Which indicators of new physics should we be examining closely, and which might be misleading?

From time to time, research findings emerge that don’t align with established theoretical expectations. In the realms of physics and astronomy, the laws of nature have been defined with such precision that any deviation from our predictions is not only noteworthy but could signal a significant shift in understanding. On one side, we have the Standard Model of particle physics governed by quantum field theory, and on the other, we have gravitational laws ruled by General Relativity.

Yet, despite the robustness of these theories, we occasionally observe results that contradict the predictions derived from their combination. Possible explanations for these discrepancies include:

• Errors in the experimental or observational processes.
• Flaws in the theoretical predictions.
• New phenomena within the confines of the Standard Model or General Relativity that haven’t yet been accounted for.
• The introduction of new physics altogether.

While the last option is enticing, it should be regarded as a last resort, as the durability and successes of our existing theories make them difficult to displace. Here, we will explore eight potential indicators of new physics that have generated considerable excitement but should be approached with skepticism.

1. Do gamma-ray bursts occur during black hole mergers? On September 14, 2015, the first gravitational wave signal detected by LIGO indicated the merger of two black holes, weighing 36 and 29 solar masses, respectively, which resulted in approximately three solar masses of energy being converted into gravitational waves. Remarkably, 0.4 seconds later, a faint signal was recorded by the Fermi Gamma-ray Burst Monitor, suggesting a possible electromagnetic counterpart.

However, after more than 50 additional black hole mergers, including some of even greater mass, no further gamma-ray bursts have been observed. The ESA’s Integral satellite, which was also operational at the time, detected nothing. These low-magnitude transient events occur in the Fermi data about once or twice daily, yielding a false positive rate of about 1 in 454. While researchers continue to investigate the potential for gamma-ray bursts accompanying black hole mergers, current evidence is generally deemed weak.

Verdict: Likely not, but possibly very rare. Most likely explanation: Observational coincidence or statistical fluctuation.

2. Is there a new low-energy particle dubbed X17? A Hungarian research team recently suggested they may have identified a new particle, termed X17. This particle potentially emerges during the decay of unstable nuclei like Beryllium-8, an intermediate in the fusion processes of red giant stars. When this decay occurs, it emits a high-energy photon that can occasionally produce an electron-positron pair, leading to a distinctive angle between the two particles.

However, measurements indicated a deviation from the Standard Model's predictions at larger angles. While initial interpretations proposed the existence of a new particle and force, skepticism remains high. Current exclusion limits from direct detection experiments already rule out such a particle, and this claim marks the fourth alleged "new particle" by this team, with previous claims being disproven.

Verdict: Unlikely. Most likely explanation: Experimental error by the researchers.

3. Is the XENON experiment finally detecting dark matter? After years of refining detection limits for dark matter interactions with protons and neutrons, the XENON detector—the world's most sensitive dark matter experiment—reported a small yet unexplained signal in 2020, exceeding the expected background from the Standard Model.

Various exciting hypotheses arose, suggesting that neutrinos could possess a magnetic moment or that the Sun might be generating a novel dark matter candidate called an axion. Alternatively, it might simply be a trace of tritium in the water, which if present in small quantities could explain the anomaly. Current astrophysical constraints challenge the neutrino and axion scenarios, but a definitive explanation for the signal excess remains elusive.

Verdict: Doubtful; likely tritium. Most likely explanation: A new effect from an unaccounted background.

4. Is the DAMA/LIBRA experiment observing dark matter? The principle that "extraordinary claims require extraordinary evidence" applies here, as the DAMA/LIBRA collaboration has claimed for over a decade to observe an annual modulation in their signal, suggesting dark matter interactions—yet no other detectors have corroborated these findings.

Much of the experiment raises questions; for instance, the team has not shared raw data for external verification, and they conduct an annual recalibration that may introduce noise misinterpretation as a signal. Recent independent attempts to replicate their results have failed, and other direct detection efforts also contradict their claims. While DAMA/LIBRA maintains its position, skepticism prevails in the broader scientific community.

