LHC collimation with a reduced beam pipe radius in ATLAS

To increase physics performance, an upgrade of the ATLAS detector has been requested. As a consequence, a new beam pipe with a smaller radius needs to be installed inside the experimental detector. Based on SixTrack simulations, we investigate the effect from collimation of a reduced beam pipe in the ATLAS experiment. Several running scenarios are studied with a range of different beam pipe radii and in each case we conclude on the minimum allowed aperture, which does not cause beam losses inside the detectors.