Could the Higgs boson have a super-partner? Or even several? Supersymmetry (SUSY) predicts that every Standard Model particle has its own heavier “super-partner” particle. The “higgsino” would be the super-partner of the Higgs boson, with many SUSY theories predicting the existence of several higgsinos. Discovering higgsinos could address several shortcomings of the Standard Model, including the unexpected mass of the Higgs boson. Indeed, if the lightest higgsino is a “stable” particle, and does not decay to other particles, it could be a natural candidate for dark matter.

ATLAS physicists are looking for signs of higgsinos with a wide range of masses. Searches for higgsinos are difficult due to their low production rates and challenging decay modes, and because multiple higgsinos could have nearly the same mass. These challenges make higgsinos the least explored SUSY particles at the LHC.

The ATLAS Collaboration has released three new searches for higgsinos using the full Run-2 dataset (recorded 2015–2018). The first considers heavier higgsino pairs decaying to the lightest stable higgsino and low-momentum (“soft”) Standard Model particles, while the second considers heavy higgsinos decaying to lighter SUSY particles and energetic Standard Model particles. The third search provides the first LHC limits on unstable higgsinos decaying directly to quarks.

A multi-leptonic leap in higgsino constraints

If the lightest higgsino is a stable particle, theorists predict that any heavier higgsinos produced in LHC collisions would only be able to decay into the lightest higgsino and soft Standard Model particles. Soft particles are not only difficult to reconstruct in the ATLAS detector, they are also challenging to identify out of thousands of similar particles arising from Standard Model processes.

To overcome these obstacles, ATLAS physicists focused their new search on final states with three electrons or muons (Figure 1 left) and introduced a new machine-learning based identification for soft electrons and muons. This improved the ATLAS’ ability to identify a higgsino signature by 20-60%. While no such signal has been observed, this search set a stringent limit on higgsino production and complements ATLAS’ previous search, as shown in Figure 2.

Boosting the sensitivity to heavier higgsinos using hadronic decays

One way to get around studying the challenging soft particles is to look for heavy higgsinos decaying into much lighter SUSY particles and Standard Model bosons. In such decays, the resulting bosons would have very high momentum – known as “boosted” bosons. Another new ATLAS search focuses on such higgsino decays, associated with two W, Z or Higgs bosons (Figure 1 centre). The analysis also considered the opposite possibility, where heavier weakly-interacting SUSY particles decay into higgsinos – such decays could explain the recently reported anomaly in the muon g-2 factor.

ATLAS physicists studied collision events with highly-boosted Standard Model bosons. When these bosons decay to quarks, the resulting “jets” of hadrons overlap significantly in the detector and can be reconstructed as a single large jet. Researchers developed a novel technique, studying the substructure of this jet, to effectively reconstruct the bosons and suppress Standard Model background processes.

No signals were observed. Instead, the search allowed physicists to study higgsinos with unprecedentedly high masses – as heavy as 900 GeV – and to provide important constraints on a range of SUSY scenarios. An example can be found in Figure 3, with limits set on a scenario where the lightest SUSY particle is “bino” (superpartner of Standard Model U(1)_Y field).

Decaying higgsinos or extreme Standard Model events?

The third and final new ATLAS result took an unconventional approach. Instead of looking for missing energy in the detector – which is often associated with new, undetectable particles – physicists looked for the signs of the lightest higgsino decaying to Standard Model quarks and leaving a spectacular multi-particle final signature.

Their search targeted higgsinos decaying to final states containing 6 to 8 hadronic jets, in which at least three contains b-hadrons (Figure 1 right). The event display above shows one of the collisions that met these criteria, featuring six jets (four of which are identified as containing a b-hadron). Studying these extreme topologies required a data-driven approach to accurately model the challenging Standard Model background processes. Among these backgrounds was the production of four top quarks, itself a rare process only recently studied at the LHC.

Researchers used neural-net machine-learning classifiers to help distinguish the higgsino signal from this background, and their final limits excluded higgsino masses between 200 GeV and 320 GeV. The result is the first since 2004 to be sensitive to higgsinos decaying promptly to quarks; the last sensitivity to such decays was provided by the LHC’s predecessor, the Large Electron-Positron collider (LEP).

What comes next?

While the Standard Model has survived the three new ATLAS searches described above, there remain challenging “gaps” in the limits set on higgsino masses. Exploring these gaps will require larger datasets, improved data-analysis techniques or even dedicated new searches. With the third data-taking period of the LHC due to start next year, ATLAS physicists are busy finalising the analysis of the current datasets while simultaneously preparing for future searches. Higgsinos will continue to be a key target of these efforts – stay tuned to see if they lead to future discoveries.

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