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 ATHIC 2008, Tsukuba Oct. 14, 2008 I.Introduction II.Isolated photon III.Gamma-tagged jet emissions in A+A collisions IV.Conclusions

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

3 Three kinds of hard probes of QGP 1) Single jet  Single hadron spectra 2) Dijet  Hadron-triggered away-side hadron spectra 3) 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) 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 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) (X. -N. Wang, PRC70(2004)031901) With energy loss without loss The modified fragmentation functions:

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

7 J. F. Owens, Rev. Mod. Phys. 59, 465(1987); H. Baer, J. Ohnemus, and J. F. Owens, Phys. Rev. D. 42, 61(1990) 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. Jet Gamma An “isolation” cut (IC) are often applied on the electromagnetic signal to separate the prompt photons from other sources.

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

9 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. Focus on isolated photons now 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. Theory prompt photons Ex. IC photons

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

11 III. Gamma-tagged jet emissions 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 The per-trigger photon-hadron spectra

12 Data from “Matthew Nguyen for PHENIX, talk at QM2008” 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. Gamma-triggered hadrons in p+p:

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

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

15 NLO N  h > 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 The averaged distance for the gamma-triggered parton jets passing through the quark matter. Surface versus Volume emission Small zt probes the matter deeper than large zt, so more sensitive. Surface emission Volume emission

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

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

20 Comparing small-zt gamma-jets to dijets Gamma-jet Dijet small z T 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 e.g. Trig=8GeV, zt=0.5 hadr:8 jet:12 jet:12 assoc: 4 gamm:8 jet:8 assoc: 4 p+p: Per trigger Why hadron-triggered FFs are greater than gamma-triggered FFs in pp/AA?

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

23 IV. Conclusions 1)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. 2)Within the same energy loss formalism, our numerical results for photon- hadron fit data well, providing another evidence of jet quenching. 3)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. 4)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!