The HERA Accelerator and Detectors

The HERA accelerator, located at DESY in Hamburg, is an electron-proton collider. It is $6.3~{\rm\ km}$ in circumference and collides positrons (or electrons) at $\approx 27{\rm \ GeV}$with protons at $820{\rm \ GeV}.$ There are four interaction regions: two containing general purpose, hermetic detectors (H1 and ZEUS); another experiment (HERMES) investigating the spin distributions of the quarks in protons and neutrons; and another (HERA-B) planning to measure CP violation in the B-system. The H1 and ZEUS detectors took first data in 1992.

The ZEUS detector is shown in figure 1. The asymmetric design of the detector reflects the proton energy being significantly higher than that of the electron beam.

  

Figure 1: Cross sectional view of the ZEUS detector

The tracking system consists of a vertex detector (VXD) [1] and a central tracking chamber (CTD) [2] enclosed in a 1.43 T solenoidal magnetic field. Immediately surrounding the beampipe is the VXD which consists of 120 radial cells, each with 12 sense wires. The CTD, which encloses the VXD, is a drift chamber consisting of 72 cylindrical layers, arranged in 9 superlayers. Superlayers with wires parallel to the beam axis alternate with those inclined at a small angle to give a stereo view. A forward tracking detector is employed in the forward region to detect tracks in the proton direction and consists of three 12-layer planar drift chambers sandwiched with pairs of transition radiation detectors. In the rear direction there is an additional 12-layer planar drift chamber known as the rear tracking detector (RTD).

Outside the solenoid is the uranium-scintillator calorimeter (CAL) [3], which is divided into three parts: forward, barrel and rear covering the polar regions $2.6^\circ$ to $36.7^\circ$,$36.7^\circ$ to $129.1^\circ$ and $129.1^\circ$ to $176.2^\circ$, respectively. The CAL covers 99.7$\%$ of the solid angle, with holes of $20 \times 20$ cm² in the centres of the forward and rear calorimeters to accommodate the HERA beam pipe. Each of the calorimeter parts is subdivided into towers which are segmented longitudinally into electromagnetic (EMC) and hadronic (HAC) sections. The small angle rear tracking detector (SRTD) [4], which is attached to the front face of the rear calorimeter, measures the impact point of charged particles at small angles with respect to the positron beam direction.

The iron return yoke for the magnet is instrumented with proportional counters. This backing calorimeter (BAC) measures any hadronic energy which `leaks out' out of the main calorimeter. Beyond that and in the forward direction there are further detectors for muon detection.

Downstream of the main detector in the proton direction, six measuring stations are installed in the proton ring for detecting forward scattered protons. Beyond the final station, further downstream, is a forward neutron calorimeter. In the electron direction, two lead scintillator calorimeters placed $-35{\rm\ m}$ and $-107{\rm\ m}$ from the interaction point measure the luminosity and tag events with a small momentum transfer [5].

A fuller description of the ZEUS detector can be found in reference [6]. The H1 detector is of a very similar layout as ZEUS and a description can be found in reference [7].