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 la rge missing energy at LEP and future e(+)e(-) colliders requires precise pr edictions for the Standard Model irreducible background. While at LEP1 the theoretical situation is under control, going to LEP2 land beyond) some imp rovements are necessary. To approach the aimed O(1%) theoretical accuracy, the tree-level matrix elements for the processes e(+)e(-) --> <nu(nu)over b ar>n gamma, with n = 1, 2, 3, are exactly computed in the Standard Model, i ncluding the possibility of anomalous couplings for single-photon productio n. Due to the presence of observed photons in the final state, particular a ttention is paid to the treatment of higher-order QED corrections. Comparis ons with existing calculations are shown and commented on. An improved vers ion of the event generator NUNUGPV is presented. (C) 1999 Elsevier Science B.V.