Scientists weren’t convinced they’d ever find the neutrino when it was first proposed; now they’re looking for an even more elusive version of the particle.

In 1930, a group of scientists convened in Germany for a symposium on nuclear physics, and Wolfgang Pauli was invited. The Austrian physicist was credited with developing the Pauli exclusion principle, which advanced scientists’ understanding of matter and earned Pauli the Nobel Prize in Physics.

Pauli was unable to attend the German meeting due to a dispute in Zurich. Instead, he sent the guests a letter that would prove to be one of the most important correspondences in the history of physics. He predicted the existence of what would later be known as the neutrino in it.

Scientists have since identified and analyzed the theorized particle’s attributes. However, major issues remain, such as if an undiscovered form of neutrino is hiding from researchers’ detectors.

A particle that cannot be detected

Pauli revealed ideas he’d had regarding beta decay, a process that had been vexing nuclear scientists, in his letter to conference attendees.

An unstable atom releases energy in the form of a particle during beta decay (called a beta particle). Scientists examining beta decay discovered that the energy of the beta particle was insufficient to account for the overall energy lost by the decaying atom.

Pauli had an idea as to where the lost energy could be found. He proposed that an atom experiencing beta decay actually expelled more than one particle; the second particle, however, had no charge or mass and was thus undetectable by the technology of the day.

That was the issue, however. There would be no way to evaluate whether Pauli’s theory was right if the particle was undetectable. Pauli bemoaned that proposing the existence of an undetectable particle was “something no theorist should ever do,” and chose to write an informal letter rather than writing an official paper about a notion he was too uneasy to fully claim.

However, the concept of an energy-carrying “ghost particle” struck a chord with many scientists, including Enrico Fermi. A few years later, Fermi developed a complete theory of nuclear beta decay that included the new particle, which he dubbed the “neutrino.” Fermi proposed that the neutrino interacted with an undiscovered force, now known as the weak force, that interacts only at extremely short distances.

Not other physicists were as pessimistic about detecting the neutrino as Pauli. Physicist Walter Baade even bet Pauli that his undiscovered particle will be discovered one day.

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Clyde Cowan and Fred Reines of the US Department of Energy’s Los Alamos National Laboratory designed a neutrino detector in the 1950s. It would detect particles moving through and occasionally reacting with them via the weak force. They wanted to place their apparatus near the most severe group of unstable atoms undergoing beta decay that they could produce: a nuclear bomb.

Aside from the technological problems of studying a nuclear explosion, it was discovered that a nuclear weapon would emit a lot of background radiation, making it impossible to separate the signals from the neutrinos. As a result, Cowan and Reines altered their plans. Instead, they placed their detector near a nuclear reactor in South Carolina.

Despite the fact that the reactor produced neutrinos at a considerably slower rate than a bomb, the detector ultimately picked up enough signals to indicate the presence of neutrinos. To commemorate the discovery, Pauli purchased a case of champagne for Baade.

There is more to it than meets the eye

The undetectable particle had been discovered by scientists. However, there was still a lot to learn about it.

In the 1960s, astrophysicists Raymond Davis and John Bahcall used an experiment established in the Homestake Gold Mine in South Dakota to quantify neutrinos coming from the sun. They only found one-third of the particles they anticipated to see.

The cause of the missing neutrinos was discovered in the 1990s by researchers at the Sudbury Neutrino Observatory in Canada and the Super-K experiment in Japan. The neutrino could “oscillate,” or alternate between three different sorts or “flavors”: electron neutrinos, muon neutrinos, and tau neutrinos. Because oscillation indicates mass, the discovery also revealed that neutrinos were not massless as previously supposed.

Scientists had developed hypotheses about how neutrinos might oscillate, but those theories were put to the test in the Liquid Scintillator Neutrino Detector experiment at Los Alamos.

LSND observed how many neutrinos—specifically, muon antineutrinos—oscillated to a new type over a short distance. Its findings revealed that more of them than expected had converted into electron antineutrinos.

Scientists wondered if the increased number of oscillations indicated the presence of an even more elusive ghost particle. One that didn’t even interact via the weak force? Was it time to place another case of champagne wager?

