Precision study of the η → μ + μ − γ and ω → μ + μ − π 0 electromagnetic transition form-factors and of the ρ → μ + μ − line shape in NA60

The NA60 experiment studied low-mass muon pair production in proton-nucleus (p-A) collisions using a 400 GeV proton beam at the CERN SPS. The low-mass dimuon spectrum is well described by the superposition of the two-body and Dalitz decays of the light neutral mesons , , , and , and no evidence of in-medium effects is found. A new high-precision measurement of the electromagnetic transition form factors of the and was performed, profiting from a 10 times larger data sample than the peripheral In-In sample previously collected by NA60. Using the pole-parameterisation |F (M)| 2 = (1 -M 2 / 2 ) -2 we find -2 = 1.934 0.067 (stat.) 0.050 (syst.) (GeV/c 2 ) -2 and -2 = 2.223 0.026 (stat.) 0.037 (syst.) (GeV/c 2 ) -2 . An improved value of the branching ratio of the Dalitz decay + - 0 is also obtained, with B R( + - 0 ) = [1.41 0.09 (stat.) 0.15 (syst.)] 10 -4 . Further results refer to the line shape and a new limit on / interference in hadron interactions.