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

Abstract 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 − M 2 / Λ 2 ) − 2 we find Λ η − 2 = 1.934 ± 0.067 (stat.) ±0.050 (syst.) ( GeV / c 2 ) − 2 and Λ ω − 2 = 2.223 ± 0.026 (stat.) ±0.037 (syst.) ( GeV / c 2 ) − 2 . An improved value of the branching ratio of the Dalitz decay ω → μ + μ − π 0 is also obtained, with B R ( ω → μ + μ − π 0 ) = [ 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.