Signals for black body limit in coherent ultraperipheral heavy ion collisions

We argue that study of total cross section of photoabsorption and coherent photoproduction of $\rho,\rho^{\prime}$-mesons in ultraperipheral heavy ion collisions (UPC) is effective method to probe onset of black body limit(BBL) in the soft and hard QCD interactions. We illustrate the expected features of the onset of BBL using generalized vector dominance model. We show that this model describes very well $\rho$-meson coherent photoproduction at $6 \leq E_{\gamma} \leq 10 GeV$. In the case of $\rho$-meson production we find a UPC cross section which is a factor $\sim 1.5$ larger than the one found by Klein and Nystrand. The advantages of the process of coherent dijet production to probe onset of BBL in hard scattering regime where decomposition over the twists becomes inapplicable are explained and relative importance of the $\gamma +Pomeron$ and $\gamma +\gamma$ mechanisms is estimated.