Study of the electromagnetic transition form-factors in η-> μ^+μ^-γand ω-> μ^+μ^-π^0 decays with NA60

Abstract

The NA60 experiment at the CERN SPS has studied low-mass muon pairs in 158A 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^-γand ω-> μ^+μ^-π^0. Using the usual pole approximation F = (1-M_{μμ}^{2}/Λ^{2})^{-1} for the form factors, we find Λ^{-2} (in GeV^{-2}) to be 1.95+-0.17(stat.)+-0.05(syst.) for the ηand 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 10σthe strong enhancement of the ωform factor beyond the expectation from vector meson dominance. An improved value of the branching ratio BR(ω-> μ^+μ^-π^0) = [1.73+-0.25(stat.)+-0.14(syst.)]*10^{-4} has been obtained as a byproduct.

Submitted to Phys. Lett. B