Spatial Resolution Study of ALFA from the CERN Testbeam in 2006

The Roman Pot units located at a distance of 240m on each side of the ATLAS interaction point will be equipped with a scintillating fiber tracker system called ALFA (Absolute Luminosity For ATLAS). The main purpose of the ALFA system is to determine the absolute luminosity of ATLAS from elastic proton scattering at very small angles. This note describes in detail the spatial resolution study of ALFA that was performed using the data from the beam test at CERN in October 2006. The resolution performance was studied for a first ALFA prototype in the DESY testbeam in November 2005, but using off-the-shelf electronics and a 6 GeV electron beam. At the CERN testbeam a new ALFA prototype was operated with a first version of the dedicated ALFA front-end electronics and exposed to a hadron beam with energy up to 230 GeV. The resolution study shows an adequate tracking capability of ALFA using the first version of the front-end electronics and a new method to obtain alignment information is described.