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The reaction $\gamma p \rightarrow p \pi^0 \eta$ has been studied with theCBELSA/TAPS detector at the electron stretcher accelerator ELSA in Bonn forincident photon energies from threshold up to 3.1 GeV. This paper has beenmotivated by the recently claimed observation of a narrow structure in theM$_{N\eta}$ invariant mass distribution at a mass of 1678 MeV/$c^2$. Theexistence of this structure cannot be confirmed in the present work. Instead,for E$_{\gamma}$ = 1400 - 1500 MeV and the cut M$_{p\pi^0} \le 1190 $ MeV/$c^2$a statistically significant structure in the M$_{p\eta}$ invariant massdistribution near 1700 MeV/$c^2$ is observed with a width of $\Gamma\approx 35$MeV/$c^2$ while the mass resolution is $\sigma_{res}$ = 5 MeV/$c^2$. Increasingthe incident photon energy from 1420 to 1540 MeV this structure shifts in massfrom $\approx$ 1700MeV/c$^2$ to $\approx$ 1725 MeV/$c^2$; the width increasesto about 50 MeV/$c^2$ and decreases thereafter. The cross section associatedwith this structure reaches a maximum of $\approx$ 100 nb around E$_{\gamma}\approx$ 1490 MeV (W $\approx $ 1920 MeV), which coincides with the $p a_0$threshold. Three scenarios are discussed which might be the origin of thisstructure in the M$_{p\eta}$ invariant mass distribution. The most likelyinterpretation is that it is due to a triangular singularity in the $\gamma p\rightarrow p a_0 \rightarrow p \pi^0 \eta$ reaction

european physical journal. a, hadrons and nuclei

Observation of a structure in the M$_{p\eta}$ invariant mass  distribution near 1700 MeV/$c^2$ in the $\mathbf{\gamma p \rightarrow p \pi^0  \eta} $ reaction