Measurement of the branching fractions of ψ ( 2 S ) → 3 ( π + π − ) and J / ψ → 2 ( π + π − )

Using data samples collected at {radical}(s)=3.686 GeV and 3.650 GeV by the BESII detector at the BEPC, the branching fraction of {psi}(2S){yields}3({pi}{sup +}{pi}{sup -}) is measured to be [5.45{+-}0.42(stat){+-}0.87(syst)]x10{sup -4}, and the relative branching fraction of J/{psi}{yields}2({pi}{sup +}{pi}{sup -}) to that of J/{psi}{yields}{mu}{sup +}{mu}{sup -} is measured to be [6.01{+-}0.20(stat){+-}0.48(syst)]% via {psi}(2S){yields}{pi}{sup +}{pi}{sup -}J/{psi},J/{psi}{yields}2({pi}{sup +}{pi}{sup -}). The electromagnetic form factor of 3({pi}{sup +}{pi}{sup -}) is determined to be 0.19{+-}0.02 and 0.24{+-}0.02 at {radical}(s)=3.650 GeV and 3.686 GeV, respectively.