No. As of October 4, 2026, scientists have not established a fifth fundamental force. Recent results—including Fermilab’s highly precise muon measurement and an ATLAS collider search—test particular possibilities, but neither is a confirmed discovery.
What scientists mean by a “fifth force”
Physics recognizes four established fundamental interactions: gravity, electromagnetism, the strong interaction and the weak interaction. “Fifth force” is a broad label for a proposed interaction beyond those four, not the name of one confirmed force.
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There is no single experiment that can rule out every possible fifth force. Different proposals predict different effects, particle mediators and ways of interacting with matter. A result that constrains one model or coupling does not settle all the others.
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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesWhat the muon g−2 result does—and does not—show
The muon is a subatomic particle whose spin precesses in a magnetic field. The quantity called its magnetic anomaly, aμ = (g−2)/2, can be affected by known particles and potentially by hypothetical ones. Comparing a precise measurement with a sufficiently precise Standard Model prediction can therefore test for physics beyond the Standard Model. Even a confirmed mismatch would be a clue to investigate, not by itself proof of a fifth force or identification of its cause.
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On June 3, 2025, the Fermilab Muon g−2 collaboration announced its third and final measurement. It reported aμ = 0.001 165 920 705, with statistical and systematic uncertainties of 0.000 000 000 114 and 0.000 000 000 091, respectively. The measurement reached 127 parts-per-billion precision, exceeding the experiment’s original 140 parts-per-billion design goal, and agreed with the collaboration’s 2021 and 2023 results. Fermilab’s announcement describes the measurement; the U.S. Department of Energy also summarized the final result.
The interpretation depends on theory as well as measurement. Different methods of calculating the Standard Model prediction have produced different comparisons with the experimental value. Fermilab’s account explains that newer computational work yields a prediction closer to the measurement, reducing the apparent case for new physics compared with some data-driven approaches. A 2026 review in Annual Review of Nuclear and Particle Science discusses the final result and the Muon g−2 Theory Initiative’s second white paper, and says further improvements to the prediction are needed to fully exploit the experiment’s sensitivity. The measurement is a major precision achievement; it does not establish a fifth force. The 2026 review and the Department of Energy summary provide additional context.
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What other fifth-force searches look for
Researchers use different experimental methods because proposed interactions can affect particles and matter in different ways. A 2025 Reviews of Modern Physics article surveys searches for spin-dependent exotic interactions mediated by hypothetical spin-0 or spin-1 bosons. Candidate mediators discussed include axions or axionlike particles, Z′ bosons, dark photons and paraphotons. The reviewed methods include atomic comagnetometers, torsion balances, nitrogen-vacancy spin sensors, and precision atomic and molecular spectroscopy.
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These experiments test particular couplings and interaction types. To understand what a result means, compare the proposed mediator and its couplings, the experimental signature, and the range of masses or other parameters the experiment can probe. A review of these methods and existing constraints is available in the 2025 APS article.
What ATLAS’s 2026 dark-sector search found
On June 12, 2026, the ATLAS Collaboration reported a search for soft unclustered energy patterns, or SUEPs. In some hidden-dark-sector models, such a pattern could result from a new strong-like interaction. ATLAS examined 140 inverse femtobarns of proton-proton collisions at 13 TeV from the full Run-2 dataset, looking for events with many charged particles and an unusually isotropic distribution of muons.
The search found two events resembling the proposed signature. Their number remained compatible with expected Standard Model backgrounds, and the reported local excess significance was 1.7 sigma. That is not a confirmed signal. ATLAS used the analysis to set limits on specified mediator models; those limits constrain those scenarios, not every possible fifth force. The collaboration’s search summary reports a cross-section reach of 0.05 femtobarns for a specified 750 GeV mediator scenario and a restriction of approximately 0.2% on the Higgs boson’s decay probability to SUEPs in the scenario it describes.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.When would a fifth force count as discovered?
A suggestive result or a limit is not enough. A discovery claim would need a robust signal that cannot be adequately explained by known backgrounds or uncertainties, along with evidence that survives scrutiny and is consistent with other measurements. Because proposed fifth forces differ, independent experiments would also need to test the same underlying interaction rather than merely observe unrelated anomalies.
- A measurement reports an observed quantity, such as the muon magnetic anomaly.
- A discrepancy is a mismatch between measurement and prediction; its meaning depends on the reliability of both.
- A limit rules out or constrains specified values or models within an experiment’s reach.
- A discovery requires compelling evidence for a new effect, followed by checks that establish what it is and exclude alternative explanations.
By those distinctions, the Fermilab result is a precision measurement and the ATLAS result is a background-compatible search with model-specific limits. Neither establishes a fifth force.
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