Confidence Intervals and Upper Bounds for Small Signals in the Presence   of Background Noise
  We discuss a new method for setting limits on small signals in the presence of background noise. The method is based on a combination of a two dimensional confidence region and the large sample approximation to the likelihood ratio test statistic. It automatically quotes upper limits for small signals and two-sided confidence intervals for larger samples. We show that this method gives the correct coverage and also has good power. 
Wolfgang A. Rolke and Angel M. Lopez