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Surface versus volume emissions in photon-hadron correlations

Surface versus volume emissions in photon-hadron correlations. Han-Zhong Zhang Institute of Particle Physics, Huazhong Normal University, China Collaborators: J. Owens, E. Wang and X.-N. Wang. Introduction Isolated photon Gamma-tagged jet emissions in A+A collisions Conclusions.

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Surface versus volume emissions in photon-hadron correlations

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  1. Surface versus volume emissions in photon-hadron correlations Han-Zhong Zhang Institute of Particle Physics, Huazhong Normal University, China Collaborators: J. Owens, E. Wang and X.-N. Wang • Introduction • Isolated photon • Gamma-tagged jet emissions in A+A collisions • Conclusions ATHIC 2008, Tsukuba Oct. 14, 2008

  2. I. Introduction leading particle hadrons q q hadrons leading particle Leading particle suppressed hadrons q q hadrons N-N collision A-A collision leading particle suppressed What happens for a jet propagating inside QGP? Jet quenching: X.-N.Wang and M.Gyulassy, Phys.Rev.Lett.68,1480(1992) The hard jet loses a significant amount of its energy via gluon radiation induced by multiple scattering.

  3. Three kinds of hard probes of QGP • Single jet  Single hadron spectra • Dijet  Hadron-triggered away-side hadron spectra • Gamma-jet  Photon-triggered away-side hadron spectra Single jet Dijet Gamma-jet H.Z. Zhang, J.F. Owens, E. Wang and X.-N. Wang , PRL 98(2007)212301 ? Motivation

  4. Gamma-jet by NLO pQCD parton model LO : 22 tree level: NLO corrections: 22 one loop +23 tree level therefore leading to hadrons with transverse momentum larger than that of the photons

  5. FFs modification due to jet quenching The modified fragmentation functions: With energy loss without loss (X. -N. Wang , PRC70(2004)031901) The jet energy loss in a 1D expanding system: Energy loss parameter (a parameterization form of theory calculations)Enke Wang , X. -N. Wang , PRL87(2001)142301)

  6. II. Isolated photon Direct photon sources in p+p: 1) Prompt photon from hard scattering LO NLO 22 tree level Annihilation Compton 23 tree level 22 one-loop Annihilation Compton • Fragmentation or e. g. • bremsstrahlung contributions

  7. Most accompanying hadrons are within a cone of half-angle Isolation Cut (IC) Generally, the bremsstrahlung photons are always accompanied by nearly collinear hadrons on the same side. Gamma An “isolation” cut (IC) are often applied on the electromagnetic signal to separate the prompt photons from other sources. Jet J. F. Owens, Rev. Mod. Phys. 59, 465(1987); H. Baer, J. Ohnemus, and J. F. Owens, Phys. Rev. D. 42, 61(1990)

  8. NLO results for prompt photons in p+p Data with IC: PHENIX, PRL 98 (2007) 012002 “the measured photon samples … are expected to be isolated from parton jet activity.” Because of IC selected at RHIC, most fragmentation contributions from parton jets are taken out.

  9. Focus on isolated photons now If we only consider the events where the photon has no nearly collinear hadrons accompanying on the same side, high p_T photon/photon-hadron will be dominated by annihilation and Compton processes. Theory prompt photons Ex. IC photons In order to give a simple and clear jet quenching picture in A+A, we will only consider such gamma-jet events in which the photons are isolated or only contributed by annihilation and Compton processes.

  10. High p_T prompt photon dominates in central Au+Au Other sources in AA: conversion photons; induced photons; thermal photons Turbide, Gale, Jeon, Moore, Phys. Rev. C. 72 (2005) 014906 With isolation cut applied, the “clean” photons are separated from the “unclean” EM signals in A+A collisions.

  11. III. Gamma-tagged jet emissions The per-trigger photon-hadron spectra D_AA gives such a fragmentation function of the final parton jets, which is just weighted with the invariant cross sections of the correlated photons. Sometimes we call D_AA as the photon-triggered hadron fragmentation functions

  12. Gamma-triggered hadrons in p+p: Per-trigger yield as a function of the p_T of the triggered photon: NLO pQCD results describe the behavior of the data for photon-hadron production in p+p collisions. Data from “Matthew Nguyen for PHENIX, talk at QM2008”

  13. Gamma-triggered hadrons in A+A: Within the same energy loss formalism as that in our previous studies on single/dihadron spectra in A+A collisions. Data from “A. Hamed for STAR, talk at QM2008 and HP2008” Simultaneous fit of single, dihadron and photon-hadron productions.----Another evidence of jet quenching!

  14. Per-trigger yield for photon-hadron in central Au+Au Nuclear modification factor: Energy loss parameter, which is introduced to describe the uncertainty of the medium density. NLO Qualitatively, Iaa in small z_T region is slightly more sensitive to epsilon_0 than Iaa in large z_T region. LO

  15. NLO Ngh > 0 at z_T>1: surface emission At large z_T: the contributions with energy loss vanish due to jet quenching, dominated by the contributions without energy loss. For LO, the jet’s energy can’t exceed the gamma’s energy, no contributions for z_T>1 region. For NLO, because of 2->3 processes, have z_T>1 contributions. However 2->2 (tree level + one loop) dominate.

  16. For small z_T: Volume emission At small z_T: both contribute. The jets near the center with energy loss dominate.

  17. Surface versus Volume emission The averaged distance <L> for the gamma-triggered parton jets passing through the quark matter. Volume emission Surface emission Small zt probes the matter deeper than large zt, so more sensitive.

  18. Single hadron Dihadron Photon-hadron NLO More sensitive probe? Comparing small-zt gamma-jets to single jets and dijets

  19. Comparing small-zt gamma-jets to single jets Gamma-jet small zT Single jet Gam-jets for small zt probes the matter deeper than single jets.

  20. Comparing small-zt gamma-jets to dijets Gamma-jet Dijet small zT Because of punch-through jets for dihadrons, it is not sure whether small-zt gam-jets are more sensitive than dijets.

  21. Hadron-triggered FFs are greater than gamma-triggered FFs Why hadron-triggered FFs are greater than gamma-triggered FFs in pp/AA? e.g. Trig=8GeV, zt=0.5 hadr:8 jet:12 jet:12 assoc: 4 p+p: Per trigger gamm:8 jet:8 assoc: 4

  22. Hadron-triggered FFs are greater than gamma-triggered FFs Why hadron-triggered FFs is greater than gamma-triggered FFs in pp/AA? e.g. Trig=8GeV, zt=0.5 hadr:8 12 12 assoc: 4 Tangential ~pp Au+Au: Per trigger gamm:88 6 assoc: 4 Volume emission

  23. IV. Conclusions • With isolation cut applied, the “clean” photons are separated from the “unclean” EM signals in A+A collisions. With the “clean” photons tagged, the back-to-back jets are “clearly” stared for checking their energy loss. • Within the same energy loss formalism, our numerical results for photon-hadron fit data well, providing another evidence of jet quenching. • The suppression factor for hadrons with large z_T is controlled mainly by the surface emission of the gamma-jet events, while small z_T region will be volume emission bias. • Gamma-jets for small z_T region probe the dense matter deeper than those for large z_T region, so the gamma-jets for small z_T region are slightly more sensitive to the dense matter properties. Thanks for your attention!

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