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

Abstract

We present the first measurement of the exclusive reaction ${\hat{I}}^{3}p\to a_2(1320{)}^{0}p$ in the photon energy range $3.5$-$5.5$ 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 ${\hat{I}}^{3}p\to \ddot{I}{€}^0\hat{I}\cdot p$ and reconstructing the $\ddot{I}{€}^0\hat{I}\cdot$ invariant mass. The differential cross section $d\ddot{I}ƒ/dt$ 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.

Version accepted for publication in PRC, "Rapid Communications" section