Impact of four-quark shape functions on inclusive B decay spectra

Abstract.

It has recently been pointed out that a new class of subleading shape functions involving B-meson matrix elements of non-local four-quark operators contributes at order $\Lambda_{\rm QCD}/m_b$ to $\bar{B}\to X_u l^-\bar{\nu}$ decay distributions in the endpoint region. The corresponding functions $f_u(\omega)$ and $f_v(\omega)$ are estimated using the vacuum-insertion approximation. A numerical analysis of various $\bar{B}\to X_u l^-\bar{\nu}$ decay spectra suggests that these power corrections are very small, below present theoretical uncertainties due to other subleading shape-function contributions.