Search for out-of-time decays of stopped particles at the ATLAS detector

On July 4th 2012, the ATLAS and CMS experiments announced the discovery of a new particle, later declared to be one of possibly many Higgs bosons. The Higgs mechanism has been so successful explaining several striking features of fundamental particle physics it was the topic of the 2013 Nobel Prize in physics. However, this mechanism provides a few problems of it's own. Most importantly, both the mass of the new Higgs boson and the cosmological constant must be extremely fine-tuned to produce a universe remotely similar to the one we observe today. Supersymmetry, a hypothetical extension to the current theory, addresses many problems in theoretical and experimental physics including the fine-tuned Higgs mass. In this work, a variant, called Split-Supersymmetry, is investigated; it avoids some problems in standard Supersymmetry while explicitly leaving the Higgs mass fine-tuned. A experimentally unique feature of Split-Supersymmetry is the production of R-hadrons---composite, massive, long-lived, particles. Indeed such long-lived states are predicted in several scenarios of physics beyond the Standard Model, and this search is sensitive to them as well. This dissertation describes the ATLAS searches using 2010, 2011 and 2012 data for gluino and squark R-hadrons which have come to rest within the ATLAS detector, particularly the calorimeter, and decay at some later time to jets or "ttbar" and a neutralino. Candidate decay events are triggered in the empty bunch crossings in order to remove collision backgrounds. Selections based on jet shape and muon-system activity are applied to discriminate events from backgrounds, the largest of which are cosmic and beam-halo muons. In the absence of a excess, limits are placed on the new particle mass as a function of its lifetime, for various neutralino masses and decay types.