Charmonium Decays to Axialvector plus Pseudoscalar Mesons

A sample of 3.79 million ψ(2S) events is used to study the decays of charmonium to axialvector plus pseudoscalar mesons. The branching fraction for the decay ψ(2S) → b±1 (1235)π agrees with expectations based on scaling the corresponding J/ψ branching fraction. Flavor-SU(3)-violating K1(1270)K1(1400) asymmetries with opposite character for ψ(2S) and J/ψ decays are observed. This contrasting behavior can not be accommodated by adjustments of the singlet-triplet mixing angle. In perturbative QCD, the dominant process for hadronic decays of both the J/ψ and the ψ(2S) is annihilation into three gluons followed by the hadronization of these gluons into physically observable hadrons. The similarity between the parton-level final states has led to the conjecture that the ratio of the J/ψ and ψ(2S) decay branching fractions into any exclusive final state Xh is given by the ratio of the square of the wave function at the origin of the constituent cc quark state, which is well determined from the dilepton decay rates [1]: B[ψ(2S) → Xh] B[J/ψ → Xh] ≃ B[ψ(2S) → e e] B[J/ψ → e+e−] = 0.141± 0.012. This conjecture is sometimes referred to as the 14% rule [2]. Although this conjecture seems to work reasonably well for a number of decay channels, it fails badly in the case of ψ(2S) two-body decays to vector plus pseudoscalar meson (V P ) final states—the decay ψ(2S) → ρπ is suppressed relative to the 14% rule expectation by more than a factor of fifty [3,4]. This conundrum is commonly called the ρπ puzzle [5]. In addition, the BES group has reported suppressions by factors of at least three in the vector plus tensor meson (V T ) final states: KK 2 , ρa2, ωf2 and φf ′ 2 [6]. To date, no convincing evidence has been uncovered for hadronic ψ(2S) decays that are enhanced relative to the 14%-rule expectation. Since at least one explanation for the ρπ puzzle involves a mechanism that suppresses all ψ(2S) decays to lowest lying two-body mesons final states [7], it is useful to examine all possibilities. Here we report first measurements of ψ(2S) decays to axialvector plus pseudoscalar mesons. There are two lowest-lying axialvector-meson octets. These correspond to the singlet (P1) and triplet ( P1) spin configurations of two quarks in a P-wave orbital angular momentum state. The non-strange, isospin I = 1 members of the two octets have opposite G-parity: the b1(1235) is in the P1 octet and has G = +1, while the a1(1260) is in the P1 octet and has G = −1. Since strong decays of the J/ψ and ψ(2S) conserve G-parity, decays to the axialvector pseudoscalar (AP ) pair b1π are allowed and seen in J/ψ decays; decays to a1π final states are forbidden and not seen in J/ψ decays. The strange members of the P1 and P1 octets, the KA and KB, respectively, are mixtures of the observed physical states, the K1(1270) and the K1(1400), where KA = cos θK1(1400) + sin θK1(1270) (1) KB = cos θK1(1270)− sin θK1(1400), (2) and the mixing angle is near θ ≃ 45 [8]. The dominant K1(1270) decay mode is to Kρ (Br = 42± 6%); the K1(1400) decays almost always to Kπ (Br = 94± 6%).