Supersymmetry in the third generation

Despite the current absence of direct experimental evidence, supersymmetry (SUSY) at the weak scale remains among the most motivated and studied extensions of the Standard Model.

A common feature of many models is that third-generation SUSY particles – the stop (t~), sbottom (b~) and stau (τ~), which are the partners of the third-generation quarks and of the τ lepton – are lighter than the partners of the first two generations. Hence, they can be produced at large rates via pair production or in the decay of gluinos, the scalar partners of gluons. Furthermore, they should decay to heavy quarks (t, b) or τ leptons, providing characteristic and striking experimental signatures. The ATLAS collaboration recently presented the results from several searches for third-generation SUSY particles based on 2 fb^–1 of data (ATLAS 2012). Different strategies are used in each of these analyses, which rely on signatures with one or two hadronic taus in τ~ searches, b-jets with a lepton veto in b~ searches and two same-sign leptons or b-jets with a lepton in t~ searches. In the models considered, each SUSY decay-chain ends with the production of a stable, lightest supersymmetric particle (LSP), which is only weakly interacting and escapes detection. Therefore, high missing transverse-momentum is also required in all of these analyses.

The searches found no significant excess over the Standard Model background and provide the most stringent limits to date on models that are characterized by the decay of third-generation SUSY particles. The figure shows, as an example, that the exclusion limits obtained in the search for scalar bottom pair-production – using events with exactly two b-jets – extend the existing limits on the b~ mass by about 150 GeV.

In the coming months and with increasing amounts of data, these analyses will probe unexplored regions, corresponding for example to high-LSP or gluino masses, and so may shed light on the existence of third-generation SUSY particles.