Reduction of the statistical power per event due to upper lifetime cuts in lifetime measurements

A cut on the maximum lifetime in a lifetime fit does not only reduce the number of events, but it also, in some circumstances dramatically, decreases the statistical significance of each event. The upper impact parameter cut in the hadronic B trigger at CDF, which is due to technical limitations, has the same effect. In this note we describe and quantify the consequences of such a cut on lifetime measurements. We find that even moderate upper lifetime cuts, leaving event numbers nearly unchanged, can dramatically increase the statistical uncertainty of the fit result.