CDF trigger final balance: Offline resolution at low level selections to fight against Tevatron increasing luminosity

The CDF detector at Tevatron collider is at present the most long-lasting high energy physics experiment. Since its first data taking in 1992 it has produced many results of primary importance, such as the discovery of top quark and, more recently, the observations of Bs oscillations and single-top production. None of them would have been possible without a fast and efficient trigger system. Based on a three level architecture, the CDF trigger takes decisions on simple calorimetric and tracking objects and assures both high efficiency on signal events and low dead time. It reduces the data flow rate from 2.53 MHz, the collision rate, to 150 Hz, the current limit on tape writing and is flexible enough to be easily adapted to the continuously growing instantaneous luminosity. In the last years the Tevatron instantaneous luminosity has rapidly increased and is now reaching 4×1032cm-2s-1. The CDF trigger system has been widely upgraded to cope with increasing trigger rates. The upgrade result is online reconstruction of missing transverse energy, jets and tracks with a quality comparable to the offline one. Jet energy and direction can be precisely determined and tracks can be subjected to 3-D reconstruction with good resolution. These upgrades reduce high trigger rates to acceptable levels and have provided invaluable tools to increase the purity of the collected samples. They also represent a helpful experience for LHC experiments where background rates will be much more demanding.