In late 2011, ATLAS launched a dedicated programme targeting searches for the supersymmetric partner of the top quark – the scalar top, or “stop” – which could be pair-produced in high-energy proton–proton collisions. If not much heavier than the top quark, this new particle is expected to play a key role in explaining why the Higgs boson is light.
While earlier supersymmetry (SUSY) searches at the LHC have already set stringent exclusion limits on strongly produced SUSY particles, these generic searches were not very sensitive to the stop. If it exists, the stop could decay in a number of ways, depending on its mass and other SUSY parameters. Most of the searches at the LHC assume that the stop decays to the lightest SUSY particle (LSP) and one or more Standard Model particles. The LSP is typically assumed to be stable and only weakly interacting, making it a viable candidate for dark matter. Events with stop-pair production would therefore feature large missing transverse momentum as the two resulting LSPs escape the detector.
The first set of results from the searches by ATLAS were presented at the International Conference on High-Energy Physics (ICHEP) in 2012. A stop with mass between around 225 and 500 GeV for a nearly massless LSP was excluded for the simplest decay mode. Exclusion limits were also set for more complex stop decays.
These searches revealed a sensitivity gap when the stop is about as heavy as the top quark – a scenario that is particularly interesting and well motivated theoretically. Such a “stealth stop” hides its presence in the data, because it resembles the top quark, which is pair-produced roughly six times more abundantly.
Use of the full LHC Run-1 data set, together with the development of novel analysis techniques, has pushed the stop exclusion in all directions. The figure shows the ATLAS limits as of the ICHEP 2014 conference, in the plane of LSP mass versus stop mass for each of the following stop decays: to an on-shell top quark and the LSP (right-most area); to an off-shell top quark and the LSP (middle area); to a bottom quark, off-shell W boson, and the LSP (left-most grey area); or to a charm quark and the LSP (left-most pink area). The exclusion is achieved by the complementarity of four targeted searches (ATLAS Collaboration 2014a–2014d). The results eliminate a stop of mass between approximately 100 and 700 GeV (lower masses were excluded by data from the Large Electron–Positron collider) for a light LSP. Gaps in the excluded region for intermediate stop masses are reduced but persist, including the prominent region corresponding to the stealth stop.
Standard Model top-quark measurements can be exploited to get a different handle on the potential presence of a stealth stop. The latest ATLAS high-precision top–antitop cross-section measurement, together with a state-of-the-art theoretical prediction, has allowed ATLAS to exclude a stealth stop between the mass of the top quark and 177 GeV, for a stop decaying to a top quark and the LSP.
The measurement of the top–antitop spin correlation adds extra sensitivity because the stop and the top quark differ by half a unit in spin. The latest ATLAS measurement (ATLAS Collaboration 2014e) uses the distribution of the azimuthal angle between the two leptons from the top decays, together with cross-section information, to extend the limit for the stealth stop up to 191 GeV.
The rigorous search programme undertaken by ATLAS has ruled out large parts of interesting regions of the stop model and closed in on a stealth stop. It leaves the door open for discovery of a stop beyond the current mass reach, or in remaining sensitivity gaps, at the higher-energy and higher-luminosity LHC Run 2.
