Design and construction of a vertex chamber and measurement of the average B-hadron lifetime

Four parameters describe the mixing of the three quark generations in the Standard Model of the weak charged current interaction. These four parameters are experimental inputs to the model. A measurement of the mean lifetime of hadrons containing b-quarks, or B-Hadrons, constrains the magnitudes of two of these parameters. Measurement of the B-Hadron lifetime require a device that can measure the locations of the stable particles that result from B-Hadron decay. This device must function reliably in an inaccessible location, and survive high radiation levels. We describe the design and construction of such a device, a gaseous drift chamber. Tubes of 6.9 mm diameter, having aluminized mylar walls of 100 μm thickness are utilized in this Vertex Chamber. It achieves a spatial resolution of 45 μm, and a resolution in extrapolation to the B-Hadron decay location of 87μm. Its inner layer is 4.6 cm from e+- colliding beams. The Vertex Chamber is situated within the MAC detector at PEP. We have analyzed both the 94 pb-1 of integrated luminosity accumulated at √s = 29 GeV with the Vertex Chamber in place as well as the 210 pb-1 accumulated previously. We require a lepton with large momentum transverse to the event thrust axis to obtain a sample of events enriched in B-Hadron decays. The distribution of signed impact parameters of all tracks in these events is used to measure the B-Hadron flight distance, and hence lifetime. If b-c dominates b-quark decay the corresponding weak mixing matrix element |Vcb| = 0.47 ± 0.006 ± 0.005, where the first error is from this experiment, and the second theoretical uncertainty. If b-u dominates, |Vub| = 0.033 ± 0.004 ± 0.12