As the LHC delivered proton–proton collisions at the record energy of 13 TeV during the summer and autumn of last year, the experiments were eagerly scrutinising the new data. They were on alert for the signatures that would be left by new heavy particles, indicating a breakdown of the Standard Model in the new energy regime. A few days before CERN closed for the Christmas break, and only six weeks after the last proton–proton collisions of 2015, the ATLAS collaboration released the results of seven new searches for supersymmetric particles.
Supersymmetry (SUSY) predicts that, for every known elementary particle, there is an as-yet-undiscovered partner whose spin quantum number differs by half a unit. In most models, the lightest SUSY particle (LSP) is stable, electrically neutral and weakly interacting, hence it is a good dark-matter candidate. SUSY also protects the Higgs boson mass from catastrophically large quantum corrections, because the contributions from normal particles and their partners cancel each other out. The cancellation is effective only if some of the SUSY particles have masses in the range probed by the LHC. There are therefore well-founded hopes that SUSY particles might be detected in the higher-energy collisions of LHC Run 2.
The data collected by the ATLAS detector in 2015 are just an appetiser. The 3.2 fb–1 of integrated luminosity available are an order of magnitude less that collected in Run 1, and a small fraction of that expected by 2018. The first Run 2 searches for SUSY particles have focused on the partners of the quarks and gluons (called squarks and gluinos). They would be abundantly produced through strong interactions with cross-sections up to 40 times larger than in Run 1. The sensitivity has also been boosted by detector upgrades (in particular, the new “IBL” pixel layer installed near to the beam pipe) and improvements in the data analysis.
Squarks and gluinos would decay to quarks and the undetectable LSP, producing an excess of events with energetic jets and missing transverse momentum. The seven searches looked for such a signature, with different selections depending on the number of jets, b-tagged jets and leptons, to be sensitive to different production and decay modes. Six of the searches found event rates in good agreement with the Standard Model prediction, and placed new limits on squark and gluino masses. The figure shows the new limits for a gluino decaying to two b quarks and a neutralino LSP. For a light neutralino, the Run 1 limit of 1300 GeV on the gluino mass has been extended to 1780 GeV by the new results.
The seventh search looked for events with a Z boson, jets and missing transverse momentum, a final state where a 3σ was observed in the Run 1 data. Intriguingly, the new data show a modest 2σ excess over the background prediction. This intriguing excess, and a full investigation of all SUSY channels, make the upcoming 2016 data eagerly awaited.