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First Measurement of the rho Spectral Function in High-Energy Nuclear Collisions

NA60 Collaboration·R. Arnaldi
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Abstract

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 pi+pi-->rho-->mu+mu- annihilation. The associated space-time averaged rho spectral function shows a strong broadening, but essentially no shift in mass. This may rule out theoretical models linking hadron masses

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NA60 Collaboration, R. Arnaldi. 2006-05-10. First Measurement of the rho Spectral Function in High-Energy Nuclear Collisions. https://doi.org/10.1103/physrevlett.96.162302

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