medium? 2) What does this tell us about the medium ? • Density • Nature of scattering centers? (elastic vs radiative; mass of scatt. centers) • Time-evolution?

  6. Medium-induced radition Zapp, QM09 Lc = tf,max Radiation sees length ~tf at once Landau-Pomeranchuk-Migdal effect Formation time important radiated gluon propagating parton If l < tf, multiple scatterings add coherently Energy loss depends on density: and nature of scattering centers (scattering cross section) Transport coefficient

  7. Testing volume (Ncoll) scaling in Au+Au Direct g spectra PHENIX, PRL 94, 232301 PHENIX Centrality Scaled by Ncoll Direct g in A+A scales with Ncoll A+A initial state is incoherent superposition of p+p for hard probes

  8. p0 RAA – high-pT suppression : no interactions RAA = 1 Hadrons: energy loss RAA < 1 : RAA = 1 0: RAA≈ 0.2 Hard partons lose energy in the hot matter

  9. 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 ‘Energy loss’ ‘Absorption’ p+p Downward shift Au+Au Shifts spectrum to left pT P(DE) encodes the full energy loss process RAA not sensitive to energy loss distribution, details of mechanism

  10. Four theory approaches • Multiple-soft scattering (ASW-BDMPS) • Full interference (vacuum-medium + LPM) • Approximate scattering potential • Opacity expansion (GLV/WHDG) • Interference terms order-by-order (first order default) • Dipole scattering potential 1/q4 • Higher Twist • Like GLV, but with fragmentation function evolution • Hard Thermal Loop (AMY) • Most realistic medium • LPM interference fully treated • No finite-length effects (no L2 dependence)

  11. Energy loss spectrum Typical examples with fixed L <DE/E> = 0.2 R8 ~ RAA = 0.2 Brick L = 2 fm, DE/E = 0.2 E = 10 GeV Significant probability to lose no energy (P(0)) Broad distribution, large E-loss (several GeV, up to DE/E = 1) Theory expectation: mix of partial transmission+continuous energy loss – Can we see this in experiment?

  12. Geometry Density along parton path Density profile Profile at t ~ tform known Longitudinal expansion dilutes medium  Important effect Space-time evolution is taken into account in modeling

  13. Determining ASW: HT: AMY: Bass et al, PRC79, 024901 Large density: AMY: T ~ 400 MeV Transverse kick: qL ~ 10-20 GeV All formalisms can match RAA, but large differences in medium density After long discussions, it turns out that these differences are mostly due to uncontrolled approximations in the calculations  Best guess: the truth is somewhere in-between At RHIC: DE large compared to E, differential measurements difficult

  14. RAA at LHC ALICE PHENIX ALICE, PLB 696, 30 RAA at LHC: increase with pT  first sign of sensitivity to P(DE) Larger ‘dynamic range’ at LHC very important – stay tuned

  15. RAA RHIC and LHC II Overlaying the two results: PHENIX p0 and ALICE h± pT-dependence not too different… N.B.: Large uncertainties in RHIC result at high pT

  16. Path length dependence: RAA vs L RAA as function of angle with reaction plane PHENIX, PRC 76, 034904 Out of Plane In Plane 3<pT<5 GeV/c Relation between RAA(j) and v2: Suppression depends on angle, path length

  17. Path length dependence and v2 PHENIX PRL105, 142301 v2 at high pT due to energy loss Most calculations give too small effect Path length dependence stronger than expected? Depends strongly on geometry – stay tuned

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

  19. Di-hadrons at high-pT: recoil suppression 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

  20. Di­hadron yield suppression trigger Near side associated Away side 8 < pT,trig < 15 GeV Near side Yield in balancing jet, after energy loss Yield of additional particles in the jet trigger STAR PRL 95, 152301 Away side associated Near side: No modification  Fragmentation outside medium? Away-side: Suppressed by factor 4-5  large energy loss

  21. Path length II: ‘surface bias’ Near side trigger, biases to small E-loss Away-side large L Away-side suppression IAA samples longer path-lengths than inclusives RAA

  22. L scaling: elastic vs radiative T. Renk, PRC76, 064905 Radiative scenario fits data; elastic scenarios underestimate suppression RAA: input to fix density Indirect measure of path-length dependence: single hadrons and di-hadrons probe different path length distributions Confirms L2 dependence  radiative loss dominates

