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, ${ 1\over2}(-1+e^{ -i\pi \alpha_{ \omega}(t)})$.
Categories: nucl-th hep-ph