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A recent Â鶹ÒùÔºical Review Letters presents a thorough analysis of MicroBooNE detector data, investigating the anomalous surplus of neutrino-like events detected by the preceding MiniBooNE experiment.

In 1990, the LSND (Liquid Scintillator Neutrino Detector) experiment observed an anomalous signal indicating the potential existence of sterile neutrinos—a fourth neutrino species beyond the three established flavors (electron, muon, and tau neutrinos).

MiniBooNE was constructed to examine this anomaly utilizing the same neutrino beam methodology. However, instead of resolving the mystery, MiniBooNE discovered an anomaly of its own.

"MiniBooNE observed too many [electromagnetic] events that looked like electron neutrinos in their detector," explained Chris Thorpe, research associate at the University of Manchester and co-author of the study. One possible cause of the MiniBooNE excess is the existence of a new kind of neutrino, but other explanations, such as mismodeled background or decaying dark matter particles, could also be behind the anomaly."

Understanding the nature of the anomaly became the focus of MicroBooNE, the next-generation detector designed specifically to investigate the MiniBooNE excess. This new analysis represents the first comprehensive test using MicroBooNE's complete five-year dataset.

Improving the experiment

The MiniBooNE experiment used a detector filled with mineral oil that identifies electromagnetic activity through Cherenkov light emissions, characteristic blue light produced by particles when traveling faster than light can travel through the oil.

However, the technology had a critical limitation: its inability to precisely track individual particles or determine the source particle for the electromagnetic signal.

"They could not distinguish an electron being directly emitted by the interaction of the neutrino from one produced as a secondary emission from an initial photon," explained Alexandra Trettin, research associate at the University of Manchester and co-author.

This ambiguity was critical since neutrino oscillations, though well-documented, proceed too gradually to account for the observed excess, indicating that the anomaly could arise from misclassified background particles rather than novel physics.

MicroBooNE was specifically designed to tackle this uncertainty using liquid argon time projection chamber technology that achieves millimeter-scale spatial resolution for charged particle path reconstruction and definitive particle species determination.

The MicroBooNE experiment

MicroBooNE operates in the same Booster Neutrino Beam at Fermilab as MiniBooNE, positioned 470 meters from the neutrino source. The approach ensures investigation of the same anomalous neutrino beam using significantly improved detection methods.

At the heart of the experiment is a liquid argon time projection chamber (LArTPC), a technology that functions like a sophisticated 3D camera for subatomic particles. Charged particles produced by neutrino-argon nucleus interactions ionize the liquid argon during their passage through the medium. Electric fields drift these ionization electrons to wire planes that record their precise positions, creating detailed tracks that reveal each particle's path, energy, and identity.

This latest analysis marks the first use of MicroBooNE's complete five-year operational dataset, corresponding to 1.11 × 10²¹ protons on target, which is a 70% increase over previous studies.

The researchers focused on two complementary event categories: interactions producing one electron with no pions, further subdivided by whether visible protons were present.

"We separated data samples based on the presence of protons and performed the analysis on the separated samples and the combined sample, to include all possible signal topologies that the MiniBooNE detector observed," explained Fan Gao, postdoctoral scholar at the University of California, Santa Barbara, and a co-author of the study.

The separation was critical because while protons were invisible to MiniBooNE due to being too slow to produce Cherenkov emissions, MicroBooNE could identify them precisely.

Testing the anomaly

To directly test whether the MiniBooNE excess could be explained by electron neutrinos, the researchers developed two empirical models that translated the observed anomaly into predictions for what MicroBooNE should see.

The first model, used in previous analyses, assumed the excess resulted from an energy-dependent enhancement in electron neutrino flux. This approach unfolded the MiniBooNE data as a function of neutrino energy.

However, this first model had a significant drawback. The model failed to accurately reproduce the specific characteristics of electromagnetic showers that MiniBooNE actually observed, particularly their energy and directional properties.

The team developed a second, more sophisticated model that focused on the actual observables MiniBooNE measured: the energy and angle of electromagnetic showers.

The results were unambiguous.

When compared against both signal models, the MicroBooNE data showed no evidence supporting an electron neutrino interpretation of the MiniBooNE anomaly.

"We made predictions with and without including the MiniBooNE signal and compared these with our data," said Thorpe. "Our data favors the prediction that does not include the MiniBooNE signal, and our analysis indicates there is less than a 1% chance this is a statistical fluke."

The exclusion was particularly strong for the second signal model, which was ruled out at greater than 99% confidence level across multiple kinematic variables. The statistical significance reached 2.9σ for the first model and up to 3.8σ for the second model—well above the threshold typically considered significant in particle physics.

Unanswered questions

Although this thorough analysis conclusively rules out the main explanation for the MiniBooNE anomaly, the excess itself stays unexplained, maintaining the potential for other causes.

"The anomaly itself remains a statistically highly significant observation in need of an explanation, and many other hypotheses are still on the table," said Trettin.

Several alternative explanations remain under investigation. Given MiniBooNE's inability to separate photon and electron signals, the excess could arise from photon production rather than electrons.

Beyond-Standard Model exotic physics offers other explanations, including sterile neutrinos with anomalous interactions, or particles from hypothetical dark sectors.

"Some other physics processes that are beyond current particle physics knowledge can also potentially cause this excess. All of these different types of explanations are being actively investigated by MicroBooNE and the new Fermilab Short Baseline Program," noted Gao.

Looking ahead, the Short Baseline Near Detector (SBND), positioned much closer to the neutrino source, will provide significantly more data to further constrain systematic uncertainties and test remaining hypotheses with even greater precision.