Improving estimates of the number of fake leptons and other mis-reconstructed objects in hadron collider events: BoB's your UNCLE. (Previously "The Matrix Method Reloaded")

TL;DR

This paper evaluates the matrix method for estimating backgrounds from misidentified objects in collider experiments, identifies its limitations, and proposes a new strategy that balances accuracy and computational efficiency.

Contribution

It introduces a new approach that combines the speed of the matrix method with the accuracy of a more rigorous background estimation technique.

Findings

The matrix method has statistical limitations affecting limit setting.

A more rigorous method improves fake rate estimates but is computationally costly.

A new hybrid strategy offers a practical compromise.

Abstract

We consider current and alternative approaches to setting limits on new physics signals having backgrounds from misidentified objects; for example jets misidentified as leptons, b-jets or photons. Many ATLAS and CMS analyses have used a heuristic matrix method for estimating the background contribution from such sources. We demonstrate that the matrix method suffers from statistical shortcomings that can adversely affect its ability to set robust limits. A rigorous alternative method is discussed, and is seen to produce fake rate estimates and limits with better qualities, but is found to be too costly to use. Having investigated the nature of the approximations used to derive the matrix method, we propose a third strategy that is seen to marry the speed of the matrix method to the performance and physicality of the more rigorous approach.