Central Arm Spectrometers

The central arm spectrometers consist of tracking systems for charged particles and electromagnetic calorimetry. They cover $\pm0.35$ in pseudorapidity ($\eta$) and each arm spans $90\grad$ in azimuthal angle ($\phi$). The magnetic field is supplied by the Central Magnet (CM) providing an axial field parallel to the beam.

The innermost tracking detectors are the Drift Chambers (DC), placed at about 2 m from the center. They are each composed of 40 planes of wires, arranged into 160 drift cells, providing precise tracking in the $x$-$y$ plane and a high resolution $p_T$ measurement from the charged particle trajectories.

There are three layers of Pad Chambers (PC1,PC2,PC3) on the west arm and two on the east arm (PC1,PC3). The Pad Chambers are pixel-based multi-wire proportional chambers that perform space point measurements of charged particles enhancing the pattern recognition capabilities. Pad Chamber measurements minimize the influence of background from albedo and non-vertex decay particles and determine the polar angle ($\theta$) of the trajectories. The Pad Chambers are described in more detail in section 2.5.

The Ring Imaging Cherenkov Counters (RICH) are the primary detectors for electron identification. They are composed of a gas vessel, thin reflection mirrors, and arrays of photo-multiplier tubes. Electron-pion discrimination is provided, since electrons above 18 MeV/c emit Cherenkov light in the gas radiator ($CO_2$), while pions below 4.9 GeV/c do not produce a signal.

The Time Expansion Chamber (TEC), placed on the east arm only, contains four planes of wires. Like the DC, the TEC provides tracking in the $r$-$\varphi$ plane. It also contributes to electron-pion discrimination via $dE/dx$ measurements, and provides additional high momentum resolution for high-$p_T$ particles. From run period 3 it will also operate as a transition radiation detector for electron identification.

The Time-Of-Flight Detector (TOF), located on the east arm only, serves as the primary particle identification device for charged hadrons. It contains plastic scintillation counters with photo-multiplier tubes, and provides high-resolution time-measurements.

The Electromagnetic Calorimeters (EMC) provide photon identification, particle energy and time-of-flight measurements. 25% of the EMC are lead-glass calorimeters (PbGl) providing good energy resolution. The rest is made up of lead-scintillator calorimeters providing good timing.