S. Schramm

Simplified models for dark matter searches at the LHC

This document outlines a set of simplified models for dark matter and its interactions with Standard Model particles. It is intended to summarize the main characteristics that these simplified models have when applied to dark matter searches at the LHC, and to provide a number of useful expressions for reference. The list of models includes both s-channel and t-channel scenarios. For s-channel, spin-0 and spin-1 mediation is discussed, and also realizations where the Higgs particle provides a portal between the dark and visible sectors. The guiding principles underpinning the proposed simplified models are spelled out, and some suggestions for implementation are presented.

ATLAS Reconstruction and Performance

When conducting an analysis, the first step is to define the topology that will be considered. This selection typically requires the presence or absence of certain objects, where an object is either a physical particle (electrons, muons, taus, photons), a representation of an underlying physical process (jets and b-jets), or the contribution of invisible processes creating an imbalance in the event (\(\mathrm {E}_{\mathrm {T}}^{\mathrm {miss}}\)). As such, detector quantities such as cells, tracks, and clusters must be translated into objects, which can then be properly calibrated and used in analyses.

Jet Reconstruction and Performance

Unlike the other objects discussed in Chap. 3, jets are not a physical particle from the SM, rather they are a tool designed to represent an underlying physical process. While electrons exist as an independent entity, jets are defined only by the method which was used to build them. When a muon is observed, all of the different reconstruction algorithms aim to reproduce the single muon to the best precision with a single set of detector observations. On the other hand, the same set of detector observations can be used to build radically different types of jets, many of which are independently useful. Examples of the independent uses for different types of jet algorithms will be provided.

Mono-jet Dark Matter Interpretation

This chapter is based on Ref. [1]. All ATLAS material, unless otherwise mentioned, is from this document.

The Mono-Jet Analysis

This chapter is based entirely on Ref. [1]. All ATLAS material, unless otherwise mentioned, is from this document.

Introduction and Motivation for Dark Matter

One of the largest remaining open questions in physics is the nature of DM. First postulated in the 1930s [1, 2], many independent astrophysical experiments have observed the effects of DM. Cosmology has even measured its abundance to be approximately five times that of the visible matter which makes up the universe [3], including all of the stars, planets, black holes, and other known sources of matter. However, there remains no experimentally verified theory that explains the origin of DM. While numerous experiments have been designed to search for DM, and some have claimed observations consistent with the signal expected from such a phenomenon [4], the nature of DM remains unknown.

The ATLAS Experiment

A substantial part of this chapter is based off of [1] for the Large Hadron Collider (LHC) and [2] for the A Toroidal LHC ApparatuS (ATLAS) detector.

The ATLAS detector: status and performance in Run-II

During the first extended shutdown of the LHC, in 2013 and 2014, the ATLAS detector has undergone several improvements. A new silicon pixel detector layer has been added inside of the existing layers, enhancing vertex identification, while the coverage of the muon detector has been significantly expanded. Many other detector systems have been upgraded to handle the higher expected pileup conditions in the coming years and to generally improve their performance. This document describes these upgrades and the resulting impact on the reconstruction and performance of standard physics objects. Preliminary results using the first ∼ 80pb−1 of 2015 data at s = 13 Tev are presented, demonstrating the capability of ATLAS to perform both searches and measurements.