Measuring the Branching Fraction for Z " + Bb Using Precision Vertex Detectors*
-We have measured the fraction of b6 events in hadronic Z” decays, Rbb, using the vertex detector system of the Mark II detector at the SLC. We tag bb events by requiring at least three tracks with significant impact parameters. This tag is 50% efficient and results in a sample of 85% purity We find R,b = 0.251 f 0.049 f 0.030, in good agreement with other measurements and the Standard Model prediction. Presented at Particle and Fields ‘91, Vancouver, BC, Canada, August 18-22, 1991 *Work supported by Department of Energy contract DE-AC03-76SFOO515. ‘Representing the Mark II Collaboration.
A Measurement of the branching ratio of the Z boson to bottom quarks using precision tracking
We use the precision vertex detectors installed in the Mark II detector to measure Rb, the fraction of 2’ bosons decays into hadrons that include bottom quarks. The Z” bosons were created via e+eannihilation at 91 GeV by the Stanford Linear Collider during the 1990 running, The Mark II tracking system, including the Silicon Strip Vertex Detector and Drift Chamber Vertex Detector, achieves a track position resolution of better than 20 microns for high momentum tracks at the collision point. We describe procedures used to obtain this resolution, including alignment of the Silicon Strip Vertex Detector, and techniques used to quantify the achieved resolution. We can, for the first time, measure the impact parameters of tracks accurately enough to use the finite b quark lifetime to tag a sample of events containing b quarks. This is done by requiring events which contain at least three tracks with impact parameter greater than three times the calculated impact parameter error. The b quark tagging efficiency is 48% and 85% of the tagged sample events contain b quarks. Using the 29 events tagged out of a total of 220 identified hadronic Z0 decays, we measure I$= Br(ZO+bb) Br(Z"+hadrons) = 0.230+0.048+0.030 where the first error is statistical and the second is systematic. This value of Rb is in good agreement with existing measurements made using other methods and the Standard Model prediction of 0.22 using a sin28w value of 0.225.
Thermal incompatibility analysis of metal-ceramic systems based on flexural displacement data.
The feasibility of simple tests or analytical methods for prediction of residual stress states in metal-ceramic (MC) prostheses has not been demonstrated. Biomaterial metal-ceramic strips have been proposed to provide sensitive measures of transient and residual stress states through the measurement of midpoint deflection after cooling from the ceramic sintering temperature. The objective of this study was to apply the elastic-viscoelastic analogy to calculate transient and residual midpoint deflections in MC biomaterial strips and to compare these values with deflections measured with a beam-bending viscometer (BBV). Calculations and measurements were made for five MC systems that were found from a clinical study to be "thermally compatible" systems. Metal strips, 64 mm in length, 3 mm wide, and either 0.5 mm, 1.0 mm, or 2.0 mm in thickness, were veneered with four 0.25-mm thick layers of opaque porcelain. Midpoint deflection of the MC strips (ceramic oriented in the posterior position) was measured during cooling from an initial temperature of 700 degrees C. In general, the directions of the measured residual deflections did not agree with the "textbook" convention that negative deflections are associated with positive thermal contraction mismatch (alpha(M) - alpha(c) > 0) regardless of metal thickness. For a metal thickness of 0.5 mm, the residual midpoint deflection for all thermal contraction mismatch cases, except one, was positive (upward deflection) whereas the residual midpoint deflections were all negative when the metal thickness was increased to 1 or 2 mm, independent of the thermal contraction mismatch. The best agreement between calculated and measured values of residual midpoint deflection (+16 microns vs. +14 +/- 2.3 microns, respectively was obtained for MC biomaterial strips with a Ni-Cr alloy (0.5 mm thick) while the largest difference (+346 microns vs. +61 +/- 43.8 microns) was obtained for MC bimaterial strips with a Au-Pd allow (0.5 mm thick). In all but one case, changes in deflection direction as a function of metal thickness were correctly predicted by the viscoelastic analysis. The results of this study indicate that a viscoelastic model is useful for estimating thermal compatibility conditions of MC systems.