Supersymmetry (SUSY) offers an elegant solution to the limitations of the Standard Model, extending it to give each elementary particle a “superpartner” with different spin properties. Yet SUSY also contains interactions that would cause phenomena not observed in nature, such as the decay of protons. This has traditionally been avoided by requiring the conservation of a property known as “R-parity” (or “matter-parity”), which incorporates the baryon number, lepton number and spin. As a direct consequence, SUSY particles cannot decay only into Standard Model particles and each decay chain includes the lightest stable SUSY particle, usually assumed to be invisible to the ATLAS experiment.

However, this may not be the only solution. ATLAS physicists are also considering SUSY models with R-parity violation (or “RPV”), which would allow the lightest SUSY particle to be observed decaying directly into Standard Model particles. These new models would forbid the violation of the baryon number or the lepton number, but not both — thus maintaining the proton’s stability.

The ATLAS Collaboration has released two new searches for SUSY with RPV, studying the full LHC Run-2 dataset at 13 TeV proton–proton collision energy (2015–2018). The analyses looked for the production of pairs of SUSY particles, each decaying fully to observable Standard Model particles through different RPV interactions.

ATLAS physicists are also considering SUSY models with matter-parity violation, which would allow the lightest SUSY particle to be observed decaying directly into Standard Model particles. 

Searching for new phenomena with many b-jets

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Figure 1: A diagram of the pair production of two stop particles targeted by the search for multiple b-jets. (Image: ATLAS Collaboration/CERN)

ATLAS physicists considered RPV interactions that allow for baryon-number violation in their new search for pairs of top squarks (or “stops”), the supersymmetric partners of the well-known top quarks. If present in nature and not too heavy, the stop pair could be produced at the LHC at a high rate and would leave a unique signature in the ATLAS experiment. The stop pair would decay via RPV interactions to eight “jets” – collimated sprays of particles – with six originating from the decay of bottom quarks (“b-jets”) (see Figure 1). These b-jets inherit very special features from the bottom quark, such as presence of particles with measurably long lifetime, making it possible to identify them experimentally. In the Standard Model, events with this many b-jets are extremely rare, and thus a signal would easily stand out (see Figure 2).

However, a very challenging aspect of this search is predicting the Standard Model background processes. If not properly identified, such processes could mimic a SUSY signal. For this new analysis, ATLAS physicists employed a sophisticated "data-driven” method to tackle this problem, using data with fewer b-jets to predict the background with many b-jets. No signs of stops were found in this search. As shown in Figure 3, the results were used to limit the range of stop masses where SUSY particles could have escaped this search.

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Figure 2: The expected background and observed data in the different jet (j) and b-jet (b) multiplicity bins. The signal model has a larger number of b-jets than the background. (Image: ATLAS Collaboration/CERN)
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Figure 3: Observed (expected) limits on the stop pair production are shown by the red line (black dashed line). The mass of stop is shown on the x-axis, while the mass of the intermediary chargino is shown on the y-axis. (Image: ATLAS Collaboration/CERN)

In one analysis, physicists searched for SUSY particles created via electroweak interactions. These would be very rarely produced at the LHC – and are only now accessible thanks to the enormous dataset recorded during Run 2.

Exploring decays with three leptons

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Figure 4: A diagram of the pair production of a chargino and a neutralino targeted by the search for a trilepton resonance. (Image: ATLAS Collaboration/CERN)

Supersymmetry may also be hiding in processes that violate lepton-number conservation. Another new ATLAS search looked for the rare production of pairs of new SUSY particles – charginos and neutralinos – whose nearly identical masses allow both to decay through RPV interactions. These SUSY particles are produced through the electroweak interaction and are thus rare at the LHC – only now accessible thanks to the enormous dataset recorded during Run 2.

ATLAS physicists searched for a chargino decaying to a lepton and a Z boson that subsequently decays to leptons, giving a signature with a spectacular three-lepton (trilepton) resonance (see Figure 4). The chargino is produced alongside a second chargino or neutralino that would also decay, leading to a number of different signatures in the ATLAS detector. In events where both decays could be fully reconstructed, physicists were able to use the lack of mass asymmetry of both SUSY particles as a powerful handle for identification.

This was the first 13 TeV search for new resonances in a trilepton mass distribution. As shown in Figure 5, the trilepton mass distribution should peak sharply for a signal over the expected Standard Model distribution. No significant excess was found, and the results were used to place constraints on the masses of charginos and neutralinos.

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Figure 5: The trilepton invariant-mass distribution in the fully-reconstructed signal region, which is seen to peak sharply for signal events of three example masses as opposed to the smooth distribution from Standard Model processes. (Image: ATLAS Collaboration/CERN)
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Figure 6: Observed and expected 95% exclusion limits on chargino–chargino and chargino–neutralino production for equal branching fractions to electrons, muons and tau-leptons. The grey hashed region shows masses and branching fractions to Z bosons that are excluded. (Image: ATLAS Collaboration/CERN)

The strength of the exclusion depends on how often the SUSY particles decay into each lepton flavour (electron, muon and tau lepton) and into each boson type (W, Z or Higgs boson). The mass limits are most sensitive to the decay fraction to Z bosons, and are seen in Figure 6 to exclude masses up to 975 GeV for a 100% decay fraction to Z bosons and democratic decay fractions to each lepton flavour.