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No. As of October 4, 2026, scientists have not established a fifth fundamental force. Recent precision measurements and searches test ideas that could point to new physics, but the results described here do not amount to a confirmed discovery.
What is a fifth force?
Physics recognizes four fundamental interactions: gravity, electromagnetism, the strong interaction and the weak interaction. “Fifth force” is a broad label for a proposed interaction beyond those four; it does not refer to one specific, confirmed force.
Different proposals involve different particles, couplings and effects. A fifth-force search therefore tests a particular model or a defined range of possibilities, not every conceivable new interaction at once.
Why did the muon g−2 result attract attention?
The muon g−2 experiment measures how a muon’s spin precesses in a magnetic field. Its magnetic anomaly is written as aμ = (g−2)/2. Known and hypothetical particles can contribute to this quantity, so a dependable mismatch between measurement and Standard Model prediction could signal new physics. It would not, by itself, identify a fifth force.
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On June 3, 2025, Fermilab announced the collaboration’s third and final measurement. Its precision was 127 parts per billion, better than the original 140-parts-per-billion design goal. The reported central value was 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 result agreed with the collaboration’s 2021 and 2023 measurements. Fermilab’s announcement describes it as a measurement of exceptional precision, not as evidence that a new force has been found.
The prediction matters as much as the measurement
To decide whether a measurement disagrees with the Standard Model, physicists must also calculate the Standard Model’s prediction accurately enough. Different data-driven and computational approaches to that prediction have produced different interpretations of the earlier muon g−2 tension. Fermilab’s account notes that newer computational work gives a prediction closer to the measurement, reducing the apparent evidence for new physics.
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A 2026 review of the result and the Muon g−2 Theory Initiative’s second white paper says further improvements to the prediction are needed to fully exploit the experiment’s sensitivity. That makes the interpretation an active theory question, not proof of a fifth force. The 2026 Annual Reviews article discusses the measurement and the state of the prediction.
What other fifth-force searches test
Searches address different candidate interactions and parameter ranges. A review published by the American Physical Society in 2025 covers spin-dependent exotic interactions mediated by hypothetical spin-0 or spin-1 bosons. Candidates discussed include axions or axionlike particles, Z′ bosons, dark photons and paraphotons. Experimental methods include atomic comagnetometers, torsion balances, nitrogen-vacancy spin sensors, and precision atomic and molecular spectroscopy. These methods constrain particular couplings and interaction types; they are not interchangeable tests of one universal force.
The APS review surveys the approaches and existing constraints. A limit means that a particular model or parameter range is restricted; it does not rule out every possible fifth-force proposal.
What did ATLAS find in its 2026 dark-sector search?
On June 12, 2026, ATLAS reported a search for soft unclustered energy patterns (SUEPs), a possible signature of models involving a hidden dark sector and a new strong-like interaction. The analysis used 140 fb−1 of Run-2 proton-proton collision data at 13 TeV. It selected events with many charged particles and an unusually isotropic distribution of muons.
Two events resembled the proposed signature, but the observed yield was compatible with Standard Model backgrounds. The local excess significance was 1.7 sigma, which is not a confirmed discovery. ATLAS set limits for specified mediator models: its reported reach included a cross section of 0.05 fb for a 750 GeV mediator, and a restriction of approximately 0.2% on the Higgs boson’s decay probability to SUEPs in the scenario described by the collaboration. Those limits constrain the stated scenarios, not all possible fifth forces. ATLAS’s search summary gives the analysis details.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to tell a search result from a discovery
A striking measurement, a candidate-like event or a model limit is not automatically evidence of a new force. When assessing a claim, check what was actually measured and how it compares with known physics:
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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Clear out junk files and repair common Windows errorsFree Scan →- Measurement: an experiment determines an observable, such as the muon magnetic anomaly. Precision alone does not establish a discrepancy.
- Interpretation: a possible discrepancy depends on the reliability of the theoretical prediction and may have explanations other than a new force.
- Search or limit: an experiment looks for a particular signature or constrains specified models and parameter ranges. A background-compatible excess is not a discovery.
- Discovery claim: a new interaction would need compelling, independently checked evidence that survives comparison with the Standard Model and alternative explanations. The results described above have not reached that point.
The practical answer is therefore no: the muon g−2 measurement and the ATLAS search are important tests, but neither establishes a fifth force of nature. The theoretical interpretation of muon g−2 remains sensitive to improvements in the Standard Model prediction, while other experiments continue to test distinct proposed interactions.
For a concise account of Fermilab’s final measurement, the U.S. Department of Energy also published a summary on June 3, 2025: DOE’s overview of the result.
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