A similar test at Fermi National Accelerator Laboratory, titled after Enrico Fermi, was conducted in 2002. The MiniBooNE experiment employed a different experimental methods and operated at a different energy level than LSND; it also observed an excess in electron neutrinos.

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The results could be explained if neutrinos oscillated in unusual ways, such as to more than three flavors. Scientists dubbed the potential missing neutrino flavor a “sterile” neutrino since it would interact with matter much less strongly.

According to Harvard physicist Carlos Argüelles-Delgado, there does not have to be a single form of sterile neutrino. Many particles and processes, for example, appear in groups of three in the Standard Model of physics; perhaps sterile neutrinos do as well.

Anomalies that contradict each other

Argüelles-Delgado is a researcher who works on the IceCube experiment. IceCube, which is run by the University of Wisconsin and is stationed in Antarctica, studies neutrinos released by the sun or other astronomical phenomena such as supernovae.

Although sterile neutrinos are not its primary goal, IceCube is one of numerous experiments contributing to the current quest. According to Argüelles-Delgado, IceCube has primarily only tightened limitations around what sterile neutrinos could be.

“IceCube has not discovered any solid evidence of oscillations compatible with MiniBooNE,” he explains. “We also discovered no solid evidence for a sterile neutrino. However, we have discovered evidence of sterile neutrinos… We have discovered something that points us in the correct direction.”

Another experiment that has contributed to the search is MINOS, which analyzed a beam of neutrinos produced at Fermilab from 2005 to 2012, sampling the particles both close to the origin of the beam and far away in a mine in Minnesota. The experiment yielded no results that could imply the existence of sterile neutrinos.

Fermilab particle scientist Pedro Machado believes it’s nearly impossible to establish cohesive evidence for sterile neutrinos with LSND and MiniBooNE getting different results from IceCube and MINOS.

A collection of experiments dating back to the initial detection of neutrinos, such as Daya Bay in China, Double Chooz in France, and RENO in Korea, are also contributing to the discussion.

According to Virginia Tech physicist Patrick Huber, they have yet to find any convincing evidence to support MiniBooNE or LSND. They have, however, been embroiled in their own confrontation between theory and experiment. In 2011, a group of theorists revised the predicted number of electron antineutrinos observed in reactor experiments. They discovered that their forecast did not correspond to the experimental measurement.

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Researchers at Daya Bay, as well as theorists and other experimentalists, have continued to work on their models and explore the decay mechanisms that produce these antineutrinos since the discovery of the possible anomaly.

Experiments are on the way

All of these investigations provide insight into the subject of sterile neutrinos, but they do so from distinct perspectives, employing different methods and exploring different neutrino sources. In the near future, scientists may receive responses from experiments that attempt to match the original instigators of the sterile neutrino debate’s perspectives.

The LSND observation will be tested by an experiment dubbed JSNS2 at the Japan Proton Accelerator Research Complex. “With the same experiment as LSND, we hope to establish or disprove the existence of a sterile neutrino,” says Takasumi Maruyama, a J-PARC researcher on JSNS2. “There are numerous experiments, but we must understand what is happening with the same neutrino source and neutrino interaction.

“So, 20 years after the LSND results, I think it’s a good moment to revisit the LSND experiment.”

MicroBooNE, a Fermilab experiment, tries to replicate MiniBooNE’s measurements in greater detail. The first results are expected later this year, according to the researchers.

MicroBooNE is also a component of the Short Baseline Neutrino Program at Fermilab, which consists of three detectors. The three detectors will examine oscillations at three different distances from the source of a neutrino beam to create a thorough picture of neutrino behavior. This fall, the ICARUS detector, which is the furthest away from the source, will begin gathering physics data. The Short Baseline Near Detector, which is the nearest to the source, is being built.

Huber believes that sterile neutrinos will remain concealed for the foreseeable future, maybe until even more precise detectors can be created and installed. It’s possible they’ll never be discovered, either because they don’t exist or because Pauli’s anticipated particle has an undetectable side after all.

According to Argüelles-Delgado, whether or not subsequent tests uncover sterile neutrinos, the hunt will assist science.

“When there are suggestions in particle physics, you have to investigate those hints,” he says, “because some hints will wind up being challenges that only allow you to better your detector technology and techniques—and other hints will accomplish that while also allowing you to find new physics.” As a result, you always win.”

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