Evidence of a dibaryon spectrum in coherent $\pi^0\pi^0 d$ photoproduction at forward deuteron angles

The coherent reaction, $\gamma d \rightarrow \pi^0\pi^0 d$ was studied with the BGOOD experiment at ELSA from threshold to a centre-of-mass energy of 2850\,MeV. A full kinematic reconstruction was made, with final state deuterons identified in the forward spectrometer and $\pi^0$ decays in the central BGO Rugby Ball. The strength of the differential cross section exceeds what can be described by models of coherent photoproduction and instead supports the three isoscalar dibaryon candidates reported by the ELPH collaboration at 2.38, 2.47 and 2.63\,GeV/c$^2$. A low mass enhancement in the $\pi^0\pi^0$ invariant mass is also observed at the $d^*(2380)$ centre-of-mass energy which is consistent with the ABC effect. At higher centre-of-mass energies, a narrow peak in the $\pi^0 d$ invariant mass at 2114\,MeV/c$^2$ with a width of 20\,MeV/c$^2$ supports a sequential two-dibaryon decay mechanism.