The ATLAS Forward Physics Program

The central ATLAS detector consists of an inner tracking detector (|η| < 2.5), electromagnetic and hadronic calorimeters (|η| < 4.9) and the muon spectrometer (|η| < 2.7). In addition to these, there are a number of sub-detectors that measure far-forward particle production at ATLAS. These are the LUCID, ZDC and ALFA detectors. The LUCID detectors [2] are located 17 m from the interaction point, one on each side of ATLAS, and provide coverage of 5.6 < |η| < 6.0 for charged particles. Each LUCID detector is a symmetric array of polished aluminium tubes that surround the beam-pipe. Each tube is 15mm in diameter and filled with C4F10 gas, which results in Cerenkov emission from charged particles crossing the tube. The Cerenkov light is read out by photo-multiplier tubes. An upgrade strategy for LUCID is to provide full azimuthal coverage, which is incomplete at installation and for early data taking. The Zero Degree Calorimeter (ZDC) [3] is located 140 m from the interaction point in the TAN region (target absorber for neutrals), which is the point where the single beam-pipe splits into two, and provides coverage of |η| > 8.3 for neutral particles. The ZDC consists of one electromagnetic and 3 hadronic tungsten/quartz calorimeters. Vertical quartz strips provide the energy measurements and horizontal quartz rods are used for coordinate readout. At LHC startup, when there are few bunches in the beam, the electromagnetic calorimeter is not installed and the space it would occupy is used by the LHCf experiment. After initial running, LHCf is removed and the full ZDC installed. The ALFA roman pot (RP) spectrometers are located 240 m from the interaction point [4]. Unlike other detectors, the RP spectrometers are not fixed relative to the beam. At injection, the ALFA detectors are in a withdrawn position far from the beam. After the beam has stabilized, the detectors are moved to within 1.5 mm of the beam. Elastic and diffractive protons which are not in the beam pass through arrays of scintillating fibre trackers (20×64 fibres in each array), which measure the distance of the proton to the beam. ALFA is used during special LHC runs at low luminosities with high β∗ optics.