The ρ − π Puzzle of J / ψ and ψ ′ Decays March 23 , 1999

The recent BES Collaboration data on ψ ′ → P V , particularly the isospin violating mode ψ ′ → π 0 ω 0 and finding of a finite number for B(ψ ′ → K * 0 K 0), enable us now to deal more precisely about the challenges to theory concerning this extraordinary and remarkable so called ρ − π Puzzle of J/ψ and ψ ′ Decays. In terms of existing data, and deploying the simplest phenomenol-ogy, measurement of ψ ′ → π + π − and whether a finite number for the K * + K − mode might require a significantly larger accumulation of data, remain interesting questions. The strong suppression of ψ ′ → ρπ and K * K decays is well known and has been the subject for considerable experimental and theoretical activity. No evidence is seen for ψ ′ → ρπ in the BES group's 3.8 million ψ ′ event sample, in striking contrast to the case for the J/ψ, where J/ψ → ρπ is the dominant decay. This conundrum is referred to as the " ρπ " puzzle and remains one of the most intriguing mysteries in quarkonium physics. The BES group [1] has the largest event sample of ψ ′ decays thus affording it the opportunity to undertake a systematic investigation of ψ ′ decays to all of the lowest lying vector-plus pseudoscalar-meson (VP) final states. Of particular interest is the identification of the isospin violating mode ψ ′ → π 0 ω 0 and the finding of a finite value for the branching ratio B(ψ ′ → K * 0 K 0) which is likely an isospin violating VP case. These new numbers impel us by creating a " crisis " to deal in a systematic manner both theoretically and experimentally to pin down the essential nature of this so called ρ − π Puzzle of J/ψ and ψ ′ decays. Indeed interesting further but pedestrian tests appear feasible for the BES sample within a phenomenological framework, e.g. measurement of