ATLAS Experiment Narrows Search for Supersymmetric Higgsinos
Geneva – Physicists at the ATLAS experiment at the Large Hadron Collider (LHC) have achieved a significant milestone in the hunt for supersymmetry (SUSY), surpassing previous limitations set by the Large Electron–Positron (LEP) collider experiments over two decades ago. The new results focus on the search for compressed higgsinos, hypothetical particles that could explain the mass of the Higgs boson and potentially account for the universe’s dark matter.
The Quest for Supersymmetry and Higgsinos
Supersymmetry proposes that every known particle has a heavier “superpartner.” Higgsinos are the superpartners of the Higgs boson. Finding evidence of their existence is a key goal in particle physics, as they could help explain the mass of the Higgs boson and potentially account for the universe’s dark matter, making them attractive targets in searches for physics beyond the Standard Model.1
Challenges in Detecting Higgsinos
Higgsinos are not expected to appear in isolation. Instead, they mix to form neutralinos and charginos, making their detection incredibly challenging. The search is particularly difficult in the “compressed mass spectrum,” where the mass difference between these particles is modest. This makes reconstructing and identifying them incredibly difficult.2
ATLAS Collaboration’s Breakthrough
The ATLAS collaboration has employed advanced machine-learning techniques to analyze data from the LHC’s Run-2 dataset, focusing on the pair production of the lightest higgsino-like states: a chargino (χ̃±1) and two neutralinos (χ̃01 and χ̃02).3 Researchers conducted two distinct searches targeting different mass-splitting regimes.
Displaced Track Search
The “displaced track” search focused on mass splittings of 0.3–1 GeV between the chargino and the neutralino χ̃01. In such scenarios, the chargino decays into an invisible neutralino χ̃01 and a low-momentum charged pion, resulting in a pion track “displaced” from the original collision point. Physicists used dedicated neural networks to enhance signal sensitivity, focusing on event kinematics and displaced track characteristics.1
One-Lepton-One-Track (1L1T) Search
The “one-lepton-one-track” (1L1T) search targeted larger mass splittings between 1 GeV and 3 GeV. Here, the heavier neutralino χ̃02 decays into the lightest neutralino χ̃01 and two low-momentum leptons, one of which is difficult to identify. Researchers developed neural-network-based identification algorithms to spot these elusive tracks. A parameterized neural network was then used for event selection, enhancing the signal sensitivity.1
New Limits on Higgsino Masses
The observed data are consistent with Standard Model predictions. The 1L1T search excluded scenarios where the mass difference between the chargino and the lightest neutralino χ̃01 lies between about 0.8 and 2 GeV – extending previous LEP limits up to a chargino mass of 132 GeV for a mass splitting of 1.8 GeV. The displaced track search extends previous ATLAS exclusion limits by about 30 GeV, reaching chargino masses up to 199 GeV for a mass splitting of 0.6 GeV. Both searches excluded chargino masses below 126 GeV at the 95% confidence level.1 These new limits supersede LEP experiment results in all mass splitting regimes.2
Looking Ahead
The ATLAS Collaboration has established constraints across the full range of higgsino mass splittings, marking a significant step forward in the search for supersymmetry. The new Run-3 dataset and evolving analysis techniques will allow the ATLAS Collaboration to further advance these searches, potentially paving the way to the discovery of physics beyond the Standard Model.1