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 requires 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 pm thickness are utilized in this Vertex Chamber. It achieves a spatial resolution of 45pm, and a resolution in extrapolation to the B-Hadron decay location of 87pm. Its inner layer is 4.6 cm from e+ecolliding beams. The Vertex Chamber is situated within the MAC detector at PEP. We have analyzed both the 94pb -’ of integrated luminosity accumulated at fi = 29 GeV with the Vertex Chamber in place as well as the 2lOpb-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. . The trimmed mean signed impact parameters are 130 & 19pm for data accumulated with the Vertex Chamber, and 162 & 25pm for previous data. Together these indicate an average B-Hadron lifetime of [ +0.22 70 = 1.37 -0.19 stat.hO.11 sys. I x (1 310.15 sys.) psec. We separate additive and multiplicative systematic errors because the second does not degrade the statistical significance of the difference of the result from 0. If &c dominates b-quark decay the corresponding weak mixing matrix element lVc.l = 0.047 3~ 0.006 rt 0.005, where the first error is from this experiment, and the second theoretical uncertainty. If &.L dominates, lVubl = 0.033 -+ 0.004 -+ 0.12.