CMS hunts for supersymmetry in uncharted territory

18 March 2016

The CMS collaboration is continuing its hunt for signs of supersymmetry (SUSY), a popular extension to the Standard Model that could provide a weakly interacting massive-particle candidate for dark matter, if the lightest supersymmetric particle (LSP) is stable.

With the increase in the LHC centre-of-mass energy from 8 to 13 TeV, the production cross-section for hypothetical SUSY partners rises; the first searches to benefit are those looking for the strongly coupled SUSY partners of the gluon (gluino) and quarks (squarks) that had the most stringent mass limits from Run 1 of the LHC. By decaying to a stable LSP, which does not interact in the detector and instead escapes, SUSY particles can leave a characteristic experimental signature of a large imbalance in transverse momentum.

Searches for new physics based on final states with jets (a bundle of particles) and large transverse-momentum imbalance are sensitive to broad classes of new-physics models, including supersymmetry. CMS has searched for SUSY in this final state using a variable called the “stransverse mass”, MT2, to measure the transverse-momentum imbalance, which strongly suppresses fake contributions due to potential hadronic-jet mismeasurement. This allows us to control the background from copiously produced QCD multi-jet events. The remaining background comes from Standard Model processes such as W, Z and top-quark pair production with decays to neutrinos, which also produce a transverse-momentum imbalance. We estimate our backgrounds from orthogonal control samples in data targeted to each. To cover a wide variety of signatures, we categorise our signal events according to the number of jets, the number of jets arising from bottom quarks, the sum of the transverse momenta of hadronic jets (HT), and MT2. Some SUSY scenarios predict spectacular signatures, such as four top quarks and two LSPs, which would give large values for all of these quantities, while others with small mass splittings produce much softer signatures.

Unfortunately, we did not observe any evidence for SUSY in the 2015 data set. Instead, we are able to significantly extend the constraints on the masses of SUSY partners beyond those from the LHC Run 1. The gluino has the largest production cross-section and many potential decay modes. If the gluino decays to the LSP and a pair of quarks, we exclude gluino masses up to 1550–1750 GeV, depending on the quark flavour, extending our Run 1 limits by more than 300 GeV. We are also sensitive to squarks, with our constraints summarised in figure 1. We set limits on bottom-squark masses up to 880 GeV, top squarks up to 800 GeV, and light-flavour squarks up to 600–1260 GeV, depending on how many states are degenerate in mass.

Even though SUSY was not waiting for us around the corner at 13 TeV, we look forward to the 2016 run, where a large increase in luminosity gives us another chance at discovery.

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