Measurement of the phase between strong and electromagnetic amplitudes in the decay $\textit{J/ψ → ϕη}$

The first direct measurement of the relative phase between the strong and electromagnetic amplitudes for a $\textit{J/ψ}$ decaying into a vector-pseudoscalar final state is performed using 26 energy points of $\textit{e}$ $\textit{e}$ annihilation data between 3$\textit{.}$00 GeV and 3.12 GeV. The data sets were collected by the BESIII detector with a total integrated luminosity of 452 pb . By investigating the interference pattern in the cross section lineshape of $\textit{e}$ $\textit{e}$ $\textit{→ ϕη}$, the relative phase between the strong and electromagnetic amplitudes of $\textit{J/ψ}$ decay is determined to be within [133$\textit{°,}$ 228$\textit{°}$] at 68% confidence level.