Universal extra dimensions, radiative returns and the inverse problem at a linear e+e- collider

In a five-dimensional model with one universal extra dimension, signals for a pair of n=1 Kaluza-Klein excitations could be easily observed at a future e+e- collider if the process in question is kinematically allowed. However, these signals would prove difficult to distinguish from those predicted in other models, such as those with an extended gauge symmetry or supersymmetry. A much better power of discrimination is provided by the fact that the same machine could also produce the n=2 gauge bosons γ2 and Z2 as resonances in fermion pair production without any upgrade in the collision energy √(s). Assuming a fixed √(s)--as is expected at upcoming e+e- machines--we investigate the role of beam radiation in helping to excite such resonances through radiative returns. We then show how these resonances could yield unambiguous signals for a universal extra dimension, if taken in conjunction with the production of Z1 pairs, identified by their decays to leptons and missing transverse energy.