deposition. The ratio of the signal strength in these channels differs for electron and nuclear recoil interactions, allowing discrimination between incident particle species and the efficient rejection of most background events. However, as is the case for all direct dark matter search experiments, single elastic scattering of neutrons and WIMPs generate the same signature, rendering the former a particularly problematic background. Furthermore, even where electron recoil discrimination is excellent, it is probabilistic and there always remains a small probability for those electron recoils to be misidentified as nuclear recoils. An external plastic scintillator based veto detector can help decrease these backgrounds by removal of coincident $\gamma$-ray and neutron induced nuclear recoil events (WIMPs are extremely unlikely to scatter in both detectors). In addition, by providing an independent measurement of $\gamma$-ray and neutron rates in the WIMP target, it will also help decrease the systematic uncertainty in the estimation of these background rates, thereby increasing the significance of a non-zero observation. Experiencing the same physical conditions as ZEPLIN–III, the veto system also provides valuable environmental diagnostic information as well as independent [*in situ*]{} measurements of ambient background radioactivity levels.

The first science run of ZEPLIN–III resulted in the exclusion, with 90% 