The ATLAS Forward Physics Program

After a brief review of the approved ATLAS forward detector system we describe the main ATLAS forward physics program. This program currently includes such topics as soft and hard diffraction, double pomeron exchange a , central exclusive production, rapidity gap survival, two photon physics, the determination of the total cross-section and the determination of the absolute luminosity A possible high luminosity upgrade program involving new forward proton detectors is also briefly reviewed. This program opens up a new vista of forward physics for ATLAS that includes SM/MSSM/NMSSM Higgs boson studies, W pair production, slepton production and gluino pair production, etc. 1 The ATLAS Forward Detector System The central ATLAS detector consists of an inner tracking detector (|η| < 2.5), electromagnetic (EM) and hadronic calorimeters ((|η| < 4.9)and a muon spectrometer (|η| < 2.7). In addition, there are a number of sub- detectors that measure far-forward particle production at ATLAS. They are, in order of distance from the ATLAS IP, the LUCID [1], ZDC [2] and ALFA [3] detectors. The LUCID detectors [1] are located ±17 m from the interaction point, and provide a coverage 5.5 < |η| < 6.0 for charged particles. Each LUCID detector is a symmetric array of polished aluminium tubes that surround the beam-pipe and point toward the ATLAS IP. Each tube is 15mm in diameter and filled with C4F10 gas, which results in a maximum of Cerenkov emission from charged particles from the IP which traverse the full length of the the tube. The Cerenkov light is read out by photo-multiplier tubes. A high luminosity scenario upgrade strategy for LUCID, that enhances its efficacy in the forward physics program, is being explored. The Zero Degree Calorimeters (ZDCs) provide coverage of the region |η| > 8.3 for neutral particles. They reside in a slot in the TAN (Target Absorber Neutral) absorber, which would otherwise contain copper shielding. The TAN is located at ± 140m from the interaction point, at the place where the straight-section of the beam-pipe divides into two independent beam- pipes. The ZDC consists of one EM and three hadronic tungsten/quartz calorime- ters. Vertical quartz strips provide the energy measurements and horizontal quartz rods are used for coordinate readout. At LHC startup, when the luminosity is low, the ZDC EM calorimeter is not installed and the space it would occupy is filled by the LHCf experiment. After initial running, LHCf is removed and the full ZDC installed. The ALFA (Absolute Luminosity For ATLAS) Roman Pot (RP) spectrometers are located ± 240 m from the interaction point [3]. The RP spectrometers are not fixed relative to the beam. At injection, the ALFA detectors are moved away from the beam. After the beam has stabilized, the detectors are moved back to within 1.5 mm of the beam. Elastic and diffractive protons which are not in the beam pass through arrays of scintillating fibre * On behalf of the ATLAS Collaboration. a The lowest order prototype of the Pomeron (IP) is, in perturbative QCD, a colour neutral system of two gluons DIS 2009