  23. Intermediate pT So far, focused on high-pT Where factorisation may hold pT > 1-4 GeV • Enhanced baryon/meson ratio • Hadronisation by coalescence? • Enhanced near-side yield at large Dh ‘ridge’ • Triangular flow? • Away-side double-peak structure • Mach cone? • Triangular flow? Some other ‘puzzling’ (i.e. not dominated by jet fragmention+energy loss) observations at intermediate pT:

  24. Baryon excess STAR Preliminary B. Mohanty (STAR), QM08 High pT: Au+Au similar to p+p  Fragmentation dominates Intermediate pT, 2 – 6 GeV Large baryon/meson ration in Au+Au Baryon/meson = 0.2-0.5

  25. Hadronisation through coalescence Fries, Muller et al Hwa, Yang et al fragmenting parton: ph = z p, z<1 R. Belmont, QM09 recombining partons: p1+p2=ph Recombination of thermal (‘bulk’) partonsproduces baryons at larger pT Recombination enhancesbaryon/meson ratio Note also: v2 scaling Baryon pT=3pT,parton MesonpT=2pT,parton Hot matter

  26. Near-side Ridge  trigger 3 < pt,trig< 4 GeV/c Jet-like peak 4 < pt,trig < 6 GeV/c pt,assoc. > 2 GeV/c Au+Au 0-10% STAR preliminary Au+Au 0-10% STAR preliminary J. Putschke et al, QM06 `Ridge’: associated yield at large , small Df associated Weak dependence of ridge yield on pT,trig  Relative contribution reduces with pT,trig Ridge softer than jet – medium response  probably v3

  27. Away-side shapes Preliminary 3.0 < pTtrig < 4.0 GeV/c 4.0 < pTtrig < 6.0 GeV/c 6.0 < pTtrig < 10.0 GeV/c 1.3 < pTassoc < 1.8 GeV/c Au+Au 0-12% 0-12% M. Horner, M. van Leeuwen, et al Low pTtrig: broad shape, two peaks High pTtrig: broad shape, single peak Fragmentation becomes ‘cleaner’ as pTtrig goes up Suggests kinematic effect?

  28. v3, triangular flow Alver and Roland, PRC81, 054905 Participant fluctuations lead to triangular component of initial state anisotropy This may well be the underlying mechanism for both ‘ridge’ and ‘Mach cone’

  29. Jet reconstruction Single, di-hadrons: focus on a few fragments of the shower  No information about initial parton energy in each event Jet finding: sum up fragments in a ‘jet cone’ Main idea: recover radiated energy – determine energy of initial parton Feasibility depends on background fluctuations, angular broadening of jets Need: tracking or Hadron Calorimeter and EMCal (p0)

  30. Generic expectations from energy loss • Longitudinal modification: • 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?

  31. Modified fragmentation functions Fragmentation function ratio Fragmentation function ‘Hump-backed plateau’ A. Majumder, MvL, arXiv:1002.2206 Borghini and Wiedemann z =ln(EJet/phadron) z = ph||/Ejet Expect softening of fragmentations: fewer fragments at high pT, more at low pT

  32. Jet shapes q-Pythia, Eur Phys J C 63, 679 Energy distribution in sub-jets Energy loss changes radial distribution of energy Several ‘new’ observables considered Discussion: sensitivity  viability … ongoing

  33. Fixing the parton energy with g-jet events  Input energy loss distribution T. Renk, PRC74, 034906 Away-side spectra in g-jet Eg = 15 GeV Nuclear modification factor Away-side spectra for g-jet are sensitive to P(DE) g-jet: know jet energy  sensitive to P(DE) RAA insensitive to P(DE)

  34. Direct-g recoil suppression  8 < ET,g < 16 GeV STAR, arXiv:0912.1871 DAA(zT) IAA(zT) = Dpp(zT) Large suppression for away-side: factor 3-5 Reasonable agreement with model predictions NB: gamma pT = jet pT still not very large

  35. Thermal photons Idea: hot quark-gluon matter radiates photons which escape PHENIX, PRL 104, 132301 • Difficult measurement: • Large background p0→gg • Thermal photons at low pT Excess of photons seen at RHIC

