Observation of a structure in the M$_{p\eta}$ invariant mass distribution near 1700 MeV in the $\mathbf{\gamma p \rightarrow p \pi^0 \eta} $ reaction
2021
The reaction $\gamma p \rightarrow p \pi^0 \eta$ has been studied with the
CBELSA/TAPS detector at the electron stretcher accelerator ELSA in Bonn for
incident photon energies from threshold up to 3.1 GeV. This paper has been
motivated by the recently claimed observation of a narrow structure in the
M$_{N\eta}$ invariant mass distribution at a mass of 1678 MeV. The existence 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 a
statistically significant structure in the M$_{p\eta}$ invariant mass
distribution near 1700 MeV is observed with a width of $\Gamma\approx 35$ MeV
while the mass resolution is $\sigma_{res}$ = 5 MeV. Increasing the incident
photon energy from 1420 to 1540 MeV this structure shifts in mass from $\approx
1700 $ to 1725 MeV; the width increases to about 50 MeV and decreases
thereafter. The cross section associated with 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 this structure in the M$_{p\eta}$ invariant mass
distribution. The most likely interpretation is that it is due to a triangular
singularity in the $\gamma p \rightarrow p a_0 \rightarrow p \pi^0 \eta$
reaction
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