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

R. Arnaldi , R. Averbeck , K. Banicz , J. Castor (1) , B. Chaurand (2) , C. Cicalò , A. Colla , P. Cortese , S. Damjanovic , A. David , A. de Falco , A. Devaux (1) , A. Drees , L. Ducroux (3) , H. En'Yo , J. Fargeix (1) , A. Ferretti , M. Floris , A. Förster , P. Force (1) , N. Guettet (1) , A. Guichard (3) , H. Gulkanian , J.-M. Heuser , M. Keil , L. Kluberg (2) , C. Lourenço , J. Lozano , F. Manso (1) , A. Masoni , P. Martins , A. Neves , H. Ohnishi , C. Oppedisano , P. Parracho , Philippe Pillot (3) , G. Puddu , E. Radermacher , P. Ramalhete , P. Rosinsky , E. Scomparin , J. Seixas , S. Serci , R. Shahoyan , P. Sonderegger , H. J. Specht , R. Tieulent (3) , G. Usai , R. Veenhof , H. K. Wöhri

1 LPC - Laboratoire de Physique Corpusculaire - Clermont-Ferrand
2 LLR - Laboratoire Leprince-Ringuet
3 IPNL - Institut de Physique Nucléaire de Lyon

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-->µ+µ- 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 directly to the chiral condensate.

Mots clés: quantum chromodynamics; pion-pion interactions; rho mesons; meson leptonic decay; heavy ion-nucleus reactions

Domaines: Physique des Hautes Energies - Expérience [hep-ex]

R. Arnaldi, R. Averbeck, K. Banicz, J. Castor, B. Chaurand, et al.. First Measurement of the $\rho$ Spectral Function in High-Energy Nuclear Collisions. Physical Review Letters, 2006, 96, pp.162302. ⟨10.1103/PhysRevLett.96.162302⟩. ⟨in2p3-00026024⟩