Study of the electromagnetic transition form-factors in eta ---> mu+ mu- gamma and omega ---> mu+ mu- pi0 decays with NA60.

The NA60 experiment at the CERN SPS has studied low-mass muon pairs in 158A GeV In-in collisions. The mass and p(T) 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 eta -> mu(+)mu(-)gamma and omega -> mu(+)mu(-)pi(0). Using the usual pole approximation F = (1 - M-2/Lambda(2))(-1) for the form factors, we find Lambda(-2) (in GeV-2) to be 1.95 +/- 0.17(stat.) +/- 0.05(syst.) for the eta and 2.24 +/- 0.06(stat.) +/- 0.02(syst.) for the omega. While the values agree with previous results from the Lepton-G experiment, the errors are greatly improved, confirming now on the level of 10 sigma the strong enhancement of the omega form factor beyond the expectation from vector meson dominance. An improved value of the branching ratio BR(omega -> mu(+)mu(-)pi(0)) = [1.73 +/- 0.25(stat.) +/- 0.14(syst.) 1 x 10(-4) has been obtained as a byproduct.