Observation of the Isospin-Violating Decay J/ψ → φπ⁰ f₀(980)
Using a sample of 1.31 x 10⁹ J/ψ events collected with the BESIII detector at the BEPCII collider, the decays J/ψ → φπ⁺π⁻π⁰ and J/ψ → φπ⁰π⁰π⁰ are investigated. The isospin-violating decay J/ψ → φπ⁰f₀(980) with f₀(980) → ππ is observed for the first time. The width of the f₀(980) obtained from the dipion mass spectrum is found to be much smaller than the world average value. In the π⁰f₀(980) mass spectrum, there is evidence of f₁(1285) production. By studying the decay J/ψ → φη', the branching fractions of η' → π⁺π⁻π⁰ and η' →π⁰π⁰π⁰, as well as their ratio, are also measured.