Precision study of the $\eta \to {\mu }^{+}{\mu }^{-}\gamma$ and $\omega \to {\mu }^{+}{\mu }^{-}{\pi }^0$ electromagnetic transition form-factors and of the $\rho \to {\mu }^{+}{\mu }^{-}$ 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 $\eta$, $\rho$, $\omega$, ${\eta }^{\prime }$ and $\varphi$, 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 $BR(\omega \to {\mu }^{+}{\mu }^{-}{\pi }^0)=[1.41\pm 0.09(stat.)$ $\pm 0.15(syst.)]\times 10^{-4}$. Further results refer to the $\rho$ line shape and a new limit on $\rho /\omega$ interference in hadron interactions.

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• Measurement of sigma(e+ e- -> pi+ pi-) from threshold to 0.85 GeV^2 using Initial State Radiation with the KLOE detector
• Study of the electromagnetic transition form-factors in η-> μ^+μ^-γand ω-> μ^+μ^-π^0 decays with NA60
• Evidence for the production of thermal muon pairs with masses above 1 GeV/c^2 in 158A GeV Indium-Indium Collisions
• NA60 results on the rho spectral function in In-In collisions
• First measurement of the rho spectral function in nuclear collisions
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