![\[starttime\_merge\]Pulse time distribution from 14–19 $\mu$s in the veto detector when triggered by ZEPLIN–III. The peak at 17.7 $\mu$s is from pulses in prompt coincidence with the [*S1*]{}. Smaller signals in the WIMP target, which trigger on [*S2*]{} instead, are contained within a 16 $\mu$s region before the peak. Pulses occurring after this peak cannot be in prompt coincidence, and represent the background rate in the veto detector itself. Insert: the pulse start times in the veto detector for the full 320 $\mu$s timelines recorded.](vstart_merge.eps){width="8.6cm"}

A number of factors affect the timing resolution of the target-veto combination. Naturally, the limited sampling speed of the veto digitisers (100 ns sampling) dominates this, as illustrated in Figure \[starttime\_merge\]. In addition, there is a smaller contribution from the timing jitter in ZEPLIN–III itself (2 ns sampling), both in determining the arrival time of [*S1*]{} and, more importantly, in measuring the [*S1*]{}–[*S2*]{} separation. The latter applies to [*S2*]{} triggers and is a few tens of ns, reflecting the slower build up of the electroluminescence response in the target and charge diffusion whilst drifting to the liquid surface. Taking these factors into account, a coincidence window of 0.4 $\mu$s has been defined as the prompt 