Average kinetic energy of a heavy quark in semileptonic [Formula presented] decay

Within the ACCMM model the average kinetic energy of the heavy quark in a heavy-light meson is calculated as [Formula presented] solely from the fact that the Gaussian momentum probability distribution has been taken in the ACCMM model. Therefore, the Fermi momentum parameter [Formula presented] of the ACCMM model is not a truly free parameter, but is closely related to the average kinetic energy of the heavy quark, which is theoretically calculable in principle. In this context, we determine [Formula presented] by comparing the theoretical prediction of the ACCMM model with the model-independent lepton energy spectrum of [Formula presented] from the recent CLEO analysis, and find that [Formula presented] GeV. We also calculate [Formula presented] in the relativistic quark model by applying the quantum mechanical variational method, and obtain [Formula presented] GeV. We show the correspondences between the relativistic quark model and the heavy quark effective theory. We then clarify the importance of the value of [Formula presented] in the determination of [Formula presented].