Pushing the limits on supersymmetry

A report from the ATLAS experiment

Supersymmetry (SUSY) introduces a new fermion–boson symmetry that gives rise to supersymmetric “partners” of the Standard Model (SM) particles, and “naturally” leads to a light Higgs boson with mass close to that of the W and Z. SUSY partners that are particularly relevant in these “natural SUSY” scenarios are the top and bottom squarks, as well as the SUSY partners of the weak SM bosons, the neutralinos and charginos.

Despite the theory’s many appealing features, searches for SUSY at the LHC and elsewhere have so far yielded only exclusion limits. With LHC Run 2 completed as of the end of 2018, the ATLAS experiment has recorded 139 fb^-1 of physics-quality proton–proton collisions at a centre-of-mass energy of 13 TeV. Three recent ATLAS SUSY searches highlight the significant increase in sensitivity offered by this dataset.

The first search took advantage of refinements in b-tagging to search for light bottom squarks decaying into bottom quarks, Higgs bosons and the lightest SUSY partner, which is assumed to be invisible and stable (a candidate for dark matter). The data agree with the SM and lead to significantly improved constraints, with bottom squark masses now excluded up to 1.5 TeV. 

If the accessible SUSY particles can only be produced via electroweak processes, the resulting low-production cross sections present a challenge. The second search focuses on such electroweak SUSY signatures with two charged leptons and a significant amount of missing momentum carried away by a pair of the lightest SUSY partners. The current search places strong constraints on SUSY models with light charginos and more than doubles the sensitivity of the previous analysis (figure 1).

A third recent analysis considered less conventional signatures. Top squarks – the bosonic SUSY partner of the top quark – may evade detection if they have a long lifetime and decay at macroscopic distances from the collision point. This search looked for SUSY particles decaying to a quark and a muon, looking primarily for long-lived top squarks that decayed several millimetres into the detector volume. The observed results are consistent with the background-only expectation.

These analyses represent just the beginning of a large programme of SUSY searches using the entirety of the Run-2 dataset. With a rich signature space left to explore, there remains plenty of room for discovery in mining the riches from the LHC.