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 $\eta$, $\rho$, $\omega$, $\eta'$ 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 $|F(M)|^2 = (1 -M^2/\mathrm{\Lambda}^2)^{-2}$ we find $\mathrm{\Lambda}_\eta^{-2} = 1.934\ \pm\ 0.067$~(stat.) $\pm\ 0.050$~(syst.)~(GeV/$c^2$)$^{-2}$ and $\mathrm{\Lambda}_\omega^{-2} = 2.223\ \pm\ 0.026$~(stat.) $\pm\ 0.037$~(syst.)~(GeV/$c^2$)$^{-2}$. An improved value of the branching ratio of the Dalitz decay $\omega \to \mu^+\mu^-\pi^0$ is also obtained, with $BR(\omega \to \mu^+\mu^-\pi^0) = [1.41~\pm~0.09~\mathrm{(stat.)}$ $\pm~0.15~\mathrm{(syst.)}] \times 10^{-4}$. Further results refer to the $\rho$ line shape and a new limit on $\rho/\omega$ interference in hadron interactions.
Highlights
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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
The present paper reports on a new measurement of the electromagnetic transition form-factors of the η and ω Dalitz decays, improving with a still higher precision the previous NA60 results based on peripheral In–In collisions [1]
Summary
This sample is almost a factor of 10 larger than the indium–indium peripheral data exploited in [1] With this data, a comprehensive and detailed study of the production of the light neutral mesons has been performed in p–A collisions, providing the most precise measurement currently available for the electromagnetic transition form factors in the η → μ+ μ− γ and ω → μ+ μ− π 0 decays, together with a new measurement of the branching ratio of the Dalitz decay ω → μ+ μ− π 0 , the study of the line shape of the ρ meson and the investigation of ρ /ω quantum interference effects
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