Measurement of the first hadronic spectral moment from semileptonic B decays

A preliminary determination of the first moment of the hadronic mass distribution <(M_X)^2 - bar{m_D}^2> 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 BBbar event s 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 distrib ution 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 lambda_1 and Lambdabar 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 Lambdabar and lambda_1 than obtained from earlier measurements based on lepton momentum spectra and the photon spectrum in b --> s gamma transitions.