LAUSR

The BESIII Collaboration recently reported the observation of the a0(980)0-f0(980) mixing in the isospin-breaking decay J/ψ→ηπ0ϕ. In the Dalitz plot for that decay with the η reconstructed from two photons, there is a band around 1.4 GeV on the π0ϕ distribution. In general, this peak can be due to a resonance or a kinematic effect. In this paper, we study the effects of a set of K*KK¯ triangle diagrams and show that due to triangle singularities such diagrams can lead to a peak around 1.4 GeV in the π0ϕ invariant mass distribution, which is a model-independent conclusion. The Dalitz plot induced by such a mechanism has a feature consistent with the BESIII observation; namely, events along the band accumulate at both ends close to the Dalitz plot boundary. The effects of the same mechanism on the J/ψ→π0π0ϕ and J/ψ→ηπ0K+K- decays are also investigated. We suggest to take more data for the J/ψ→ηπ0ϕ→ηπ0K+K- and check whether the structure around 1.4 GeV persists for the K+K- invariant mass away from the ϕ mass region. This is crucial for understanding whether the band is due to triangle singularities or due to a resonance. Were it the latter, the band should remain, while it would not if it is due to the former.