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 ${\chi }_{c2}\to K^{\ast }\bar{K}$, $\chi_{c2}\to a_{2}^{\pm}(1320)\pi^{\mp}/a_{2}^{0}(1320)\pi^{0}$and$\chi_{c2}\to \rho(770)^{\pm}\pi^{\mp}$aremeasured\mathrm{.}Here$K^{\ast}\overline{K}$ denotes both $K^{\ast \pm }{K}^{\mp }$ and $K^{\ast 0}\bar{K}{}^0+c\mathrm{.}c\mathrm{.}$, and $K^{\ast }$ denotes the resonances $K^{\ast }(892)$, $K_2^{\ast }(1430)$ and $K_3^{\ast }(1780)$. 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.