Performance of tracking and vertexing techniques for a disappearing track plus soft track signature with the ATLAS detector

Several theories beyond the Standard Model predict the existence of massive long-lived charged particles with an extremely compressed mass spectra. This note presents improved tracking techniques for searches of long-lived charged particles decaying to a low momentum charged particle and a massive weakly interacting particle within the tracking volume of the ATLAS inner detector. This signature is characterized by a disappearing high momentum track with as few as three hits in the innermost layers of the ATLAS pixel detector, a low momentum charged track reconstructed in the outer layers of the ATLAS micro-strip tracker, and a displaced secondary vertex formed by the combination of these two tracks. Dedicated tracking and vertexing algorithms are developed to target this challenging signature, and the performance of these algorithms are characterized using a supersymmetric higgsino model, where the lightest chargino ($\tilde{\chi}^{\pm}_{1}$) is nearly degenerate with the lightest neutralino ($\tilde{\chi}^{0}_{1}$), and decays primarily $\tilde{\chi}^{\pm}_{1}$~$\rightarrow$~$\pi^{\pm}~\tilde{\chi}^{0}_{1}$.