Observation of the Helicity-Selection-Rule Suppressed Decay of the χ(c2) Charmonium State

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) → KK̅, χ(c2) → a₂(±)(1320)π(∓)/a₂⁰(1320)π⁰, and χ(c2) → ρ(770)(±)π(∓) are measured. Here, KK̅ denotes both K*(±)K(∓) and its isospin-conjugated process K⁰K̅⁰ + 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.