Design and Construction of a Vertex Chamber and Measurement of the Average B-Hadron Lifetime PDF Download

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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 .mu.m thickness are utilized in this Vertex Chamber. It achieves a spatial resolution of 45 .mu.m, and a resolution in extrapolation to the B-Hadron decay location of 87 .mu.m. Its inner layer is 4.6 cm from ee− colliding beams. The Vertex Chamber is situated within the MAC detector at PEP. We have analyzed botht he 94 pb−1 of integrated luminosity accumulated at .sqrt.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. 106 refs., 79 figs., 20 tabs.