The CDF L2 Track Trigger Upgrade

Since 2001 the CDF II detector has been collecting data used to carry out a rich physics program at the Fermilab Tevatron pp̄ collider. The quality and purity of the data relies heavily on the high performance and efficiency of the CDF trigger system. The online trigger most heavily relied on for physics analyses is the eXtremely Fast Tracker (XFT), a track trigger system used to identify charged tracks. Tracks are identified based on the data from sense wires in the central tracking chamber (COT) which are read out in time for the Level 1 trigger decision. The tracks are extrapolated to the calorimeter and muon chambers to form electron and muon trigger candidates. The steady increase of the Tevatron instantaneous luminosity results in higher detector occupancy from multiple pp̄ interactions. For the track trigger, this produces overlapping patterns of hits which can be identified as high momentum track, leading to the rapid growth of track trigger rates with instantaneous luminosity. The control of trigger rates by either raising the thresholds or prescaling the triggers results in signficicant loss in acceptance for important physics signatures with a negative impact on the physics program. An upgrade of the original system was thus required. Without the track trigger upgrades the physics potential of CDF would have be significantly compromised. A description of the axial XFT system and the Level 1 track trigger upgrade can be found elsewhere [1]. Here we describe the Level 2 part of the XFT system. The rough sequence of the data is that track segments identified at Level 1 are delivered, with a higher resolution, to the Level 2 processor, where more available time allows for full 3D-track reconstruction. This improves not only the track trigger purity but also allows the use of the track φ and cot θ information.