Abstract
The magnitude of the CKM matrix element Vub is determined by measuring the inclusive charmless semileptonic branching fraction of beauty hadrons at OPAL based on b -> Xu l nu event topology and kinematics. This analysis uses OPAL data collected between 1991 and 1995, which correspond to about four million hadronic Z decays. We measure Br(b -> Xu l) to be (1.63 +/- 0.53 +0.55/-0.62) x 10^(-3). The first uncertainty is the statistical error and the second is the systematic error. From this analysis, Vub is determined to be: |Vub| = (4.00 +/- 0.65(stat) +0.67/-0.76(sys) +/- 0.19(HQE)) x 10^(-3). The last error represents the theoretical uncertainties related to the extraction of |Vub| from Br(b -> Xu l) using the Heavy Quark Expansion.
Highlights
The CKM matrix [1] describes the relation between quark weak and mass eigenstates, with the element Vub describing decays of the b to u quark
The dominant background to b → Xu ν comes from b → Xc ν decays because the branching ratio of b → Xc ν is more than 50 times greater than that of b → Xu ν
We describe the determination of |Vub| using the inclusive b → Xu ν decay rate from the OPAL data taken at center of mass energies near the Z resonance
Summary
The CKM matrix [1] describes the relation between quark weak and mass eigenstates, with the element Vub describing decays of the b to u quark. Given that the branching fraction of inclusive b → u semileptonic decay is of order 10−3, a large number of b hadrons are required to measure |Vub|. The inclusive method developed by ARGUS [2] and CLEO [3] is to extract |Vub|/|Vcb| from the excess of events in the 2.3 to 2.6 GeV/c region of the lepton momentum spectrum in the B meson rest frame, where the b → Xc ν contributions vanish. This technique uses only a small fraction of the lepton phase space and so has considerable model dependence in extrapolating to the entire lepton spectrum in the B rest frame.
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