The statistics of background rejection in direct detection experiments for dark matter.

A number of the new generation of dark matter direct detection experiments, now underway, employ gamma-background rejection techniques to improve their performance. Event discrimination permits the setting of limits on dark matter event rates orders of magnitude below the raw background event rates. However, proper attention must be paid to systematic, as well as statistical errors, during data analysis. We illustrate the importance of using detectors with good intrinsic discrimination if the impact of systematic errors is to be minimised.