Measurement of

Of special importance is the purely leptonic decay . Measuring the width of this reaction determines the decay constant . This decay constant can be calculated theoretically as can the decay constant . In fact, lattice gauge calculations have made estimates of both numbers. Knowledge of is crucial in testing Standard Model parameters. It is, in fact, very difficult to measure directly since the purely leptonic decays needed to measure it directly proceed through the channel and have extremely small branching ratios. It is possible, however, to measure through its dominant decay channel. CLEO II has made the first measurement of the ratio =, using 's tagged by the decay [51]. In order to extract a value for , the value of the branching ratio must be known. Using an estimate relating semileptonic decays to semileptonic decays, CLEO II finds MeV. The last error is due to the uncertainty on the branching ratio, (3.7 0.9)%, which is consistent with more recent estimates[52]. CLEO III will be able to substantially reduce the errors on both the partial width ratio and the absolute branching ratio.