I have done a little study to asses the experimental uncertainty on the
mb parameter.
There are measurements of lambda_1 and Lambda_bar from the hadron mass
moments. The extraction yields a result in which the parameters are
correlated to a fair amount (0.7 or so).
The error ellipse of these two parameter can be mapped onto the contour
of a and m_b. Due to the non-linear mapping this leads a much distorted
contour. As a consequence the variation towards smaller values of m_b
has to be assumed almost twice as big as the upward variation.
http://www.slac.stanford.edu/~elsen/amb.pdf
With resolution of a few 100 MeV in m_X we should make a serious
attempt of extracting a branching ratio below say 1.55 GeV and quote
that as a solid number in the paper which will remain valid in the
future. It would be a pity to have the theoretical uncertainty dominate.
Eckhard
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