Study of the decay (Formula presented)

Using (Formula presented) (Formula presented) events collected with the BESIII detector operating at the BEPCII collider, we have observed the decay (Formula presented) for the first time with a statistical significance of (Formula presented). Its branching fraction is measured to be (Formula presented), where the first uncertainty is statistical and the second is systematic. The result is consistent with the previous measurement of its isospin partner process (Formula presented), aligning with the naive expectation of isospin symmetry. In addition, we search for potential intermediate states in the (Formula presented) ((Formula presented)) invariant mass distribution and a possible threshold enhancement in the (Formula presented) system, but no conclusive evidence of them is observed.