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Humboldt-Universität zu Berlin - Mathematisch-Naturwissen­schaft­liche Fakultät - Experimentelle Elementarteilchenphysik

Humboldt-Universität zu Berlin | Mathematisch-Naturwissen­schaft­liche Fakultät | Institut für Physik | Experimentelle Elementarteilchenphysik | BaBar | Theses | Determination of the CKM matrix element |Vcb|, the B → Xsγ decay rate, and the b-quark mass

Florian Bernlochner (2011)

Determination of the CKM matrix element |Vcb|, the B → Xsγ decay rate, and the b-quark mass

PHD-Thesis, HU-Berlin.

   In this work, the preliminary measurements of two fundamental parameters of the Standard Model of particles physics are presented:

the CKM matrix element |Vcb | , and the b-quark mass.  The measurement of the absolute value of the CKM matrix element Vcb uses

the full set of recorded data of 429.06 fb−1 of B B mesons of the BABAR experiment. The CKM matrix element is obtained by

measuring the branching fractions and non-perturbative shape parameters of the two transitions into the charmed 1S ground states,

B → D l νl and B →D∗ l νl , respectively. The kinematic of the produced lepton is measured and the kinematics of the short-lived

charmed mesons is reconstructed from kaon and pion candidates. By combining the reconstructed three-momenta of both particles

with the angular information of the decay, three independent variables can be obtained. The measured distributions in these variables

are analyzed in a three-dimensional global fit, which simultaneously extracts the decay parameters and branching fractions of both

charmed transitions. We find that  
            B → D l νl :   |Vcb |    =  (36.14 ± 0.57stat. ± 1.30sys. ± 0.80theo. ) × 10−3 ,
          B → D ∗ l νl :   |Vcb |    =   (39.71 ± 0.26stat. ± 0.73sys. ± 0.74theo. ) × 10−3 ,
where the uncertainties are statistical, systematic, and theoretical, respectively. In the Standard Model, both measured values

of |Vcb | can be averaged to further minimize the uncertainties. We find
                      Combined :          |Vcb |         =  (38.29 ± 0.26stat. ± 0.64sys. ± 0.52theo. ) × 10−3 .
Furthermore, several scenarios are explored how possible future unquenched lattice QCD points can be incorporated into the

measurement, to further reduce the uncertainty on |Vcb |.    The b-quark mass is determined by analyzing measured B → Xs γ

photon energy spectra from BABAR and Belle . Due to the sizeable background contributions from other processes which produce

high energetic photons, the B → Xs γ decay rate is best measured near the kinematic limit where the photon recoils against a light

jet-like hadronic state. Unfortunately, the heavy quark expansion breaks down in this kinematic limit and non-perturbative corrections

arise. This behaviour in the theory can be remedied by summing the most divergent contributions of the heavy quark expansion

into a single function, the shape function. At the time being, the shape function cannot be determined by first principles. This work

includes the first direct measurement of the shape function with absorbed 1/mb corrections, and uses the applied framework for a

precision test of the Standard Model prediction for the B → Xs γ decay by determining the absolute value of the leading Wilson

coefficient C7 . The b-quark mass is related to the first, second, and third moment of the shape function. .
For |C7 | and the b-quark mass we find,
                                          |C7 Vtb Vts | × 10-3   =15.003 ± 0.537exp. ± 0.303theo. ,
                                         m1S_b     =4.764 ± 0.027exp. ± 0.016theo.   GeV/c2 ,

where the uncertainties are experimental, and theoretical, respectively. The extracted value of |C7 | is in excellent agreement with

the Standard Model expectation and the precision on the b quark mass in the 1S scheme, m1S_b , is of the same order as the

uncertainties on  m1S_b from global B → Xc l νl moment fits.