The shape of the lepton spectrum in inclusive semileptonic $B\to X\ell {\overline{\nu }}_{\ell }$ decay is sensitive to the matrix elements of the heavy quark effective theory, $\overline{\Lambda }$ and ${\lambda }_1$. From CLEO data we extract $\overline{\Lambda }=0.39\mathrm{}\pm 0.11\mathrm{GeV}$ and ${\lambda }_1=-0.19\mathrm{}\pm 0.10{\mathrm{GeV}}^2$, where the uncertainty is the $1\sigma$ statistical error only. Systematic uncertainties are discussed. These values for $\overline{\Lambda }$ and ${\lambda }_1$ are used to determine $|V_{\mathrm{cb}}|$ and the $\overline{\mathrm{MS}}$ bottom and charm quark masses. We discuss the theoretical uncertainties related to order $({\Lambda }_{\mathrm{QCD}}/m_b{)}^3$ effects and higher orders in the perturbative expansion.

• Received 15 March 1996

DOI:https://doi.org/10.1103/PhysRevLett.77.20