Forward Detectors in ATLAS: ALFA, ZDC and LUCID

In order to determine the experimental cross sections for the observed physics processes, an estimation of the absolute luminosity is needed. In fact a careful study of “well known” processes will be one of the first steps of the LHC experiments as it can provide possible signatures of new physics which consist in deviations with respect to the Standard Model (SM) predictions. The methodologies for luminosity monitoring and total cross section estimation at the LHC will be reviewed in this talk along with the dedicated detectors of the ATLAS experiment. ATLAS will make extensive usage of the detectors in the forward region each one with a different task: LUCID (LUminosity measurement using Cherenkov Integrating Detector) is a system of 40 (2 x 20) Cherenkov tubes, surrounding the beam pipe at about 17 m from the interaction region. It will be able to monitor the collision-by-collision luminosity by detecting and counting the number of charged particles coming from the impact point. ALFA (Absolute Luminosity for ATLAS) is a system of ultra-small-angle detectors, located at 240 m on either side of the ATLAS interaction region. It consists of 4 station of Roman Pots (RP) inserts in the beam pipe equipped with position sensitive scintillating fibers. Its aim is to measure elastic scattering at small angles. This will provide a measurement on both the absolute luminosity and the total cross section. ZDC (Zero Degree Calorimeter) is a Tungsten-quartz fiber calorimeter situated at about 140 m from the interaction region on each side and housed in transverse aperture of neutral particle absorber (TAN). The ZDC detector will be able to study both heavy ions and pp physics (η>8.3) and will be an effective beam-tuning device. Over the last years a big effort to assure ATLAS a “good” estimation of luminosity was done. Different methods for luminosity determination are performed and will be reported in the talk.