The ATLAS experiment has released several new results in its search for supersymmetry (SUSY) using the full 13 TeV LHC data set from 2015 and 2016, obtaining sensitivity for certain new particles with masses exceeding 2 TeV.
SUSY is one of the most studied extensions of the Standard Model (SM) and, if realised in nature, it would introduce partners for all the SM particles. Under the assumption of R-parity conservation, SUSY particles would be pair-produced and the lightest SUSY particle (LSP) would be stable. The strongly produced partners of the gluon and quarks, the gluino and squarks, would decay to final states containing energetic jets, possibly leptons, and two LSPs. If the LSP is only weakly interacting, which would make it a dark-matter candidate, it would escape the detector unseen, resulting in a signature with missing transverse momentum.
A recent ATLAS analysis [1] searched for this signature, while a second [2] targets models where each gluino decays via the partner of the top quark (the “stop”), producing events with many jets originating from a b quark (b jets). Both analyses find consistency with SM expectations, excluding squarks and gluinos from the first two generations at 95% confidence level up to masses of 2 TeV (see figure). Pair-produced stops could decay to final states containing up to six jets, including two b jets, or through the emission of a Higgs or Z boson. Two dedicated ATLAS searches [3, 4] find no evidence for these processes, excluding stop masses up to 950 GeV.
SUSY might alternatively be manifested in more complicated ways. R-parity violating (RPV) SUSY features an LSP that can decay and hence evade missing transverse momentum-based searches. Moreover, SUSY particles could be long-lived or metastable, leading to unconventional detector signatures. Two dedicated searches [5, 6] for the production of gluino pairs and stop pairs decaying via RPV couplings have recently been studied by ATLAS, both looking for final states with multiple jets but little missing transverse momentum. In the absence of deviations from background predictions, strong exclusion limits are extracted that complement those of R-parity conserving scenarios.
The production of metastable SUSY particles could give rise to decay vertices that are separated by from the proton–proton collision point in a measurable way. An ATLAS search [7] based on a dedicated tracking and vertexing algorithm has now ruled out large regions of the parameter space of such models. A second search [8] exploited the new layer of the ATLAS pixel tracking detector to identify short track segments produced by particles decaying close to the LHC beam pipe, yielding sensitivity to non-prompt decays of SUSY charginos with lifetimes of the order of a nanosecond. The result constrains an important class of SUSY models where the dark-matter candidate is the partner of the W boson.
The ATLAS SUSY search programme with the new data set is in full swing, with many more signatures being investigated to close in on models of electroweak-scale supersymmetry.
