Electroweak Sudakov form factors and nonfactorizable soft QED effects at NLC energies

We study the leading log infrared behavior of electroweak corrections at Te V scale energies, that will be reached by next generation of linear collide rs (NLC). We show that, contrary to what happens at typical LEP energies, i t is not anymore possible to disentangle ''pure electroweak'' from "photoni c'' corrections. This means that soft QED effects do not factorize and ther efore cannot be treated in the usual "naive" way they were accounted for in the LEP-era. The nonfactorizable effects come up first at the two loop LL level, that we calculate explicitly for a fermion source that is neutral un der the SU(2) circle times U(1) gauge group (explicitly, a Z' decay into tw o fermions). The basic formalism we set up can be used to calculate LL effe cts at any order of perturbation theory. The results of this paper might be important for future calculations of electroweak corrections at NLC energi es. (C) 2000 Published by Elsevier Science B.V. All rights reserved.