Measurement of the phase between strong and electromagnetic amplitudes of $J/\psi$ decays

Abstract

Using 16 energy points of $e^{+}{e}^{-}$ annihilation data collected in the vicinity of the $J/\psi$ resonance with the BESIII detector and with a total integrated luminosity of around 100 pb${}^{-1}$, we study the relative phase between the strong and electromagnetic amplitudes of $J/\psi$ decays. The relative phase between $J\psi$ electromagnetic decay and the continuum process ($e^{+}{e}^{-}$ annihilation without the $J/\psi$ resonance) is confirmed to be zero by studying the cross section lineshape of ${\mu }^{+}{\mu }^{-}$ production. The relative phase between $J/\psi$ strong and electromagnetic decays is then measured to be $(84.9\pm 3.6{)}^{\circ }$ or $(-84.7\pm 3.1{)}^{\circ }$ for the $2({\pi }^{+}{\pi }^{-}){\pi }^0$ final state by investigating the interference pattern between the $J/\psi$ decay and the continuum process. This is the first measurement of the relative phase between $J/\psi$ strong and electromagnetic decays into a multihadron final state using the lineshape of the production cross section. We also study the production lineshape of the multihadron final state $\eta {\pi }^{+}{\pi }^{-}$ with $\eta \to {\pi }^{+}{\pi }^{-}{\pi }^0$, which provides additional information about the phase between the $J/\psi$ electromagnetic decay amplitude and the continuum process. Additionally, the branching fraction of $J/\psi \to 2({\pi }^{+}{\pi }^{-}){\pi }^0$ is measured to be $(4.73\pm 0.44)\%$ or $(4.85\pm 0.45)\%$, and the branching fraction of $J/\psi \to \eta {\pi }^{+}{\pi }^{-}$ is measured to be $(3.78\pm 0.68)\times 10^{-4}$. Both of them are consistent with the world average values. The quoted uncertainties include both statistical and systematic uncertainties, which are mainly caused by the low statistics.

References in corpus (2)

• Measurement of Γ_{ee}(J/ψ)*Br(J/ψ->e^+e^-) and Γ_{ee}(J/ψ)*Br(J/ψ->μ^+μ^-)
• On resonance parameter measurement and luminosity determination at e+e- collider

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