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Detailed measurement of the e+e pair continuum in p+p and Au+Au collisions at sNN=200 GeV and implications for direct photon production

A. Adare et al. (PHENIX Collaboration)
Phys. Rev. C 81, 034911 – Published 29 March 2010
Physics logo See Viewpoint: Taking the temperature of extreme matter

Abstract

PHENIX has measured the e+e pair continuum in sNN=200 GeV Au+Au and p+p collisions over a wide range of mass and transverse momenta. The e+e yield is compared to the expectations from hadronic sources, based on PHENIX measurements. In the intermediate-mass region, between the masses of the ϕ and the J/ψ meson, the yield is consistent with expectations from correlated c production, although other mechanisms are not ruled out. In the low-mass region, below the ϕ, the p+p inclusive mass spectrum is well described by known contributions from light meson decays. In contrast, the Au+Au minimum bias inclusive mass spectrum in this region shows an enhancement by a factor of 4.7±0.4stat±1.5syst±0.9model. At low mass (mee<0.3 GeV/c2) and high pT (1<pT<5 GeV/c) an enhanced e+e pair yield is observed that is consistent with production of virtual direct photons. This excess is used to infer the yield of real direct photons. In central Au+Au collisions, the excess of the direct photon yield over the p+p is exponential in pT, with inverse slope T=221±19stat±19syst MeV. Hydrodynamical models with initial temperatures ranging from Tinit300600 MeV at times of 0.60.15 fm/c after the collision are in qualitative agreement with the direct photon data in Au+Au. For low pT<1 GeV/c the low-mass region shows a further significant enhancement that increases with centrality and has an inverse slope of T100 MeV. Theoretical models underpredict the low-mass, low-pT enhancement.

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  • Received 1 December 2009

DOI:https://doi.org/10.1103/PhysRevC.81.034911

©2010 American Physical Society

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Taking the temperature of extreme matter

Published 29 March 2010

Researchers at the Relativistic Heavy Ion Collider have now measured the temperature of hot dense matter created in a nuclear collision.

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Vol. 81, Iss. 3 — March 2010

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