Characterisation and mitigation of beam-induced backgrounds observed in the ATLAS detector during the 2011 proton-proton run

TL;DR

This paper characterizes beam-induced backgrounds in the ATLAS detector during 2011 proton-proton collisions, develops tagging tools to identify contaminated events, and demonstrates their importance for new physics searches.

Contribution

It introduces new background tagging methods based on detector observables and evaluates their effectiveness in cleaning data for physics analyses.

Findings

Background tagging tools achieve high efficiency.

Correlations between backgrounds and machine conditions are established.

Background mitigation improves sensitivity in physics searches.

Abstract

This paper presents a summary of beam-induced backgrounds observed in the ATLAS detector and discusses methods to tag and remove background contaminated events in data. Trigger-rate based monitoring of beam-related backgrounds is presented. The correlations of backgrounds with machine conditions, such as residual pressure in the beam-pipe, are discussed. Results from dedicated beam-background simulations are shown, and their qualitative agreement with data is evaluated. Data taken during the passage of unpaired, i.e. non-colliding, proton bunches is used to obtain background-enriched data samples. These are used to identify characteristic features of beam-induced backgrounds, which then are exploited to develop dedicated background tagging tools. These tools, based on observables in the Pixel detector, the muon spectrometer and the calorimeters, are described in detail and their efficiencies are evaluated. Finally an example of an application of these techniques to a monojet analysis is given, which demonstrates the importance of such event cleaning techniques for some new physics searches.