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Physical Review Letters 100 (2008) 022302
Evidence for radial flow of thermal dileptons in high-energy nuclear collisions
R. Arnaldi, K. Banicz, J. Castor1, B. Chaurand2, C. Cicalo, A. Colla, P. Cortese, S. Damjanovic, A. David, A. De Falco, A. Devaux1, L. Ducroux3, H. En'yo, J. Fargeix1, A. Ferretti, M. Floris4, A. Forster, P. Force1, N. Guettet1, A. Guichard3, H. Gulkanian, J.M. Heuser, M. Keil, L. Kluberg2, C. Lourenco2, J. Lozano, F. Manso1, P. Martins, A. Masoni, A. Neves, H. Ohnishi, C. Oppedisano, P. Parracho, P. Pillot3, T. Poghosyan, G. Puddu, E. Radermacher, P. Ramalhete, P. Rosinsky, E. Scomparin, J. Seixas, S. Serci, R. Shahoyan, P. Sonderegger, H.J. Specht, R. Tieulent3, G. Usai, R. Veenhof
NA60 Collaboration(s)

The NA60 experiment at the CERN SPS has studied low-mass dimuon production in 158 AGeV In-In collisions. An excess of pairs above the known meson decays has been reported before. We now present precision results on the associated transverse momentum spectra. The slope parameter Teff extracted from the spectra rises with dimuon mass up to the rho, followed by a sudden decline above. While the initial rise is consistent with the expectations for radial flow of a hadronic decay source, the decline signals a transition to an emission source with much smaller flow. This may well represent the first direct evidence for thermal radiation of partonic origin in nuclear collisions.
1 :  LPC - Laboratoire de Physique Corpusculaire [Clermont-Ferrand]
2 :  LLR - Laboratoire Leprince-Ringuet
3 :  IPNL - Institut de Physique Nucléaire de Lyon
4 :  INFN, Sezione di Cagliari - Istituto Nazionale di Fisica Nucleare, Sezione di Cagliari
Physique/Physique Nucléaire Expérimentale
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