Observation of the Helicity-Selection-Rule Suppressed Decay of the χ(c2) Charmonium State
Achasov, M. N.
Ai, X. C.
Ambrose, D. J.
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The decays of χ(c2) → K⁺K⁻ π⁰, K(S)K(±)π(∓), and π⁺π⁻π⁰ are studied with the ψ(3686) data samples collected with the Beijing Spectrometer ( BESIII). For the first time, the branching fractions of χ(c2) → K*K̅, χ(c2) → a₂(±)(1320)π(∓)/a₂⁰(1320)π⁰, and χ(c2) → ρ(770)(±)π(∓) are measured. Here, K*K̅ denotes both K*(±)K(∓) and its isospin-conjugated process K*⁰K̅⁰ + c.c., and K* denotes the resonances K*(892), K₂*(1430), and K₃*(1780). The observations indicate a strong violation of the helicity selection rule in χ(c2) decays into vector and pseudoscalar meson pairs. The measured branching fractions of χ(c2) → K*(892)K̅ are more than ten times larger than the upper limit of χ(c2) → ρ(770)(±)π(∓), which is so far the first direct observation of a significant U-spin symmetry breaking effect in charmonium decays.
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