NA60 results on thermal dimuons

R. Arnaldi, K. Banicz, K. Borer, J. Castor, B. Chaurand, W. Chen, C. Cicalò, A. Colla, P. Cortese, S. Damjanovic, A. David, A. De Falco, A. Devaux, L. Ducroux, H. En'Yo, J. Fargeix, A. Ferretti, M. Floris, A. Förster, P. ForceN. Guettet, A. Guichard, H. Gulkanian, J. M. Heuser, M. Keil, L. Kluberg, Z. Li, C. Lourenço, J. Lozano, F. Manso, P. Martins, A. Masoni, A. Neves, H. Ohnishi, C. Oppedisano, P. Parracho, P. Pillot, T. Poghosyan, G. Puddu, E. Radermacher, P. Ramalhete, P. Rosinsky, E. Scomparin, J. Seixas, S. Serci, R. Shahoyan, P. Sonderegger, H. J. Specht, R. Tieulent, G. Usai, R. Veenhof, H. K. Wöhri

Research output: Contribution to journalArticle

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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<M<2.6 GeV. The mass spectrum for M<1 GeV is consistent with a dominant contribution from π + π -→ρ→μ + μ - annihilation. The associated ρ 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 T eff associated with the transverse momentum spectra rises with mass up to the ρ, 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 p T and the lack of polarization, is consistent with a global interpretation of the excess dimuons as thermal radiation. We conclude with first results on ω in-medium effects.

Original languageEnglish
Pages (from-to)711-720
Number of pages10
JournalEuropean Physical Journal C
Volume61
Issue number4
DOIs
Publication statusPublished - 2009 Jun 1
Externally publishedYes

ASJC Scopus subject areas

  • Engineering (miscellaneous)
  • Physics and Astronomy (miscellaneous)

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    Arnaldi, R., Banicz, K., Borer, K., Castor, J., Chaurand, B., Chen, W., Cicalò, C., Colla, A., Cortese, P., Damjanovic, S., David, A., De Falco, A., Devaux, A., Ducroux, L., En'Yo, H., Fargeix, J., Ferretti, A., Floris, M., Förster, A., ... Wöhri, H. K. (2009). NA60 results on thermal dimuons. European Physical Journal C, 61(4), 711-720. https://doi.org/10.1140/epjc/s10052-009-0878-5