1 Surface （表层） versus volume （深层） emission in photon-hadron correlations Han-Zhong Zhang Institute of Particle Physics, Huazhong Normal University, China Collaborators: E. Wang, J. Owens and X.-N. Wang The international workshop for QCD/HIC July 10-12, 2008 I.Introduction II.Analysis on photon-hadron correlations III.Conclusion

2 I. Introduction Jet quenching: The hard jet loses a significant amount of its energy via radiating gluon 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)212301

4 Gamma-jets were suggested for studying jet energy loss in dense matter. X. -N. Wang, Z. Huang, and I. Sarcevic, PRL 77(1996) The NLO study of the photon-triggered away-side hadron spectra will help to obtain the detailed picture of jet quenching in the whole $z_T$ region. The sensitivity of Gamma-jets to probe the dense matter. Motivation

5 Gamma-jet by NLO pQCD parton model LO: NLO corrections: (e.g. 2  3) therefore leading to hadrons with transverse momentum larger than that of the photons

6 The fragmentation of the jets off the dense matter 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) medium vacuum

7 II. High p_T photon-triggered away-side hadron spectra within a NLO pQCD parton model in heavy ion collisions High p_T photon sources in p+p: 1) Direct photon from hard scattering Annihilation Compton 2  3 one-loop LO NLO J. F. Owens, Rev. Mod. Phys. 59, 465(1987); H. Baer, J. Ohnemus, and J. F. Owens, Phys. Rev. D. 42, 61(1990)

8 2) Fragmentation (bremsstrahlung) contributions (accompanied by nearly collinear hadrons on the same side) 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

9 isolation cuts (IC): Isolated photons in p+p at RHIC Because of IC selected at RHIC, most fragmentation contributions from parton jets are taken out. The left are mainly from annihilation and Compton processes, direct photon. PRL 98 (2007)

10 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. Only consider on annihilation and Compton photons !!! Focus on isolated photons now No consideration for energy loss of jets fragmentated into photons in AA. The left direct photons don’t encounter energy loss. Quenching picture is simply and clearly exhibited by the correlated parton jets.

11 Turbide, Gale, Jeon, Moore, Phys. Rev. C. 72 (2005) High p_T direct photon dominates in central Au+Au at RHIC Annihilation and Compton processes dominate for high p_T photons in AA.

12 Per-trigger yield for photon-hadron production in p+p 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 produced in p+p at 200GeV

13 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 Why?

14 NLO N  h > 0 at z_T>1: surface emission At large z_T: medium contributions vanish due to jet quenching, dominated by vacuum contributions. For LO, the jet’s energy can’t exceed the gamma’s energy, no contributions for z_T>1 region. For NLO, because of contributions from 2->3 processes, the jet’s energy can exceed the gamma’s energy, have z_T>1 contributions.

15 For small z_T: Volume emission At small z_T: both contribute

16 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

17 Data from “A. Hamed for STAR, talk at QM2008”

18 Single hadron Dihadron Photon-hadron More sensitive probe? NLO

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

20 Small-zt gamma-jets vs dijets Gamma-jet Dijet small z T Because of punch-through jets for dihadrons, it is not sure that small-zt gam-jets are more sensitive than dijets.

21 Comparisons between gamma-h and dihadron in pp/AuAu 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

22 Data from “A. Hamed for STAR, talk at HP2008” e.g. Trig=8GeV, zt=0.5 hadr: assoc: 4 gamm:8 8 6 assoc: 4 Au+Au: Per trigger Volume emission Tangential ~pp Comparisons between gamma-h and dihadron in pp/AuAu

23 III. Conclusions 1)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. 2) 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. Thank for your attention!

24 Thank for your attention! 谢谢！

25 Dominated by jets close and perpendicular to the surface Dominated by dijets close and tangential to the surface and the punch-through dijets dihadron Color strength: single/dihadron yield from the jets originating from the square Thickness of the outer corona single hadron Spatial transverse distribution of the initial parton production points that contribute to the single and dihadron along a given direction at RHIC 25% contribution

26 1)No parton jet energy loss 2)Isospin effects 3)Shadowing effects NLO direct photon in central Au+Au at RHIC Data from “PRL. 98 (2007) ”

27 For large z_T, “NLO I_AA ” > “LO I_AA” for large z_T

28 Parton jet energy loss per unit length: E. Wang, X. -N. Wang, PRL87(2001)142301) B. B. Back for PHOBOS, nucl-ex/ v1 Initial gluon density coefficient Energy loss parameter (a parameterization form of theory calculations)