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Physical Letters B 584, 251–259. doi.org/10.1016/j.physletb.2004.01.041

The collaboration selected 730 candidates with an estimated $58\pm 8$ background events and measured a branching fraction $(1.16\pm 0.05_{\mathrm{stat}}\pm 0.06_{\mathrm{syst}})\times 10^{-3}$. This is an observed weak radiative channel, not an electromagnetic flavour-conserving decay.