Sensitivity of the ANUBIS and ATLAS Detectors to Neutral Long-Lived Particles Produced in $pp$ Collisions at the Large Hadron Collider

For the past fourteen years, the Large Hadron Collider (LHC) has been colliding particles at nearly the speed of light in an attempt to understand and test the Standard Model of particle physics. The Standard Model, however, is not a complete theory; for example, it lacks explanations for the observed matter-antimatter asymmetry, as well as for the nature of the dark matter which populates our universe. Many proposed extensions to the Standard Model postulate the existence of massive long-lived particles with lifetimes ranging up to one second. Despite their theoretical promise, however, searches for long-lived particles have yet to find experimental evidence. This lack of observation may be due to a lack of sensitivity as the LHC’s ATLAS Experiment alone is only sensitive to particles with proper lifetimes reaching about 10 ns. The ANUBIS project proposes installing a novel set of detectors in the existing service shaft of the LHC’s ATLAS Experiment and/or in its cavern at a modest civil engineering cost. This thesis presents the ANUBIS project by discussing various proposed layouts for its detectors and the physics potential of these configurations. These studies show that the combined ANUBIS and ATLAS detectors could be sensitive to branching ratios of 10, 20, 30, and 40 GeV Higgs portal long-lived particle production reaching $\mathcal{O}\left(10^{−6}\right)$ for particles with $c\tau$ of 3, 4, 6, and 10 m, respectively. This demonstrates an improvement of several orders of magnitude over the sensitivity of the ATLAS Experiment alone. Studies of the background events with which ANUBIS will have to contend are also presented, as is a discussion of its prototype detector, proANUBIS, which will seek to validate these studies.