Role of $\omega$-meson exchange in scaling of the $\gamma p\to\pi^0 p$ process from a Regge-type model with resonances

Abstract

The scaling of photoproduction $\gamma p\to\pi^0 p$ is investigated in the Reggeized model with $N^*$ and $\Delta$ resonances included to describe resonance peaks up to photon energy $E_\gamma$= 3 GeV. Given the $t$-channel exchanges $\rho(770)+\omega(780)+b_1(1235)+h_1(1170)$ Reggeized for the background contribution, the resonances of the Breit-Wigner form are introduced to agree with cross sections for total, differential and beam asymmetry in the low energy region. The scaled differential-cross sections $s^7{d\sigma/ dt}$ are reproduced to agree with the recent JLab data, revealing the production mechanism of the big bump structure around $W\approx 2.2$ GeV by the deep-dip pattern of the $\omega$ exchange that originates from the zeros of the trajectory $\alpha_\omega(t)=0$ in the canonical phase, ${1\over2}(-1+e^{-i\pi \alpha_{\omega}(t)})$.