Precision study of the η → μ+μ-γ and ω → μ+μ-π0 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 eta, rho, omega, eta' and phi, and no evidence of in-medium effects is found. A new high-precision measurement of the electromagnetic transition form factors of the eta and omega was performed, profiting from a 10 times larger data sample than the peripheral In-In sample previously collected by NA60. Using the pole-parameterisation vertical bar F(M)vertical bar(2) = (1 - M-2/Lambda(2))(-2) we find Lambda(-2)(eta) = 1.934 +/- 0.067 (stat.) +/- 0.050 (syst.) (GeV/c(2))(-2) and Lambda(-2)(omega) = 2.223 +/- 0.026 (stat.) +/- 0.037 (syst.) (GeV/c(2))(-2). An improved value of the branching ratio of the Dalitz decay omega -> mu(+)mu(-)pi(0) is also obtained, with BR(omega -> mu(+)mu(-)pi(0)) = [1.41 +/- 0.09 (stat.) +/- 0.15 (cyst.)] x 10(-4). Further results refer to the rho line shape and a new limit on rho/omega interference in hadron interactions. (C) 2016 The Authors. Published by Elsevier B.V.