The Tevatron proton-antiproton collider, the highest-energy particle collider currently operational anywhere in the world, delivered 125 pb-1 integrated luminosity between 1990 and 1995. This period is refered to as 'Run I' and produced a large number of exciting high energy physics results, led by the discovery of the top quark in 1995. After an upgrade phase of more than six years, the Tevatron has been switched on again in March 2001. Major efforts have been taken in order to increase the luminosity and therby the physics potential of the collider, which will remain the world's only top factory until startup of the LHC. By redesigning the accelerator chain, the number of antiprotons available under normal operation and thus the luminosity have been siginificantly increased. The centre-of-mass energy has been raised from 1.8 TeV to 1.96 TeV. Up to now, the Tevatron has delivered a data sample corresponding to an integrated luminosity of aout 1 fb-1, colliding 36 proton on 36 antiproton bunches. This coresponds to a bunch crossing rate of 2.5 MHz and 396 ns between each crossing. The total integrated luminosity before the final shut down is expected at about 8 fb-1. This increase of about a factor 0f 80 with respect to Run I represents a major improvement, fostering the hope for exciting precision measurements and new physics discoveries in upcoming years.
The collisions produced by the Tevatron are studied by two multipurpose particle detectors, the D0 detector and the CDF detector. Here, the upgrade of the D0 detector with respect to Run I will be briefly described.
Substantial part of the upgrade has been the replacement of most electronics in order to conform with the small bunch spacing forseen for Run II. A three-level trigger system is incorporated in order to manage the huge raw event rates. On average, each bunch crossing produces one collision; this coresponds to a raw input rate of 7.6 MHz. Approximatly 50 Hz can be written to disk. Therefore, the trigger system is responsible for rejecting events which appear not to be interesting in light of the physical goals of the experiment. Each trigger level provides rate reduction sufficient to allow processing in the next level with minimal deadtime.
The most intriguing improvement to the physics capabilites of the D0 detector are the addition of a 2 T magnetic field and the installation of a new inner tracking system. The new silicon tracking system provides high precision track reconstruction at low radii and enables secondary vertex reconstruction, crucial for a variety of physics goals! The scinitilation fiber tracker extends to radii of 50 cm and further enhances the track reconstruction capabilites of the detector. Together, these two system measure charged particle momenta (from track curvature in the magnetic field) in the region |η| < 1.7.
The muon system has been extended to cover the region |η| < 2.0. The completely new system in the forward region (1.0 < |η| < 2.0) consists of three layers of drift tubes and scintilators. The later provide timing information and therefore help reject cosmics and reduce fake track rates. Standalone momentum measurement is possible.
The new scintillator-based preshower detectors located outside the magnet solenoid enhance electron identification both offline and at trigger level in combination with calorimeter information. They extend to |η| < 2.5 and are expected to provide a factor of 3-5 reduction in trigger rate.
The uranium liquid-argon calorimeter extends to |η| < 4.0 and thereby provides hermetic coverage needed for excellent missing transverse energy resolution and efficient jet identification and jet energy reconstruction. Its understanding is the key to all precision measurements regarding the typically jet-rich top antitop event topology.