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

TL;DR

The paper investigates beam-induced backgrounds observed by ATLAS during 2011, identifying primary sources such as tertiary halo and beam-gas interactions near the LHC. It develops and validates background tagging tools using the Pixel detector, muon system, and calorimeters, complemented by dedicated BIB simulations (SixTrack/Fluka) to understand spatial and temporal signatures. Trigger-rate monitoring (BCM and L1 triggers) reveals correlations with vacuum conditions and ghost charge, while pixel-based tagging and muon rejection techniques provide high-purity background identification. The jet cleaning framework, including a data-driven monojet analysis, demonstrates that effective BIB cleansing is critical for preventing fake jets and ensuring reliable searches for new physics.

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.

Figures (35)

• Figure 1: The general layout of the LHC lhcfigref. The dispersion suppressors (DSL and DSR) are sections between the straight section and the regular arc. In this paper they are considered to be part of the arc, for simplicity. LSS denotes the Long Straight Section -- roughly 500 m long parts of the ring without net bending. All insertions (experiments, cleaning, dump, RF) are located in the middle of these sections. Beams are injected through transfer lines TI2 and TI8.
• Figure 2: Detailed layout of the ATLAS interaction region lhc. The inner triplet consists of quadrupole magnets Q1, Q2 and Q3. The tertiary collimator (TCT) is not shown but is located between the neutral absorber (TAN) and the D2 magnet.
• Figure 3: Schematic illustration of the LHC cleaning system. Primary and secondary collimators and absorbers in the cleaning insertions remove most of the halo. Some tertiary halo escapes and is intercepted close to the experiments by the TCT assmannfig.
• Figure 4: Inelastic beam-gas interaction rates per proton, calculated for beam-1 in LHC fill 2028. The machine elements of main interest are indicated. The pressure in the arc is assumed to be constant from 270 m onwards. The letters, a-e, identify the different sections, for which rates are given separately in other plots.
• Figure 5: Simulated distribution of the $z$-coordinates of inelastic beam-gas events from which a muon with more than 100 GeV has reached the interface plane at 22.6 m. The two curves correspond to muons at radii below and above 1 m at the interface plane.