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

A bstract The first direct measurement of the relative phase between the strong and electromagnetic 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 lineshape 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.