ALFA: Absolute Luminosity For ATLAS - Development of a scintillating fibre tracker to determine the absolute LHC luminosity at ATLAS

The ATLAS collaboration will determine the absolute luminosity of the CERN LHC at Interaction Point 1 by measuring the trajectory of protons elastically scattered at very small angles (μrad). A scintillating fibre tracker system called ALFA (Absolute Luminosity For ATLAS) is proposed for this measurement. Detector modules will be placed above and below the LHC beam axis in roman pot units at a distance of 240 m on each side of the ATLAS interaction point. They allow the detectors to approach the beam axis to millimeter distance. Overlap detectors also based on the scintillating fibre technology, will measure the precise relative position of the two detector modules. Results obtained during beam tests at DESY and at CERN validate the detectors design and demonstrate the achievable resolution. We report about preliminary results obtained with a full-scale prototype with final readout electronics in 2008.