Observation and Spin-Parity Determination of the $X(1835)$ in $J/\ensuremath{\psi}\ensuremath{\rightarrow}\ensuremath{\gamma}{K}_{S}^{0}{K}_{S}^{0}\ensuremath{\eta}$

Observation and Spin-Parity Determination of the $X (1835)$ in $J / ψ \to γ K_{S}^{0} K_{S}^{0} η$

M. Ablikim et al. (BESIII Collaboration)

Phys. Rev. Lett. 115, 091803 – Published 27 August 2015

Abstract

We report an observation of the process $J / ψ \to γ X (1835) \to γ K_{S}^{0} K_{S}^{0} η$ at low $K_{S}^{0} K_{S}^{0}$ mass with a statistical significance larger than $12.9 σ$ using a data sample of $1.31 \times 10^{9}$ $J / ψ$ events collected with the BESIII detector. In this region of phase space the $K_{S}^{0} K_{S}^{0}$ system is dominantly produced through the $f_{0} (980)$ . By performing a partial wave analysis, we determine the spin parity of the $X (1835)$ to be $J^{P C} = 0^{- +}$ . The mass and width of the observed $X (1835)$ are $1844 \pm 9 (stat)_{- 25}^{+ 16} (syst) MeV / c^{2}$ and $19 2_{- 17}^{+ 20} (stat)_{- 43}^{+ 62} (syst) MeV$ , respectively, which are consistent with the results obtained by BESIII in the channel $J / ψ \to γ π^{+} π^{-} η^{'}$ .