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NA60 results on thermal dimuons

Abstract : The NA60 experiment at the CERN SPS has measured muon pairs with unprecedented precision in 158A GeV In-In collisions. A strong excess of pairs above the known sources is observed in the whole mass region 0.2 rho -> mu+mu- annihilation. The associated rho spectral function shows a strong broadening, but essentially no shift in mass. For M>1 GeV, the excess is found to be prompt, not due to enhanced charm production, with pronounced differences to Drell-Yan pairs. The slope parameter Teff associated with the transverse momentum spectra rises with mass up to the rho, followed by a sudden decline above. The rise for M<1 GeV is consistent with radial flow of a hadronic emission source. The seeming absence of significant flow for M>1 GeV and its relation to parton-hadron duality is discussed in detail, suggesting a dominantly partonic emission source in this region. A comparison of the data to the present status of theoretical modeling is also contained. The accumulated empirical evidence, including also a Planck-like shape of the mass spectra at low pT and the lack of polarization, is consistent with a global interpretation of the excess dimuons as thermal radiation. We conclude with first results on omega in-medium effects.
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http://hal.in2p3.fr/in2p3-00373113
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Submitted on : Friday, April 3, 2009 - 1:58:27 PM
Last modification on : Thursday, March 19, 2020 - 9:55:36 AM

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R. Arnaldi, K. Banicz, K. Borer, J. Castor, B. Chaurand, et al.. NA60 results on thermal dimuons. Electromagnetic Probes of High-Energy Nuclear Collisions, Hard Probes 2008, Jun 2008, Illa da toxa, Spain. pp.711-720, ⟨10.1140/epjc/s10052-009-0878-5⟩. ⟨in2p3-00373113⟩

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