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Direct Photon-Hadron Correlations in Au+Au at 200GeV

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## Direct Photon-Hadron Correlations in Au+Au at 200GeV

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**Direct Photon-Hadron Correlations in Au+Au at 200GeV**Megan Connors For the PHENIX Collaboration Quark Matter 2009**Photon tagged jets as probes**• Photons do not interact strongly • RAA≈1 implies no medium effect • γ-h correlations should be more sensitive to modified fragmentation function and energy loss than h-h γenergy ≈ jet energy**Photon Sources**• Hadron decays • Direct photons • Compton scattering • 2 parton annihilation • Fragmentation Photons • Final state partons • Jet-thermal annihilation • Thermal radiation • Bremsstrahlung π0γγ γ γ q q g q g q Compton Annihilation At high pT prompt photons dominate! π0suppression Medium Induced**γ**q g q ? γ-jet Correlations**Measurements with Photon Tagged Jets**• Also can measure IAA, ratio of the direct photon-hadron yields X.N. Wang, Z. Huang PRC55 (1997) 3047-3061 γenergy ≈ jet energy IAA = YAA/Ypp~DAA(z)/Dpp(z)**Rg**pT [GeV] Statistical Method for Determining the Direct Photon Yield For Singles: Nincl = Ndirect + Ndecay • p0-h decayed via MC to get decay g-h For Pairs: π0 pT [GeV]**γ-h Df Correlation**• Inclusive • Decay • Direct γ-h**Away-side Yields**• Measurements have shown suppression of awayside (PHENIX arXiv:0903.3399) Run 7 ~4X statistics**Run 7 Measurements**• Suppression and modified fragmentation function**How much suppression?**• Ratio to p+p yields • Average from Run4: IAA=0.32±0.12±0.09 arXiv:0903.3399**Modified Fragmentation Function**γpT≈ jet pT IAA ~DAA(zT)/Dpp(zT)**Surface emission?**• NLO, Longitudinal expansion • Modification at low zT and suppression at high zT • Zhang et al arXiv0902.4000v1 High zT Low zT**Effect of Fluctuations**• LO, 3D hydro • ASW • Includes probability function instead of an average energy loss • Result: • Reduces the geometric dependence • Flatter IAA vs zT Priv. comm. T. R. and K. Eskola, Phys. Rev. C 75 (2007) 054910; T. R., Phys. Rev. C 78 (2008) 034904**Enhanced Low zT Multiplicity**• Analytic MLLA • Radiative Energy loss results in enhancement of soft particle multiplicity • Ratio of medium/vacuum curves is IAA Borghini and Wiedemann arXiv:hep-ph/0506218**The Comparison**• Exhibits the biggest zT dependence • Expanded awayside integration region Borghini and Wiedemann arXiv:hep-ph/0506218**Compare to p0-h**• Similar suppression in direct g-h and p0-h! See A. Adare’s Poster**Fragmentation Function**• Measurement of modified FF in Au+Au compared to p+p**Slope comparison**• Fit a universal curve for all jet energies • p+p slope of 6.89 ± 0.64 more consistent with quark fragmentation of b=8 than b=11 for gluons • Slope of Au+Au is 9.49 ±1.37 • Au+Au slope is ~1.3σ higher than p+p**Near Side Story**• Approximately zero direct γ–h yield in Au+Au • No enhancement compared to p+p direct γ–h**Conclusions**• Moving toward precision measurements of Energy loss • Similar suppression as seen in p0-h • Au+Au zT distribution appears steeper than p+p • No enhanced nearside yield observed • Not yet the final word on direct γ-h**PbSc**Electromagnetic Calorimeters PbGl How we measure γ–h correlations Drift Chamber • Trigger on a high pT (>5GeV/c) inclusive photon • Measure Df between photon and all hadrons in the event within a certain pT range • Event mixing corrects for acceptance and combinatorial pairs Δφ Underlying Event = Combinatorial Bknd + Elliptic Flow Per-Trigger Yield