Measurement of the phase between strong and electromagnetic amplitudes in the decay J/ψ → ϕη

The first direct measurement of the relative phase between the strong and electro magnetic amplitudes for a ^J/ψ decaying into a vector-pseudoscalar final state is performed using 26 energy points of e⁺e⁻ annihilation data between 3.00 GeV and 3.12 GeV. The data sets were collected by the BESIII detector with a total integrated luminosity of 452 pb⁻¹. By investigating the interference pattern in the cross section line shape of e⁺e⁻ → ϕη, the relative phase between the strong and electromagnetic amplitudes of J/ψ decay is determined to be within [133^◦, 228^◦] at 68% confidence level.