Precision study of the $η\toμ^+μ^-γ$ and $ω\toμ^+μ^-π^0$ electromagnetic transition form-factors and of the $ρ\toμ^+μ^-$ 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 $|F(M)|^2 = (1 -M^2/\mathrmΛ^2)^{-2}$ we find $\mathrmΛ_η^{-2} = 1.934\ \pm\ 0.067$~(stat.) $\pm\ 0.050$~(syst.)~(GeV/$c^2$)$^{-2}$ and $\mathrmΛ_ω^{-2} = 2.223\ \pm\ 0.026$~(stat.) $\pm\ 0.037$~(syst.)~(GeV/$c^2$)$^{-2}$. An improved value of the branching ratio of the Dalitz decay $ω\to μ^+μ^-π^0$ is also obtained, with $BR(ω\to μ^+μ^-π^0) = [1.41~\pm~0.09~\mathrm{(stat.)}$ $\pm~0.15~\mathrm{(syst.)}] \times 10^{-4}$. Further results refer to the $ρ$ line shape and a new limit on $ρ/ω$ interference in hadron interactions.