A surreal illustration of particles colliding in the ATLAS detector, symbolizing higgsinos and supersymmetry.

The Invisible Partner: Unmasking the Higgsino and its Role in Supersymmetry

Elliot Brynn in Science & Nature August 2025 • 4 min read.

"Delve into the groundbreaking research exploring the potential of higgsinos in supersymmetric models and understand how it could rewrite our understanding of the universe."

In the realm of particle physics, supersymmetry (SUSY) emerges as a compelling theoretical framework, proposing that every known particle in the Standard Model (SM) has a corresponding supersymmetric partner. This concept aims to address some of the SM's unresolved puzzles, offering a more complete picture of the universe's fundamental constituents and forces.

Among these hypothetical SUSY particles, the higgsino—a superpartner of the Higgs boson—holds a particularly intriguing position. Unlike their heavier counterparts, higgsinos are theorized to be relatively light, making them potentially the first SUSY particles detectable by current experimental capabilities. The existence of such particles could have a huge impact on explaining the stability of the Higgs boson mass and shedding light on the composition of dark matter.

The following analysis delves into a groundbreaking search conducted by the ATLAS Collaboration, focusing on the pair production of higgsinos in proton-proton collisions at the Large Hadron Collider (LHC). These collisions produce final states with at least three b-tagged jets, this experiment pushes the boundaries of our understanding of particle interactions.

Unraveling the Mystery: The Higgsino Search

A surreal illustration of particles colliding in the ATLAS detector, symbolizing higgsinos and supersymmetry.

The exploration for higgsinos is embedded in the broader context of supersymmetric models, where higgsinos intricately blend with gauginos, the super partners of electroweak gauge bosons. This mixing gives rise to mass eigenstates known as charginos (±) and neutralinos (0). In what’s described as ‘natural’ SUSY models, the lightest neutralinos and charginos exhibit a composition largely dominated by higgsinos. This leads to a scenario where the masses of the four lightest particles converge closely, creating an exciting target for experimental searches.

In specific models, sparticle production is expected to be dominated by processes involving the production of 2,2, 2, and ⁺žī. In these scenarios, heavier charginos and neutralinos are expected to decay into the lightest neutralino (7) through the exchange of off-shell W and Z bosons, which decay to immeasurably low momentum particles. The research models are studied assuming heavier higgsinos decay first to and then immediately to the LSP. This decay can occur through various channels, including a photon, Z boson, or Higgs boson, depending on the model parameters.

This search focuses on final states involving at least three b-tagged jets, resulting from the decay of Higgs bosons. The key assumptions include:

Higgsino Decay: Each higgsino decays to a Higgs boson and a gravitino.
Mass Range: Higgsinos with masses between 130 and 230 GeV and between 290 and 880 GeV are excluded at the 95% confidence level.
LHC Data: Analyzes LHC pp collision data at √s = 13 TeV, using integrated luminosities of 36.1 fb⁻¹ and 24.3 fb⁻¹ collected in 2015 and 2016.
No Excess: No significant excess is found above the predicted background, leading to limits on higgsino pair production.

This analysis hinges on particular parameter of the model is the mass of the degenerate higgsino states, denoted as mỹ. Simultaneously, the mass of the lightest supersymmetric particle (LSP) is set to an insignificantly small value. In total, the signal cross-section accounts for the combined production of the four mass-degenerate higgsino pairs and each higgsino decays to a Higgs boson (h) and a gravitino (Ğ), in the 4-b-jet + final state.

Implications and Future Directions

The ATLAS Collaboration's results set stringent constraints on the existence of higgsinos within the explored mass ranges, providing key insights for refining supersymmetric models and guiding future searches. Although no definitive signal was observed, the techniques and methodologies refined in this study pave the way for more sensitive explorations of the SUSY landscape at the LHC and future colliders.

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Everything You Need To Know

What is a higgsino and why are particle physicists so interested in finding it?

Supersymmetry (SUSY) aims to address the Standard Model's unresolved puzzles by proposing that every known particle has a supersymmetric partner. The higgsino, a superpartner of the Higgs boson, is particularly interesting because it's theorized to be relatively light, potentially making it the first SUSY particle detectable. Detecting higgsinos could help explain the Higgs boson mass stability and dark matter composition. The concept of supersymmetry introduces a symmetry between bosons and fermions, implying that for every boson, there exists a corresponding fermion, and vice versa. This symmetry helps cancel out some of the divergences that plague the Standard Model, leading to more stable predictions at high energies.

How do higgsinos interact with other particles in supersymmetric models, and what are charginos and neutralinos?

Higgsinos can mix with gauginos (superpartners of electroweak gauge bosons), resulting in mass eigenstates called charginos (±) and neutralinos (0). In 'natural' SUSY models, the lightest neutralinos and charginos are mostly higgsino in composition, meaning their masses are close together. This mass proximity makes them a good target for experimental searches. Charginos are electrically charged and interact via the electroweak force, while neutralinos are neutral and can interact through the weak force or potentially through new interactions beyond the Standard Model, depending on their composition.

How did the ATLAS Collaboration search for higgsinos at the Large Hadron Collider (LHC), and what did they find?

The ATLAS Collaboration searched for higgsino pair production in proton-proton collisions at the LHC, looking for final states with at least three b-tagged jets. They analyzed data collected in 2015 and 2016 but found no significant excess above the predicted background. This means they set limits on higgsino pair production within the explored mass ranges (130-230 GeV and 290-880 GeV at the 95% confidence level). B-tagging is a crucial technique for identifying jets originating from b-quarks, which are produced in the decay of Higgs bosons. The absence of a signal doesn't rule out the existence of higgsinos but constrains the parameter space where they could exist, guiding future searches with more sensitive experiments.

What limitations did the ATLAS Collaboration place on the existence of higgsinos, and how do these results affect future research?

The ATLAS Collaboration set limits on the existence of higgsinos within specific mass ranges by analyzing proton-proton collision data from the LHC. They excluded higgsino masses between 130 and 230 GeV and between 290 and 880 GeV at a 95% confidence level. These constraints are based on the assumption that each higgsino decays to a Higgs boson and a gravitino. While these results narrow down the possible characteristics of higgsinos, they also highlight the need for continued research using advanced methodologies at the LHC and future colliders. The results from the ATLAS experiment provide valuable guidance for refining SUSY models and planning future searches with increased sensitivity.

What decay channels and assumptions were crucial to the higgsino search involving b-tagged jets, and what do these conditions imply about the nature of the higgsino?

The search focused on final states with at least three b-tagged jets, arising from Higgs boson decay. Assumptions included each higgsino decaying to a Higgs boson and a gravitino. The signal cross-section considers the production of four mass-degenerate higgsino pairs, each decaying to a Higgs boson (h) and a gravitino (Ğ) in the 4-b-jet final state. Degenerate higgsino states refer to a scenario where multiple higgsino particles have very similar masses, which can influence their production and decay characteristics. The gravitino is the superpartner of the graviton, and it is often assumed to be the lightest supersymmetric particle (LSP) in many models, making it a candidate for dark matter.