Single- and multi-photon events with missing energy at LEP

To avoid loss of sensitivity in the search for new physics in single- and multi-photon final states with large missing energy at LEP, precise predictions for the Standard Model irreducible background are required. At LEP1 the theoretical situation is satisfactory. Going to LEP2, some improvements are necessary. To this aim, the 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. An improved version of the event generator NUNUGPV is presented.