Measurement of the phase between strong and electromagnetic amplitudes of J/ψ decays

Using 16 energy points of e + e − annihilation data collected in the vicinity of the J/ψ resonance with the BESIII detector and with a total integrated luminosity of around 100pb −1 , we study the relative phase between the strong and electromagnetic amplitudes of J/ψ decays. The relative phase between J/ψ electromagnetic decay and the continuum process (e + e − annihilation without the J/ψ resonance) is confirmed to be zero by studying the cross section lineshape of μ + μ − production. The relative phase between J/ψ strong and electromagnetic decays is then measured to be (84.9±3.6) ∘ or (−84.7±3.1) ∘ for the 2(π + π − )π 0 final state by investigating the interference pattern between the J/ψ decay and the continuum process. This is the first measurement of the relative phase between J/ψ 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 ηπ + π − with η→π + π − π 0 , which provides additional information about the phase between the J/ψ electromagnetic decay amplitude and the continuum process. Additionally, the branching fraction of J/ψ→2(π + π − )π 0 is measured to be (4.73±0.44)% or (4.85±0.45)%, and the branching fraction of J/ψ→ηπ + π − is measured to be (3.78±0.68)×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.