Single- and multi-photon final states with missing energy at e+e- colliders

The search for new physics in single- and multi-photon final states with large missing energy at LEP and future e(+)e(-) colliders requires precise predictions for the Standard Model irreducible background. While at LEP1 the theoretical situation is under control, going to LEP2 land beyond) some improvements are necessary. To approach the aimed O(1%) theoretical accuracy, the tree-level matrix elements for the processes e(+)e(-) --> <nu(nu)over bar>n gamma, with n = 1, 2, 3, are exactly computed in the Standard Model, including the possibility of anomalous couplings for single-photon production. Due to the presence of observed photons in the final state, particular attention is paid to the treatment of higher-order QED corrections. Comparisons with existing calculations are shown and commented on. An improved version of the event generator NUNUGPV is presented. (C) 1999 Elsevier Science B.V.