  36. Jet reconstruction algorithms Two categories of jet algorithms: • Sequential recombination kT, anti-kT, Durham • Define distance measure, e.g. dij = min(pTi,pTj)*Rij • Cluster closest • Cone • Draw Cone radius R around starting point • Iterate until stable h,jjet = <h,j>particles Sum particles inside jet Different prescriptions exist, most natural: E-scheme, sum 4-vectors Jet is an object defined by jet algorithm If parameters are right, may approximate parton For a complete discussion, see: http://www.lpthe.jussieu.fr/~salam/teaching/PhD-courses.html

  37. Collinear and infrared safety Illustration by G. Salam • Jets should not be sensitive to soft effects (hadronisation and E-loss) • Collinear safe • Infrared safe

  38. Jet finding in heavy ion events STAR preliminary pt per grid cell [GeV] η j ~ 21 GeV Jets clearly visible in heavy ion events at RHIC Combinatorial background Needs to be subtracted • Use different algorithms to estimate systematic uncertainties: • Cone-type algorithms simple cone, iterative cone, infrared safe SISCone • Sequential recombination algorithmskT, Cambridge, inverse kT http://rhig.physics.yale.edu/~putschke/Ahijf/A_Heavy_Ion_Jet-Finder.html FastJet:Cacciari, Salam and Soyez; arXiv: 0802.1188

  39. Jet finding with background By definition: all particles end up in a jet With background: all h-j space filled with jets Many of these jets are ‘background jets’

  40. Background subtraction STAR Preliminary Background energy density (GeV) multiplicity Background density at RHIC:60-100 GeV Strong dependence on centrality Fluctuations remain after subtraction: RMS up to 10 GeV

  41. Jets at LHC LHC: jet energies up to ~200 GeV in Pb+Pb from 1 ‘short’ run Large energy asymmetry observed for central events

  42. Jets at LHC Centrality ATLAS, arXiv:1011.6182 (PRL) Large asymmetry seen for central events Jet-energy asymmetry Energy losses: tens of GeV, ~ expected from BDMPS, GLV etc beyond kinematic reach at RHIC N.B. only measures reconstructed di-jets Does not show ‘lost’ jets Large effect on recoil: qualitatively consistent with RHIC jet IAA

  43. Jets at LHC CMS, arXiv:1102.1957 CMS sees similar asymmetries

  44. Summary • Hard processes can be used to probe quark-gluon matter • So far: main focus on energy loss of (leading) high-pT hadrons • Integrates over initial energy, energy-loss • For radiative energy loss expect DEL2 • Di-hadron recoil suppression confirms this • Azimuthal dependence of energy loss (v2 at high pT) not yet quantitatively understood • Future directions: better handle on initial parton energy • Jet finding • g-jet

  45. Extra slides

  46. Hard probes of QCD matter Heavy-ion collisions produce‘quasi-thermal’ QCD matter Dominated by soft partons p ~ T ~ 100-300 MeV Hard-scatterings produce ‘quasi-free’ partons  Initial-state production known from pQCD  Probe medium through energy loss Use the strength of pQCD to explore QCD matter Sensitive to medium density, transport properties

  47. Toy model RAA p0 spectra Nuclear modification factor PHENIX, PRD 76, 051106, arXiv:0801.4020 This is a cartoon! Hadronic, not partonic energy loss No quark-gluon difference Energy loss not probabilistic P(DE) Ball-park numbers: DE/E ≈ 0.2, or DE ≈ 2 GeVfor central collisions at RHIC Note: slope of ‘input’ spectrum changes with pT: use experimental reach to exploit this

  48. Collinear safety Illustration by G. Salam Note also: detector effects, such as splitting clusters in calorimeter (p0 decay)

  49. Infrared safety Illustration by G. Salam Infrared safety also implies robustness against soft background in heavy ion collisions

  50. Shockwave/Mach Cone T. Renk, J. Ruppert Mach-cone/shockwave in the QGP? Gyulassy et al arXiv:0807.2235 Exciting possibility! Proves that QGP is really ‘bulk matter’ Measure speed of sound? B. Betz, QM09, PRC79, 034902 Are more mundane possibilities ruled out? – Not clear yet

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