Measurement of beam background in special high-β* LHC runs at √s = 900 GeV using the ATLAS–ALFA detectors

The background measurements performed during special runs with high $\beta^\ast$ optics at the injection energy of the LHC accelerator using the ALFA detectors in the ATLAS experiment are reported. Such special runs require dedicated collimation settings to minimize the irreducible beam-induced background. Thus, the physics data-taking had to be preceded with a series of tests of different collimation settings. The note covers background measurements performed during test runs in 2017 and 2018, as well as the background measurements during the 2018 physics run. The spatial distributions of background protons during physics data-taking are also discussed. In addition, a new type of collimators based on the crystal channelling effect was used during a subset of the fills. Data taken using crystal collimation scheme are also discussed and compared to runs with use of the two-stage collimation scheme.