Precision study of the η→μ⁺μ^-γ and ω→μ⁺μ^-π⁰ electromagnetic transition form-factors and of the →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 η, ρ, ω, η^' and φ, and no evidence of in-medium effects is found. A new high-precision measurement of the electromagnetic transition form factors of the η and ω 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-M2/Λ2)-2 we find Λη-2=1.934±0.067 (stat.) ±0.050 (syst.) (GeV/c2)-2 and Λω-2=2.223±0.026 (stat.) ±0.037 (syst.) (GeV/c2)-2. An improved value of the branching ratio of the Dalitz decay ω→μ⁺μ^-π⁰ is also obtained, with BR(ω→μ⁺μ^-π⁰)=[1.41±0.09 (stat.)±0.15 (syst.)]×10^-4. Further results refer to the ρ line shape and a new limit on ρ/ω interference in hadron interactions.