Using a sample of 1.31 billion $J/\psi$ events collected with the BESIII detector at the BEPCII collider, the decays $J/\psi \to \phi \pi^{ +}\pi^{ -}\pi^{ 0}$ and $J/\psi \to \phi \pi^{ 0}\pi^{ 0}\pi^{ 0}$ are investigated. The isospin violating decay $J/\psi \to \phi \pi^{ 0} f_{ 0}(980)$ with $f_{ 0}(980) \to \pi\pi$, is observed for the first time. The width of the $f_{ 0}(980)$ obtained from the dipion mass spectrum is found to be much smaller than the world average value. In the $\pi^{ 0} f_{ 0}(980)$ mass spectrum, there is evidence of $f_1(1285)$ production. By studying the decay $J/\psi \to \phi\eta'$, the branching fractions of $\eta' \to \pi^{ +}\pi^{ -}\pi^{ 0}$ and $\eta' \to \pi^{ 0}\pi^{ 0}\pi^{ 0}$, as well as their ratio, are also measured.
Categories: hep-ex