Dergi makalesi Açık Erişim

Measurement of the phase between strong and electromagnetic amplitudes of / Decays

2019    CERN İşbirliği

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 $100\phantom{\rule{0.25em}{0ex}}{\text{pb}}^{-1}$, 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 ${\left(84.9±3.6\right)}^{\circ }$ or ${\left(-84.7±3.1\right)}^{\circ }$ for the $2\left({\pi }^{+}{\pi }^{-}\right){\pi }^{0}$ 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 $J/\psi \to 2\left({\pi }^{+}{\pi }^{-}\right){\pi }^{0}$ is measured to be $\left(4.73±0.44\right)\text{%}$ or $\left(4.85±0.45\right)\text{%}$, and the branching fraction of $J/\psi \to \eta {\pi }^{+}{\pi }^{-}$ is measured to be $\left(3.78±0.68\right)×{10}^{-4}$. 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.