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. Force; N. 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

doi:10.1140/epjc/s10052-009-0878-5

2022 Impact factor 4.4

Particles and Fields

Open Access

Eur. Phys. J. C (2009) 61: 711-720
https://doi.org/10.1140/epjc/s10052-009-0878-5

Regular Article - Experimental Physics

NA60 results on thermal dimuons

NA60 Collaboration
R. Arnaldi¹¹, K. Banicz⁴^,6, K. Borer¹, J. Castor⁵, B. Chaurand⁹, W. Chen², C. Cicalò³, A. Colla¹¹, P. Cortese¹¹, S. Damjanovic⁴^*, A. David⁴^,7, A. de Falco³, A. Devaux⁵, L. Ducroux⁸, H. En’yo¹⁰, J. Fargeix⁵, A. Ferretti¹¹, M. Floris³, A. Förster⁴, P. Force⁵, N. Guettet⁴^,5, A. Guichard⁸, H. Gulkanian¹², J. M. Heuser¹⁰, M. Keil⁴^,7, L. Kluberg⁹, Z. Li², C. Lourenço⁴, J. Lozano⁷, F. Manso⁵, P. Martins⁴^,7, A. Masoni³, A. Neves⁷, H. Ohnishi¹⁰, C. Oppedisano¹¹, P. Parracho⁴^,7, P. Pillot⁸, T. Poghosyan¹², G. Puddu³, E. Radermacher⁴, P. Ramalhete⁴^,7, P. Rosinsky⁴, E. Scomparin¹¹, J. Seixas⁷, S. Serci³, R. Shahoyan⁴^,7, P. Sonderegger⁷, H. J. Specht⁶, R. Tieulent⁸, G. Usai³, R. Veenhof⁷ and H. K. Wöhri³^,7

¹ Laboratory for High Energy Physics, Bern, Switzerland
² BNL, Upton, NY, USA
³ Università di Cagliari and INFN, Cagliari, Italy
⁴ CERN, Geneva, Switzerland
⁵ LPC, Université Blaise Pascal and CNRS-IN2P3, Clermont-Ferrand, France
⁶ Physikalisches Institut der Universität Heidelberg, Heidelberg, Germany
⁷ IST-CFTP, Lisbon, Portugal
⁸ IPN-Lyon, Univ. Claude Bernard Lyon-I and CNRS-IN2P3, Lyon, France
⁹ LLR, Ecole Polytechnique and CNRS-IN2P3, Palaiseau, France
¹⁰ RIKEN, Wako, Saitama, Japan
¹¹ Università di Torino and INFN, Torino, Italy
¹² YerPhI, Yerevan, Armenia

^* e-mail: sanja.damjanovic@cern.ch

Received: 14 September 2008
Revised: 16 December 2008
Published online: 10 February 2009

Abstract

The NA60 experiment at the CERN SPS has measured muon pairs with unprecedented precision in 158 A 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.