Supersymmetric Superpartners
It was hoped that by now physicists would have seen the lightest sorts of particles that could confirm supersymmetry. The Standard Model already has plenty of subatomic particles within it, like the well-known electrons and neutrinos. Supersymmetry would give each of these particles a partner with almost identical properties but a heavier mass.
Since there are many different supersymmetric theories, no one knows exactly what the mass of any of these partners might be. Had the LHC found one, it would have put constraints on theoretical models and perhaps given scientists hints of where to look next for more evidence of supersymmetry. Given that none have been seen, the supersymmetric partners must be much heavier than initially thought, which has forced physicist to include ugly workarounds that have dampened some of the theory’s appeal.
There is still a chance that some of these particles will show up during the LHC’s next run. If they don’t and supersymmetry is finished, it will drive physics into a crisis over how to explain the discrepancies of the Standard Model.
Image: Simulated particle events that would indicate the existence of supersymmetry. CMS experiment