Coherent production of hard dijets on nuclei in QCD - art. no. 014020

We formulate the perturbative QCD approach to coherent diffractive dijet pr oduction in pion-nucleon and pion-nucleus collisions at high energy. For ha rd dijets the Pomeron splitting mechanism in which both helicity amplitudes are proportional to the unintegrated gluon structure function of the proto n F(x,k(2)) and pion distribution amplitude phi (pi)(z) is shown to dominat e. In nuclear diffraction multiple Pomeron splitting components are found t o give antishadowing contributions at large jet momentum k. To leading twis t there is an exact cancellation of effects of nuclear attenuation and anti shadowing or broadening of multiple Pomeron splitting contributions. The ne xt-to-leading higher twist correction driven by nuclear rescatterings is ca lculable in hard QCD and proves to be numerically very large. We argue that large higher twist effects do not preclude the determination of gross feat ures of phi (pi)(z). Our results on the atomic mass number and momentum dep endence of dijet cross sections agree well with the preliminary findings fr om the E791 experiment.