The ZEUS Experiment

The ZEUS detector [142,143] makes use of a 700-ton compensating uranium sampling calorimeter, with equal sampling fractions for electromagnetic and hadronic shower components. The calorimeter is made up of layers of 2.6 mm SCSN-38 scintillator and 3.3 mm stainless-steel-clad depleted-uranium plates. One layer corresponds to 1.0 radiation length ($X_0$) and $0.04$ interaction lengths. This choice of layer thicknesses results in a sampling fraction of $4\%$ for electromagnetic and hadronic shower components, and hence compensation, and $7\%$ for minimum-ionizing particles. The compensation results in a very good hadronic energy resolution of $\sigma(E)/E \, = \, 0.35/\sqrt{E {\rm [GeV]}} \, \oplus \, 0.02$. The resolution for electromagnetic showers is $\sigma(E)/E \, = \, 0.18/\sqrt{E {\rm [GeV]}} \, \oplus \, 0.01$.

The ZEUS solenoidal coil of diameter 1.9 m and length 2.6 m provides a 1.43 T magnetic field for the charged-particle tracking volume. The tracking system consists of a central wire chamber covering the polar angular region from 15$^\circ$ to 164$^\circ$ , a forward planar tracking detector from 8$^\circ$ to 28$^\circ$ and a second planar tracking chamber in the backward direction, covering the region from 158$^\circ$ to 170$^\circ$. The momentum resolution attained is $\sigma(p_t)/p_t\,=\,0.005 \, p_t \, \oplus \, 0.015$ and a track is extrapolated to the calorimeter face with a transverse resolution of about 3 mm. Ionization measurements from the central tracking chamber also serve to identify electron-positron pairs from $J/\psi$ decays. The muon system is constructed of limited streamer tubes inside and outside of the magnetic return yoke, covering the region in polar angle from 10$^\circ$ to 171$^\circ$. Hits in the inner chambers provide muon triggers for $J/\psi$ decays.

The ZEUS trigger system consists of three layers. The first level trigger accepts events at a rate of about 300 Hz. The read out data are stored in digital or analog pipelines with a depth of 4.4$\mu$s until a global first level trigger decision is received. At the second level commercially available microprocessors analyze the digitized data of the components. The second level trigger processor functions as an asynchronous pipeline, i.e. a series of parallel processors. Beam gas background is rejected on the basis of calorimeter timing information which is available at this stage. The second level trigger is able to perform iterative calculations on large fractions of the full event information which are not possible in the pipelined structure of the first level trigger. These features enable the second level trigger to achieve a reduction of the first level trigger rate from $\sim$ 300 Hz to $\sim$ 100 Hz. The third level trigger provides a software filter in which the event rate is further reduced to the level of 15 Hz.