First measurement of the ϱ spectral function in nuclear collisions

R. Arnaldi; R. Averbeck; K. Banicz; J. Castor; B. Chaurand; C. Cicalo; A. Colla; P. Cortese; S. Damjanovic; A. David; A. De Falco; A. Devaux; A. Drees; L. Ducroux; H. En’yo; A. Ferretti; M. Floris; A. Foerster; P. Force; N. Guettet; A. Guichard; H. Gulkanian; J. Heuser; M. Keil; L. Kluberg; J. Lozano; C. Lourenço; F. Manso; A. Masoni; P. Martins; A. Neves; H. Ohnishi; C. Oppedisano; P. Parracho; P. 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

doi:10.1140/epjc/s10052-006-0060-2

EPJ

First measurement of the ϱ spectral function in nuclear collisions

R. Arnaldi¹⁰, R. Averbeck⁹, K. Banicz²^,4, J. Castor³, B. Chaurand⁷, C. Cicalo¹, A. Colla¹⁰, P. Cortese¹⁰, S. Damjanovic²^,4^*, A. David²^,5, A. De Falco¹, A. Devaux³, A. Drees⁹, L. Ducroux⁶, H. En’yo⁸, A. Ferretti¹⁰, M. Floris¹, A. Foerster², P. Force³, N. Guettet²^,3, A. Guichard⁶, H. Gulkanian¹¹, J. Heuser⁸, M. Keil²^,5, L. Kluberg²^,7, J. Lozano⁵, C. Lourenço², F. Manso³, A. Masoni¹, P. Martins²^,5, A. Neves⁵, H. Ohnishi⁸, C. Oppedisano¹⁰, P. Parracho², P. Pillot⁶, G. Puddu¹, E. Radermacher², P. Ramalhete², P. Rosinsky², E. Scomparin¹⁰, J. Seixas²^,5, S. Serci¹, R. Shahoyan²^,5, P. Sonderegger⁵, H.J. Specht⁴^,2, R. Tieulent⁶, G. Usai¹, R. Veenhof²^,5, H.K. Wöhri¹ and The NA60 Collaboration

¹ Univ. di Cagliari and INFN, Cagliari, Italy
² CERN, Geneva, Switzerland
³ LPC, Univ. Blaise Pascal and CNRS-IN2P3, Clermont-Ferrand, France
⁴ Univ. Heidelberg, Heidelberg, Germany
⁵ IST-CFTP, Lisbon, Portugal
⁶ IPN-Lyon, Univ. Claude Bernard Lyon-I and CNRS-IN2P3, Lyon, France
⁷ LLR, Ecole Polytechnique and CNRS-IN2P3, Palaiseau, France
⁸ RIKEN, Wako, Saitama, Japan
⁹ SUNY, Stony Brook, NY, USA
¹⁰ Univ. di Torino and INFN, Torino, Italy
¹¹ YerPhI, Yerevan, Armenia

Received: 4 August 2006
Published online: 2 December 2006

Abstract

The NA60 experiment at the CERN SPS has studied low-mass muon pairs in 158 A GeV In–In collisions. A strong excess of pairs is observed above the yield expected from neutral meson decays. The unprecedented sample size close to 400000 events and the good mass resolution of about 2% made it possible to isolate the excess by subtraction of the decay sources. The shape of the resulting mass spectrum shows some non-trivial centrality dependence, but is largely consistent with a dominant contribution from π⁺π^-→ϱ→μ⁺μ^- annihilation. The associated ϱ spectral function exhibits considerable broadening, but essentially no shift in mass. The p_T-differential mass spectra show the excess to be much stronger at low p_T than at high p_T. The results are compared to theoretical model predictions; they tend to rule out models linking hadron masses directly to the chiral condensate.

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EPJ

First measurement of the ϱ spectral function in nuclear collisions

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