Precision measurement of the branching fractions of J/ψ→π+π−π0 and ψ′→π+π−π0

We study the decays of the J / ψ and ψ ′ mesons to π + π − π 0 using data samples at both resonances collected with the BES III detector in 2009. We measure the corresponding branching fractions with unprecedented precision and provide mass spectra and Dalitz plots. The branching fraction for J / ψ → π + π − π 0 is determined to be ( 2.137 ± 0.004 ( stat. ) − 0.056 + 0.058 ( syst. ) − 0.026 + 0.027 ( norm. ) ) × 10 − 2 , and the branching fraction for ψ ′ → π + π − π 0 is measured as ( 2.14 ± 0.03 ( stat. ) − 0.07 + 0.08 ( syst. ) − 0.08 + 0.09 ( norm. ) ) × 10 − 4 . The J / ψ decay is found to be dominated by an intermediate ρ ( 770 ) state, whereas the ψ ′ decay is dominated by di-pion masses around 2.2 GeV/ c 2 , leading to strikingly different Dalitz distributions.