Elementary Particle Physics

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Long-lived particles

Heavy long-lived particles (LLP) are predicted in a range of extensions of the Standard Model (SM). R-parity-conserving supersymmetry (SUSY) models, such as splitSUSY, gauge-mediated SUSY breaking (GMSB) and LeptoSUSY, as well as other scenarios such as universal extra dimensions and leptoquark extensions, allow for a variety of LLP states stable enough to be directly identified by the ATLAS detector. These states include long-lived super-partners of the leptons, quarks and gluons; sleptons, squarks and gluinos, respectively; as well as charginos, which together with neutralinos are a mixture of super-partners of the Higgs and W/Z bosons, known as Higgsinos, winos and binos.
When travelling with a speed measurably slower than the speed of light, charged particles can be identified and their mass (m) determined from their measured speed (β) and momentum (p), using the relation m = p/βγ, where γ is the relativistic Lorentz factor. Three different searches are presented in this article, using time-of-flight (TOF) to measure β and specific ionisation energy loss (dE/dx), to measure βγ.
The searches are based almost entirely on the characteristics of the LLP itself, but are further optimised for different experimental signatures.

Our group is making significant contributions to the ATLAS searches for (meta-)stable sleptons, charginos and composite colourless states of a squark or gluino together with light SM quarks or gluons, called R-hadrons. These searches are both particularly interesting and challenging from an experimental point of view, as they require a complete understanding of the detector systems and dedicated reconstruction algorithms.

Feel free to contact Dr. Sascha Mehlhase if you have any questions or are interested in working on this subject.