Observation of the helicity-selection-rule suppressed decay of the $chi_{c2}$ charmonium state

The decays of $chi_{c2}to K^{+}K^{-}pi^{0}$, $K_{S} K^{pm}pi^{mp}$ and $pi^{+}pi^{-}pi^{0}$ are studied with the $psi(3686)$ data samples collected with the Beijing Spectrometer (BESIII). For the first time, the branching fractions of $chi_{c2}to K^{ast}overline{K}$, $chi_{c2}to a_{2}^{pm}(1320)pi^{mp}/a_{2}^{0}(1320)pi^{0}$ and $chi_{c2}to rho(770)^{pm}pi^{mp}$ are measured. Here $K^{ast}overline{K}$ denotes both $K^{astpm}K^{mp}$ and $K^{ast 0}overline{K}{}^{0}+c.c.$, and $K^{ast}$ denotes the resonances $K^{ast}(892)$, $K^{ast}_{2}(1430)$ and $K^{ast}_{3}(1780)$. The observations indicate a strong violation of the helicity selection rule in $chi_{c2}$ decays into vector and pseudoscalar meson pairs. The measured branching fractions of $chi_{c2}to K^{ast}(892)overline{K}$ are more than 10 times larger than the upper limit of $chi_{c2}to rho(770)^{pm}pi^{mp}$, which is so far the first direct observation of a significant $U$-spin symmetry breaking effect in charmonium decays.