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.
Highlights
In this paper, analyses of beam induced backgrounds (BIB) seen in the ATLAS detector during the 2011 proton-proton run are presented
The tertiary collimators (TCT) is the main source of BIB resulting from tertiary halo losses. Since it is in the straight section with only the D1 dipole and inner triplet separating it from the interaction point (IP), it is expected that the secondary particles produced in the TCT arrive at rather small radii at the experiment
As described by eq (6.1), the pixel cluster width distributions are normalised to the total number of pixel clusters in each pseudorapidity bin, so that the relative multiplicity of all cluster widths can Cluster width in Z direction [pixels]
Summary
Like the BCM, the ATLAS Pixel detector is very close to the beam-line, so it is sensitive to similar background events. While the BCM consists of only eight active elements, the Pixel detector has over 80 million read-out channels, each corresponding to at least one pixel This fine granularity enables a much more detailed study of the characteristics of the BIB events. BIB events identified by a significant time difference between the BCM stations on either side of ATLAS, are found to exhibit large cluster multiplicity in the Pixel detector [29]. The characterisation of BIB-like events by comparing distributions for paired and unpaired bunches, coupled with the event timing in other sub-detectors, allows parameters to be determined for the efficient identification of BIB in the Pixel detector. The BIB tagging efficiency is quantified and the tools are applied to study 2011 data
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