Measurement and Interpretation of Moments of the Combined Hadronic Mass and Energy Spectrum in Inclusive Semileptonic B-Meson Decays

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Abstract

This thesis presents first measurements of moments of the hadronic nX2 distribution measured in inclusive semileptonic decays of B mesons to final states containing a charm quark, B → Xcℓν. The variable nX2 is a combination of the invariant mass of the charmed meson mX, its energy in the B-meson rest-frame EX;BRF, and a constant ~Λ = 0.65 GeV, nX2 = mX2c4-2~ΛEX,BRF + ~Λ2. The moments with k = 2,4,6 are measured as proposed by theory to constrain assumptions made in the theoretical description of inclusive observables in semileptonic B-meson decays. This description uses Heavy Quark Expansion (HQE), an effective QCD combined with an Operator Product Expansion. The measurement is based on a sample of 231.6 million e+e- → Υ(4S) {yields} B$\bar{B}$ events recorded with the BABAR experiment at the PEP-II e+e--storage rings at SLAC. We reconstruct the semileptonic decay by identifying a charged lepton in events tagged by a fully reconstructed hadronic decay of the second B meson. Correction procedures are derived from Monte Carlo simulations to ensure an unbiased measurement of the moments of the nX2 distribution. All moments are measured requiring minimum lepton momenta between 0.8 GeV/c and 1.9 GeV/c in the rest frame of the B meson. Performing a simultaneous fit to the measured moments up to order k = 6 combined with other measurements of moments of the lepton-energy spectrum in decays B → Xcℓν and moments of the photon-energy spectrum in decays B → Xsγ, we determine the quark-mixing parameter |Vcb|, the bottom and charm quark masses, the semileptonic branching fraction β(B → Xcℓν), and four non-perturbative heavy quark parameters. Using HQE calculations in the kinetic scheme up to order 1/mb3 we find |Vcb| = (41.65 ± 0.43 ± 0.40 ± 0.58) • 10-3 and mb = (4.570 ± 0.033 ± 0.043) GeV/c2, where the first uncertainty refers to experimental contributions, the second to uncertainties in the HQE, and the third to theoretical uncertainties in the calculations of the semileptonic decay rate Λ(B {yields} Xcℓν). All obtained results are consistent with previous determinations. The inclusion of the moments decreases the uncertainty on the HQE parameters μπ2 and ρD3. Furthermore, the theoretical treatment of higher order corrections in the HQE used for the moments has been verified with these new measurements.

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