Verdict: No; likely an erroneous claim. Most likely explanation: Experimental error, substantiated by unsuccessful replications.

5. Has the LHCb collaboration found evidence against the Standard Model? The Large Hadron Collider at CERN is renowned for colliding the highest-energy particles and for discovering the Higgs boson. While its primary mission is to unveil new fundamental particles, it also produces exotic particles like mesons and baryons containing bottom quarks, which the LHCb detector specializes in analyzing.

Notably, the decay of these particles reveals differences in the properties of those containing bottom quarks versus those containing bottom antiquarks, indicating a fundamental matter-antimatter asymmetry known as CP violation. Observations show more CP violation than what the Standard Model predicts, with some anomalies exceeding the crucial 5-sigma threshold, hinting at potential new physics. This CP violation could be significant in understanding why our universe is matter-dominated.

Verdict: Uncertain, but likely reflects new parameters associated with CP violation. Most likely explanation: A new effect within the Standard Model, though new physics remains a possibility.

6. Is there an additional type of neutrino? The Standard Model posits three neutrino types: electron, muon, and tau neutrinos. Initially believed to be massless, they were later shown to oscillate between forms, indicating mass. However, accelerator-produced neutrinos from experiments like LSND and MiniBooNE present data that don't align with existing measurements, raising questions about a potential fourth neutrino type.

Theoretical constraints from Z-boson decay and Big Bang nucleosynthesis suggest only three neutrinos exist. If a fourth, sterile neutrino exists, it may only interact through oscillation. Upcoming data from MicroBooNE, ICARUS, and SBND will either confirm or refute these indications.

Verdict: Unlikely, but new experiments will clarify these anomalies. Most likely explanation: Experimental error seems most plausible, though new physics is not off the table.

7. Does the Muon g-2 experiment contradict the Standard Model? This inquiry is both contentious and novel. Physicists have endeavored to measure the magnetic moment of the muon with high precision, and earlier findings indicated a discrepancy between the expected theoretical value and experimental results. Fermilab's Muon g-2 experiment confirmed a significant difference, stirring speculation about new physics and a breakdown of the Standard Model.

While the experiment's integrity is sound and errors are quantified, the theoretical predictions may be the source of the problem. A novel theoretical approach recently emerged, aligning closely with experimental values and prompting further investigation into the discrepancies.

Verdict: Undecided; theoretical uncertainties need resolution independent of the experimental data. Most likely explanation: Theoretical calculation errors, though new physics remains conceivable.

8. Do conflicting measurements of the expanding universe suggest new physics? To determine the universe's expansion rate, two main methods are utilized. One involves measuring nearby objects and progressively identifying more distant ones through various indicators, while the other traces back to the Big Bang to analyze early signals and their evolution.

These two methods yield conflicting results, with one indicating an expansion rate of 74 km/s/Mpc and the other suggesting 67 km/s/Mpc. Neither calibration errors nor measurement inaccuracies can explain this divergence. Is this a harbinger of new physics, or is there an unidentified error that, once rectified, could harmonize the findings?

Verdict: The discrepancies between measurement techniques need further investigation. Most likely explanation: Unknown, which opens exciting possibilities for new physics.

In conclusion, we must always consider the extensive body of evidence and the established understanding before we can hope to challenge our scientific knowledge of the universe. Each new study must be contextualized within the broader framework of existing data. An anomalous finding should be viewed as part of a larger puzzle, not in isolation.

Science is fundamentally experimental; when faced with unexplainable findings, we must interrogate the theories underpinning our understanding. If we are fortunate, one of these experimental anomalies might illuminate a path toward a revolutionary understanding of reality. Presently, we have multiple signals—some compelling, others less so—that may herald significant scientific advancements. However, history shows that anomalies often stem from errors, miscalculations, or oversights.

Will any of the current indicators of new physics prove substantial? Only time and continued exploration of reality can bring us closer to uncovering the universe's ultimate truths.

Starts With A Bang is authored by Ethan Siegel, Ph.D., who has also written Beyond The Galaxy and Treknology: The Science of Star Trek from Tricorders to Warp Drive.