First measurement of the ρ spectral function in high-energy nuclear collisions

R. Arnaldi, R. Averbeck, K. Banicz, J. Castor, B. Chaurand, C. Cicalò, A. Colla, P. Cortese, S. Damjanovic, A. David, A. De Falco, A. Devaux, A. Drees, 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, C. Lourenço, J. Lozano, F. Manso, A. Masoni, P. Martins, A. Neves, H. Ohnishi, C. Oppedisano, P. Parracho, Ph Pillot, 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

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355 Citations (Scopus)


We report on a precision measurement of low-mass muon pairs in 158 AGeV indium-indium collisions at the CERN SPS. A significant excess of pairs is observed above the yield expected from neutral meson decays. The unprecedented sample size of 360□000 dimuons and the good mass resolution of about 2% allow us to isolate the excess by subtraction of the decay sources. The shape of the resulting mass spectrum is consistent with a dominant contribution from π+π-→ρ→μ+μ- annihilation. The associated space-time averaged ρ spectral function shows a strong broadening, but essentially no shift in mass. This may rule out theoretical models linking hadron masses directly to the chiral condensate.

Original languageEnglish
Article number162302
JournalPhysical Review Letters
Issue number16
Publication statusPublished - 2006


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