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A preliminary determination of the first moment of the
hadronic mass distribution (M2X-m2D} in
semileptonic B decays has been obtained as a function of the minimum
lepton momentum, ranging from 0.9 to 1.6 GeV/c. The measurement is
based on a new technique involving BB events in which one fully
reconstructed B meson decays hadronically and the recoiling
B decays
semileptonically. The mass of the hadrons in the semileptonic decay is
determined from a kinematic fit to the whole event. For different
minimum lepton momenta, the mass distribution is decomposed into
contributions from various charm resonant states and a non-resonant
contribution, allowing for the determination of the first moment. From
these moments the Heavy Quark Effective Theory (HQET) parameters l1 and
L
can be derived. For lepton momenta in the B
rest frame above 1.5 GeV/c, we find a first moment that is compatible
with existing measurements. However, if we extend the measurement to
lower values of lepton momenta, the data can only be described by
Operator Product Expansion calculations if we use significantly
different values for L
and l1 than obtained from earlier
measurements based on lepton momentum spectra and the photon spectrum
in b > sg transitions.
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