Centrality-dependent pt spectra of Direct photons at RHIC F.M. Liu 刘刘刘 Central China Normal University, Ch ina T. Hirano University of Tokyo, Jap an K.Werner University of Nantes, Fran ce Y. Zhu 刘刘 Central China Normal Univers ity, China [email protected]
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Centrality-dependent pt spectra of Direct photons at RHIC
Centrality-dependent pt spectra of Direct photons at RHIC. F.M. Liu 刘复明 Central China Normal University, China T. Hirano University of Tokyo, Japan K.Werner University of Nantes, France Y. Zhu 朱燕 Central China Normal University, China. [email protected]. Outline. - PowerPoint PPT Presentation
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Centrality-dependent pt spectra of Direct photons at RHIC
F.M. Liu 刘复明 Central China Normal University, China
The PHENIX fit of pp spectrum is used by us to calculated the nuclear modification factor of direct photons, Raa.
PRL 98, 012002 (2007)
Side proof for our results ofLO + Frag_w/o_E loss in AA.
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Raa: scale, centrality & energy loss
• Data is reproduced within theory uncertainty.
• Almost independent of centrality for pt > 6 GeV/c, why?
• Visible, but not sensitive to energy loss, why?
PRL94,232302(2005);J.Phys.G34, S1015-1018,2007
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Jet quenching & bulk volume
• Parton energy loss does play a role in fragmentation contribution and JPC.
• This is centrality-dependent, similar to the suppression to pi0 production.
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Competition btw different sources
Raa is not sensitive to E loss at high pt, due to the dominance of LO.
Raa is almost independent to centrality at high pt, due to the compensation btw the two sources, JPC & frag. when collisions move from central to peripheral.
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Information from Thermal photonsEnergy density at plasma center
• The slope of pt spectrum at low pt region (dominant by thermal photons)
can reflect the temperature (energy density, …) of the bulk.
• The yields of photons (mainly low pt ) is roughly proportional to Ncoll., which reflect the bulk volume.
Raa due to thermal source
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Discussion and Conclusion• Parton energy loss does make a visible but not so significant effect in Raa
(γ) compared to Raa (π). D=0.5 is to understand.
• The modification of Raa(γ) by jet quenching is at the same degree of that
by theory scales, which is quite different from the case of Raa (π) .
• Raa(γ) is weakly dependent on centrality for pt> 6GeV/c, due to
1) the dominance of leading order contribution
2) the compensation btw JPC and fragmentation contribution.
• As a penetrate probe, thermal photons can reflect the properties of bulk, i.
e., temperature and volume via the slope of pt spectrum and the yields.
The interaction between thermal partons gives a cross check to the infor
mation offered by parton energy loss.
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Thank you!
One more word:
v2 of direct photons is expected to offer much more Information.
Our results and experiment measurements are coming soon!
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purely leading order calculation:
Isosping mixture and nuclear shadowing make an evident decrease.