Control of theoretical uncertainties in semileptonic B decays

Present situation

Modelling uncertainties are the largest source of uncertainties in the measurement of the inclusive semileptonic branching fraction of B hadrons. The new result given by DELPHI in HEP99 is:

BR(b --> lX)= 10.65 +-0.07 +-0.024 (+0.42 -0.28) %,

where the last error comes from models of B sl decays. There are other types of theoretical uncertainties which appear when one extracts Vcb from the measurements of the inclusive b semileptonic branching fraction and lifetime:

Vcb= (40.8 +-0.4 +-2.0) * 10**-3

where the last error comes from theory. It is hoped to have a better control of theoretical errors by looking at moments of the lepton momentum, measured in the B rest frame.

What remains to be done

In practice it is not clear if one can improve on the present errors but there is a basic interest in trying to do it and to understand the origin of the limitations of present measurements. A feasibility study is thus quite desirable to see if LEP can contribute in these measurements. A priori CLEO is in a better position because B mesons are produced almost at rest, there, and it will be easier to measure moments of the lepton momentum.

Tools and contact-persons

The analysis can be done using the lepton selection studied in details by the corresponding team.

Contact persons: