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'-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 featured of the onset of BBL using generalized vector dominance model. We show that this model describes very well rho-meson coherent photoproduction at 6 less than or equal to Egamma less than or equal to 10 GeV. In the case of rho-meson production we find a UPC cross section which is a factor similar to1.5 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. (C) 2002 Elsevier Science B.V. All rights reserved.