Study of the electromagnetic transition form-factors in η → μ+ μ- γ and ω → μ+ μ- π0 decays with NA60

R. Arnaldi, K. Banicz, J. Castor, B. Chaurand, 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. GuichardH. Gulkanian, J. M. Heuser, M. Keil, C. Lourenco, 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

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147 Citations (Scopus)


The NA60 experiment at the CERN SPS has studied low-mass muon pairs in 158 A   GeV In-In collisions. The mass and pT spectra associated with peripheral collisions can quantitatively be described by the known neutral meson decays. The high data quality has allowed to remeasure the electromagnetic transition form factors of the Dalitz decays η → μ+ μ- γ and ω → μ+ μ- π0. Using the usual pole approximation F = (1 - M2 / Λ2)- 1 for the form factors, we find Λ-2 (in GeV-2) to be 1.95 ± 0.17 (stat.) ± 0.05 (syst.) for the η and 2.24 ± 0.06 (stat.) ± 0.02 (syst.) for the ω. While the values agree with previous results from the Lepton-G experiment, the errors are greatly improved, confirming now on the level of 10σ the strong enhancement of the ω form factor beyond the expectation from vector meson dominance. An improved value of the branching ratio BR (ω → μ+ μ- π0) = [1.73 ± 0.25 (stat.) ± 0.14 (syst.)] × 10- 4 has been obtained as a byproduct.

Original languageEnglish
Pages (from-to)260-266
Number of pages7
JournalPhysics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics
Issue number5
Publication statusPublished - 2009 Jun 29
Externally publishedYes


  • Conversion decays
  • Lepton pairs
  • Transition form factor

ASJC Scopus subject areas

  • Nuclear and High Energy Physics


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