The Invisible Partner: Unmasking the Higgsino and its Role in Supersymmetry
"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.
What Is a Higgsino?
Unlike the Higgs boson discovered in 2012, the Higgsino is a fermion, meaning it follows Fermi-Dirac statistics rather than Bose-Einstein statistics. Experimental searches at the ATLAS detector have ruled out higgsinos with lifetimes of a fraction of a nanosecond for masses up to 210 GeV. Physicists summarize the experimental reach for higgsino-like electroweakinos using a two-dimensional parameter space known as the Higgsino discovery plane.
The Search Strategy and Its Limits
Discovering higgsinos could address several shortcomings of the Standard Model, including the unexpected mass of the Higgs boson. If the lightest higgsino is a stable particle that does not decay to other particles, it could be a natural candidate for dark matter. However, as the neutrino fog limit is approached in direct detection experiments, higgsino purity limits raise the minimum gaugino masses to a level that will be extremely difficult for the LHC to find, even in its high-luminosity phase.
From Theory to Strategy
Light, quasi-degenerate Higgsinos have long been considered a likely ingredient in natural supersymmetric models. Researchers have proposed new strategies to uncover these particles, which remain difficult to detect due to the small mass splittings characteristic of natural SUSY. These strategies represent a key step in bridging the gap between theoretical motivation and experimental accessibility.
Unraveling the Mystery: The Higgsino Search
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.
- 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.
Pushing the Boundaries of Higgsino Detection
Higgsinos with masses near the electroweak scale can solve the hierarchy problem and provide a dark matter candidate, though detecting them at the LHC remains challenging if their mass splitting is small. ATLAS searches have excluded higgsinos in the mass range 230–770 GeV at 95% confidence level in the context of simplified models for approximately degenerate Higgsinos. In the Next-to-Minimal Supersymmetric Standard Model, the mixing effects of Higgsino, Gaugino, and Singlino make Higgsino dark matter properties markedly more intricate compared to MSSM predictions.
Why Higgsinos Remain Elusive
Higgsinos near the TeV mass range are highly motivated as they offer an elegant solution to the naturalness problem in the Standard Model. Extensive searches within the General Gauge Mediation framework have been conducted, yet exclusion limits on higgsino masses remain much less stringent than for other supersymmetric particles. In the MSSM, the mixing of Gaugino and Higgsino components influences the mass splitting between neutralinos, complicating experimental identification.
Comparing Detection Approaches
The reach of an electron-positron Higgs factory for light higgsinos via electroweak precision observables has been evaluated in the NUHM2 model, offering a complementary path to hadron collider searches. Direct detection experiments face exclusionary pressures on natural higgsino-only dark matter, highlighting the potential need for additional dark matter components or non-thermal production mechanisms. For spin-dependent scattering, the dominant contribution comes from Z boson exchange, while the Higgs portal contribution is suppressed by a factor of mN/v and can be neglected in comparison.
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.
What the Searches Have Revealed So Far
ATLAS searches have excluded higgsinos with masses as low as 130 GeV for branching fractions to a Higgs boson and gravitino as low as 36%, providing complementarity to previous searches targeting different decay modes of electroweak bosons. Two complementary analyses targeting high- and low-mass signals have been performed to maximize sensitivity to pair-produced higgsinos. In pure gravity mediation scenarios, the dark matter may take the form of an axion and/or a neutralino, with the lightest supersymmetric particle potentially being a wino, bino, or Higgsino.
Hunting Dark Matter at Future Colliders
Techniques dedicated to suppressing backgrounds induced by in-flight decays of muon beams are being developed to improve sensitivity to wino and Higgsino dark matter. These methods will be compared against results from other future collider experiments in the context of MSSM higgsino and wino dark matter searches. The development of these background-suppression techniques represents a critical frontier for next-generation collider experiments.
Compressed Spectra and Beyond-MSSM Effects
ATLAS has conducted two searches for the electroweak production of higgsinos with compressed mass spectra using 140 fb⁻¹ of √s=13 TeV proton-proton collision data. Models based on gauge-mediated supersymmetry breaking predict the electroweak production of a pair of Higgsinos, each of which can decay via a cascade process to a Higgs boson and an undetected lightest supersymmetric particle. A modest addition of a dimension-5 term to the MSSM superpotential, originating from physics beyond the MSSM, has a significant impact on the light Higgsino-LSP scenario.
The Experimental Challenge of Compressed Scenarios
MSSM compressed scenarios present a formidable experimental challenge for Higgsino searches at the LHC. Researchers have worked to surpass two-decade-old LEP limits in the hunt for these particles. The effort to detect Higgsino dark matter at hadron colliders requires Higgsinos to be realized as multiplets of neutralinos and charginos, demanding innovative detection strategies.