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.
The Mark II detector for the SLC
Abstract The Mark II detector has been upgraded in preparation for its role as the first detector to take data at the Stanford Linear Collider. The new detector components include the central drift chamber, the time-of-flight system, the coil, the endcap electromagnetic calorimeters and the beam energy and luminosity measuring devices. There have also been improvements in detector hermeticity. All of the major components were installed for a test run at the PEP storage ring (√s = 29 GeV) in 1985. This paper describes the upgraded detector, including its trigger and data acquisition systems, and gives performance figures for its components. Future improvements are also discussed.