First measurement of direct photoproduction of the $a_2(1320{)}^0$ meson on the proton

We present the first measurement of the exclusive reaction $\gamma p \rightarrow a_2(1320)^0 , p$inthephotonenergyrange$3.5$-$5.5$ GeV and four-momentum transfer squared $0.2<-t<2.0$ GeV${}^2$. Data were collected with the CEBAF Large Acceptance Spectrometer at the Thomas Jefferson National Accelerator Facility. The neutral $a_2$ resonance was detected by measuring the reaction $\gamma p\to {\pi }^0\eta p$ and reconstructing the $\pi^0 \eta$invariantmass\mathrm{.}Thedifferentialcrosssection$d\sigma/dt$ was extracted at different beam energies in each $-t$ bin. The most prominent feature of the differential cross section is a dip at $-t\simeq 0.55$ GeV${}^2$. This can be well described in the framework of Regge phenomenology, where the exchange degeneracy hypothesis predicts a zero in the reaction amplitude for this value of the four-momentum transfer.