the DELPHI detector at LEP.

Study of spectral moments in semileptonic decays of the b hadron with the DELPHI detector at LEP. The measurement of the moments of hadronic mass spectrum and of lepton energy spectrum based on a sample of semileptonic decays of the b hadron selected from Z 0 → b ¯ b events recorded with the Delphi detector at LEP, are presented. These results are interpreted in terms of constraints on the quark masses and on the b-quark kinetic energy value. Moments of hadronic mass spectrum and of lep-ton energy spectrum are sensitive to the masses of the b and c quarks as well as to the non-perturbative parameters of the Heavy Quark Expansion. They allow to improve the determination of the |V cb | element of the CKM quark coupling matrix which can be measured from inclusive semileptonic B decays by using the relation: Γ(b → cℓν) = |V cb | 2 f (par.) = BR(b → cℓν)/τ b The current experimental accuracy on the semileptonic branching ratio and B lifetime is about 1%, while the evaluation of the function f , based on Operation Product Expansion, brings an uncertainty of the order of several percent, making it the dominant error contribution. Moreover, the comparison of these results with different measurements of the same parameters provides a test of the consistency of the theoretical predictions for inclusive semileptonic B decays and of the underlying assumptions. Previous measurements of spectral moments have been performed at the Υ(4S) [1], Delphi has performed the first measurement in b hadron semileptonic decays at the Z 0. There are several advantages in the Z 0 kinematics, mainly the large boost acquired by the b quark (E B ∼ 30 GeV) which gives access in the laboratory frame to the low region of the lepton energy spectrum. The challenge in this case is the complete reconstruction of the B system. The lepton energy acceptance extending down to the lower end of the spectrum makes these results both easier to interpret and complementary to those obtained at the Υ(4S). In this study two different formulations, using different mass definitions for deriving the constraints on the OPE parameters, are used. 2. Moments of hadronic mass distribution in b hadron semileptonic decays The hadronic mass distribution of ¯ B 0 d → D * * ℓ¯ ν events have been studied [2]. D * * events have been reconstructed in …