to neutrinos. Similarly, in supersymmetric models, a process like $e^+ e^- \to \gamma \widetilde{\chi}^0_1
\widetilde{\chi}^0_1$ (where $\widetilde{\chi}^0_1$ is the lightest neutralino) would also lead to a single photon signal. Although the $Z'$ signal can be differentiated by looking for a resonant peak in the missing invariant mass, the supersymmetric signal is more difficult to disentangle.

The situation for the process considered in this paper is somewhat more encouraging. It is true that both types of new physics considered in the preceding paragraph can produce the same signal. Processes of the form $e^+ e^- \to e^+ e^- Z'$, with the $Z'$ decaying to neutrinos are one possibility. In supersymmetry, too, it is possible to pair-produce either sleptons or charginos, followed by decays to electrons (positrons) and invisible particles. The $Z'$ signal can again be differentiated by a peak in the missing invariant mass. The supersymmetric signal, in this case, always arises from production of a pair of real sparticles, which should emerge in opposite hemispheres and, if light enough, will be highly boosted. As a result, the observed electron and positron should lie, most of the time, in opposite hemispheres. For the graviton signal, however, there is no such correlation; in 