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In purely hadronic 
decays the virtual 
from the 
(or 
) transition materializes as a quark-antiquark pair. In addition,
the spectator-antiquark participates in the decay process. The result is a
4-quark system that can interact in a myriad of ways via the exchange of
gluons. In such a system, predictions of branching fractions or even ratios of
branching fractions are difficult. In semileptonic decays the virtual
materializes as 
and thus only the 
-quark (or 
-quark)
spectator-antiquark system needs to be modeled. Thus, semileptonic decays
provide the best opportunities for measurement of the CKM parameters 
and 
. For 
CLEO has shown that an excellent procedure is
to use the channels
and 
. These exclusive channels are
selected by calculating the missing mass squared recoiling against
the observed 
and 
. Missing mass distributions from the
CLEO II data for these decays are shown in Fig. 
. Thus
far, form-factor ratios have been inferred from the data and they are given in
Table .
It is important to measure both the 
dependence and the values of the
form-factors at 
in order to compare with HQET and to extract
values of 
. HQET predicts one universal form-factor
function, dependent on and absolutely normalized at
, which describes all
decays[10].
One can take advantage of HQET to measure V
by extrapolating the
dependence of 
to 
as shown in Fig. .
The intercept at 
yields 
.
Note, 
and 
.
One problem with this analysis is the background from the 
(or
) final states. This background can be eliminated by using only
events with negative missing mass squared. This, however, results in a
loss of half the signal.
Improvements in background rejection are clearly necessary. Good
charged particle identification will improve the signal; vertex
detection can be used to reduce backgrounds by requiring that the 
and the form an acceptable vertex.
Next: Rare decays
Up: decay studies
Previous: Goals of decay