Measurements and rejection strategies for non-collision backgrounds at the ATLAS experiment.

During nominal LHC collisions, protons can interact with residual gas in the beam pipe or with upstream collimators, producing showers of background particles known as Beam-Induced Backgrounds . These particles do not originate from the real proton–proton interaction point. Beam-Induced Beams can significantly impact detector performance and mimic signals in the searches for missing energy or for certain types of new physics, such as neutral long-lived particles. The ATLAS Non-Collision Background group plays a key role in developing tools to identify and reject these backgrounds. The characteristic features of Beam-Induced Background, as observed by the ATLAS Beam Conditions Monitor, the inner detector or as fake jets in calorimeters, are studied in detail in order to identify them and estimate their residual contribution. As a part of this effort, series of dedicated pressure bump tests were performed during LHC Run 2 by introducing local pressure bumps at different locations with a gas density higher by orders of magnitude than during normal operation. The rates of beam-gas events are then estimated from the pressure measurements and pressure bump profiles obtained from calculations. In this contribution, an overview of the origins of Beam-Induced Beam, the ATLAS Beam- Induced Beam online monitoring system, the pressure bump tests, and recent results based on measurements from LHC Run 2 data are provided.