Hadron formation in high energy photonuclear reactions

We present a new method to account for coherence length effects in a semiclassical transport model. This allows us to describe photoproduction and electroproduction at large nuclei $\mathrm{}(A>\mathrm{}12\mathrm{})\mathrm{}$ and high energies using a realistic coupled channel description of the final state interactions that goes beyond simple Glauber theory. We show that the purely absorptive treatment of the final state interactions can lead to wrong estimates of color transparency and formation time effects in particle production. As an example, we discuss exclusive ${\rho }^0$ photoproduction on Pb at a photon energy of 7 GeV as well as $K^{+}$ production in the photon energy range 1–7 GeV.