Charged Higgs boson effects are not helicity suppressed in the decays $B^{-}\to \tau \overline{\nu }, \mu \overline{\nu }, \mathrm{and} e\overline{\nu }$, and could enhance or suppress these modes without violating lepton universality. The present limit of $B(B^{-}\to \mu \overline{\nu })<\mathrm{}2.0\mathrm{}\times {10}^{-5\mathrm{}}$ is more stringent than the measurement $B(b\to \tau \overline{\nu }+X)=({4.20}_{-0.68\mathrm{}}^{+0.72\mathrm{}}\pm 0.46)\%$, while a factor of 2 improvement in the present limit of $B(B^{-}\to \tau \overline{\nu })<\mathrm{}1.2\%$ would be comparable. There is much room for discovery as one pushes the latter limit down to the standard model expectation of order ${10}^{-4\mathrm{}}$. A value below this may be explained without requiring $f_B\left|V_{\mathrm{ub}}\right|$ to be unduly small.

• Received 23 December 1992

DOI:https://doi.org/10.1103/PhysRevD.48.2342