Search for Charginos Nearly Mass-Degenerate with the Lightest Neutralino: Based on a Disappearing-Track Signature in pp Collisi

In this book, the anomaly mediated super symmetry breaking (AMSB) model is explored by searching for charged winos with their subsequent decays collected with the ATLAS detector at the Large Hadron Collider (LHC). The author develops a new method to detect charged winos that decay before reaching the Semiconductor Tracker (SCT) detector, called “re-tracking”. Since the nominal tracking algorithm at the ATLAS experiment requires at least seven successive hits in the inner tracking system, the sensitivity to charged winos having a fraction of a nanosecond in the past analysis was therefore limited. However, re-tracking requires a minimum of three pixel hits and provides a fully efficient tracking capability for charged winos traversing the pixel detector, resulting in around 100 times larger efficiency for charged winos with lifetime ∼ 0.2 ns than that in the past searches. The signal topology is characterized by a jet with large transverse momentum (pT), large missing transverse energy and a high-pT disappearing track. There are three types of background tracks: interacting-hadron tracks, charged leptons and tracks with mismeasured pT. A background estimation based on the Monte Carlo (MC) simulation suffers from large uncertainties due to the poor statistics and has difficulty simulating the properties of background tracks. Therefore, a data-driven approach has been developed to estimate the background track-pT spectrum. No significant excess above the background expectation is observed for candidate tracks with large transverse momentum, and constraints on the AMSB model are obtained. The author shows that in the AMSB model, a charged wino mass below 270 GeV is excluded at 95% confidence level, which also directly constraints the mass of wino dark matter.