Introduction and Goals

The Collider Detector at Fermilab is a 5000 ton multi-purpose particle physics experiment[2] dedicated to the study of proton-antiproton collisions at the Fermilab Tevatron collider. It was designed, built and operated by a team of physicists, technicians and engineers that now spans 44 institutions and includes approximately 500 official members. Previous versions of this detector have included among their complement of instrumentation silicon vertex track detectors that have added substantially to the overall physics capabilities of the experiment, especially for studies of top and bottom quarks.

Figure 1: An overview of the Collider Detector at Fermilab in its Run II configuration.

The earlier silicon vertex detectors[3] operated for CDF during collider Run I between 1992 and 1996 were composed of four layers of single-sided sensors that covered interactions within $\pm 27$ cm along the beam line of the nominal center of the experiment. These interactions were distributed approximately as a gaussian along the beam ($z$) direction, with an average standard deviation of typically 30 cm. The relatively short length of the silicon sensors limited the geometric acceptance to about 60% for single tracks[4,6], averaged over the luminous region. The angular acceptance for tracks from any given interaction was also limited by the previous geometry. Although detection of some tracks in forward and backward directions was possible for interactions that were displaced along the $z$ direction from the center of the detector[5], more complete geometric coverage of the interaction region was clearly desirable.

Figure 2: A cutaway view of one quadrant of the inner portion of the CDF II detector showing the tracking region surrounded by the solenoid and endcap calorimeters.

For the next operating period of the accelerator, to be known as collider Run II, the expected instantaneous luminosity will be approximately an order of magnitude larger than the nominal values of up to $2\times10^{31}$cm$^{-2}$s$^{-1}$ encountered during Run  I. Changes planned for the CDF detector during Run II to meet this challenge include simplification and improvement of the angular range of the calorimetry and muon coverage, improvement of the speed of the electronics and trigger system, addition of a time of flight counter, and complete replacement of the charged particle track detectors, including the silicon detectors[6]. Figure 1 shows an isometric cutaway view of the planned configuration of the experiment once these changes are made.

Operation of the earlier Run I detectors, called SVX for the period between 1992 and 1993 and SVX$^{\prime}$ for the period between 1994 and early 1996, provided CDF with substantial experience in the electronics needs for readout[7] and radiation environment[8] to be expected in Tevatron hadron collisions. Detectors and electronics that can withstand several megarads of integrated dose are required to survive the radiation fields created by the higher Run II luminosity[6]. The beam crossing interval will be reduced by up to a factor of 25 to as little as 132 ns between bunches in order to reduce number of interactions per beam crossing. To avoid losing physics signals, electronics that can operate without deadtime losses are preferred.

Goals to be achieved by this upgrade include the determination of precise 3-dimensional track impact parameters over as wide an acceptance range as possible to provide $b$-tagging for studies of top production, supersymmetry searches and the search for the Higgs boson. This detector and the associated trigger upgrades[9] will also be of great benefit to the CDF $B$ physics program. Other goals for the Run II silicon system for CDF are to improve stereo tracking, to bridge more seamlessly between the vertex detector and outer tracker than in Run I, to improve the purity and efficiency of the tracking, and to increase the angular acceptance for well-reconstructed tracks [6,10].

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