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Dependence of $|V_{ub}/V_{cb}|$ on Fermi momentum $p_{_F}$ in ACCMM model

The Gaussian width of Fermi momentum, $p_{_F}$, is the most important parameter of the ACCMM model, and its value is essential in the determination of $|V_{ub}/V_{cb}|$ because the experimental analysis is allowed only at the end-point region of inclusive semileptonic $B$-decay spectrum. We extract the value of $|V_{ub}/V_{cb}|$ as a function of $p_{_F}$. We also calculate the parameter $p_{_F}$ in the relativistic quark model using the variational method, and obtain $p_{_F} = 0.54$ GeV which is much larger than the commonly used value, $\sim 0.3$ GeV, in experimental analyses. When we use $p_{_F} = 0.5$ GeV instead of 0.3 GeV, the value of $|V_{ub}/V_{cb}|$ from ACCMM model is increased by a factor 1.81, and can give a good agreement with Isgur {\it et al.} model.