Multiplicity of electron- and photon-seeded electromagnetic showers at multipetawatt laser facilities

Electromagnetic showers developing from the collision of an ultraintense laser pulse with a beam of high-energy electrons or photons are investigated under conditions relevant to future experiments on multipetawatt laser facilities. A semianalytical model is derived that predicts the shower multiplicity, i.e., the number of pairs produced per incident seed particle (electron or γ photon). The model is benchmarked against particle-in-cell simulations and shown to be accurate over a wide range of seed particle energies (from 100 MeV to 40 GeV), laser relativistic field strengths (10