Observation of the helicity-selection-rule suppressed decay of the $\ddot{I}{‡}_{c2}$ charmonium state

Abstract

The decays of $\ddot{I}{‡}_{c2}\to K^{+}{K}^{-}\ddot{I}{€}^0$, $K_S{K}^{\pm }\ddot{I}{€}^{\mp }$ and $\ddot{I}{€}^{+}\ddot{I}{€}^{-}\ddot{I}{€}^0$ are studied with the $\ddot{I}ˆ(3686)$ data samples collected with the Beijing Spectrometer (BESIII). For the first time, the branching fractions of $\ddot{I}{‡}_{c2}\to K^{\ast }\overline{K}$, $\ddot{I}{‡}_{c2}\to a_2^{\pm }(1320)\ddot{I}{€}^{\mp }/a_2^0(1320)\ddot{I}{€}^0$ and $\ddot{I}{‡}_{c2}\to \ddot{I}(770{)}^{\pm }\ddot{I}{€}^{\mp }$ are measured. Here $K^{\ast }\overline{K}$ denotes both $K^{\ast \pm }{K}^{\mp }$ and $K^{\ast 0}\overline{K}{}^0+c.c.$, 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 $\ddot{I}{‡}_{c2}$ decays into vector and pseudoscalar meson pairs. The measured branching fractions of $\ddot{I}{‡}_{c2}\to K^{\ast }(892)\overline{K}$ are more than 10 times larger than the upper limit of $\ddot{I}{‡}_{c2}\to \ddot{I}(770{)}^{\pm }\ddot{I}{€}^{\mp }$, which is so far the first direct observation of a significant $U$-spin symmetry breaking effect in charmonium decays.

7 pages, 3 figures