Features of $ω$ photoproduction off proton target at backward angles : Role of nucleon Reggeon in $u$-channel with parton contributions

Abstract

Backward photoproduction of $ω$ off a proton target is investigated in a Reggeized model where the $u$-channel nucleon Reggeon is constructed from the nucleon Born terms in a gauge invariant way. The $t$-channel meson exchanges are considered as a background. While the $N_α$ trajectory of the nucleon Reggeon reproduces the overall shape of NINA data measured at the Daresbury Laboratory in the range of $u$-channel momentum transfer squared $-1.7<u<0.02$ GeV$^2$ and energies at $E_γ=2.8$, 3.5 and 4.7 GeV, a possibility of parton contributions is searched for through the nucleon isoscalar form factor which is parameterized in terms of parton distributions at the $ωNN$ vertex. Detailed analysis is presented for NINA data to understand the reaction mechanism that could fill up the deep dip from the nucleon Reggeon at momentum squared $u=-0.15$ GeV$^2$. The angle dependence of differential cross sections at the NINA energies above are reproduced in the overall range of $u$ including the CLAS data at forward angles in addition. The energy dependence of the differential cross section is investigated based on the NINA data and the recent LEPS data. A feature of the present approach that implicates parton contributions via nucleon form factor is illustrated in the total and differential cross sections provided by GRAAL and CB-ELSA Collaborations.