Observation of the helicity-selection-rule suppressed decay of the χc2 charmonium state

摘要: The decays of χc2 → K+K−π0, KSK±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 χc2 → K∗K, χc2 → a±(1320)π∓/a02(1320)π0 and χc2 → ρ(770)±π∓ are measured. Here K∗K denotes both K∗±K∓ and its isospin-conjugated process K∗0K0 + 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 χc2 decays into vector and pseudoscalar meson pairs. The measured branching fractions of χc2 → K∗(892)K are more than 10 times larger than the upper limit of χc2 → ρ(770)±π∓, which is so far the first direct observation of a significant U-spin symmetry breaking effect in charmonium decays.