Years of discrepancies in neutrino observations have prompted scientists to suggest a “dark sector” of unseen particles that may explain dark matter, the universe’s unexplained expansion, and other puzzles at the same time.

A few bursts of light inside a bus-size tank of oil at the Los Alamos National Laboratory in New Mexico started a detective narrative that has yet to be completed in 1993.

The Liquid Scintillator Neutrino Detector (LSND) was looking for neutrinos, the lightest and most elusive of all known elementary particles, to cause bursts of radiation. “That’s what we saw, much to our surprise,” said Bill Louis, one of the experiment’s directors.

The issue was that they were seeing far too many. Theorists proposed that neutrinos fluctuate between kinds as they travel across space, explaining a variety of astronomical findings. LSND had set out to put this theory to the test by directing a beam of muon neutrinos, one of the three types of neutrinos known, at the oil tank and measuring the number of electron neutrinos that arrived. Despite this, Louis and his colleagues discovered much more electron neutrinos in the tank than expected by the basic hypothesis of neutrino oscillations.

Since then, scores of new neutrino experiments have been developed, each one larger than the previous. Physicists have built cathedrals to these famously slippery particles amid mountains, abandoned mining caves, and the ice under the South Pole. However, when these tests looked at neutrinos from every aspect, they continued coming up with contradictory results. “The plot thickens,” Louis observed.

“It’s a perplexing story. “I call it the Garden of Forking Paths,” said Carlos Argüelles-Delgado, a Harvard University neutrino physicist. Time branches into an unlimited number of alternative futures in Jorge Luis Borges’ 1941 short tale of the same name. In the case of neutrinos, inconsistent results have led theorists down a number of avenues, leaving them confused of which evidence to believe and which may be leading them wrong. “Like in any detective fiction, there are times when you notice clues that lead you in the wrong way,” Argüelles-Delgado explained.

The emergence of a new, fourth sort of neutrino, nicknamed the sterile neutrino, that mixes up all the neutrino kinds according to new criteria was the easiest explanation for the LSND anomaly. Over the short distance to the oil tank, sterile neutrinos would allow muon neutrinos to oscillate more quickly into electron neutrinos.

The sterile neutrino, however, didn’t suit the results of subsequent tests as time went on. “We had our champion theory,” Argüelles-Delgado explained, “but the trouble was that it failed badly everywhere.” “We were stranded deep in the woods and wanted to get out.”

As physicists have been forced to retrace their steps, they’ve started pondering what’s underlying the jumble of clues and half-results. They’ve invented new hypotheses in recent years that are more difficult than the sterile neutrino, but that, if accurate, would completely change physics, addressing inconsistencies in neutrino oscillation data as well as other important physics riddles. Not to mention, the new models propose hefty extra neutrinos as a possible explanation for dark matter, the mysterious substance that surrounds galaxies and appears to be four times more prevalent than regular matter.

Now, four new analyses from the MicroBooNE experiment at the Fermi National Accelerator Laboratory near Chicago, as well as another recent study from the IceCube detector at the South Pole, both suggest that these more complex neutrino theories are on the right track — though the future is still uncertain.

“There’s something in the air,” Argüelles-Delgado added. “It’s a really stressful situation that leads to discovery.”

Los Alamos National Laboratory’s Liquid Scintillator Neutrino Detector reported an unusually high number of neutrino detections in 1993. Engineer Rick Bolton is shown crouching amid the photomultiplier tubes that would detect light from neutrino interactions after the tank was filled with mineral oil.

A Last-Ditch Effort

In 1930, Wolfgang Pauli proposed the existence of the neutrino as a “desperate solution” for explaining where energy was escaping during radioactive decay. He doubted that an experiment could ever detect his theoretical construct because it had neither mass or electric charge. He noted in his notebook at the time, “It is something no theorist should ever do.” However, the neutrino was discovered in 1956 in an experiment similar to LSND.

When scientists discovered neutrinos coming from the sun, a natural source of the particles, they discovered fewer than half the amount expected by theoretical models of stellar nuclear processes. Neutrinos were clearly acting strangely by the 1990s. The neutrinos that descend to Earth when cosmic rays clash with the upper atmosphere, as well as solar neutrinos, seemed to abruptly vanish.

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The idea that neutrinos are shape-shifters was postulated previously by Italian scientist Bruno Pontecorvo. They occur in three varieties, like many other fundamental particles: electron, muon, and tau neutrinos. So, rather of disappearing, neutrinos might transition between various kinds while traveling, according to Pontecorvo. Some of the sun’s electron neutrinos, for example, may convert to muon neutrinos and therefore appear to vanish. Theorists eventually settled on a description of how neutrinos fluctuate between kinds based on their energy and travel distance that matched evidence from the sun and sky.

Many scientists, however, were put off by the thought of shape-shifting neutrinos. The math only works if each of the three neutrino species is a quantum mechanical mixture of three distinct masses — in other words, shape-shifting necessitates the presence of mass in neutrinos. However, the Standard Model of particle physics, a well-proven set of equations that describes all known basic particles and forces, declares neutrinos to be massless.

Because the sun and its atmosphere are so complex, LSND was equipped with a specialized neutrino source to search for more conclusive evidence of shape-shifting. It was quickly discovered by researchers. “Every week or so, we’d have a candidate,” Louis remarked. The top page of The New York Times in 1995 included an article on the experiment’s shape-shifting neutrinos.

The LSND experiment has been criticized for suspected influence from natural neutrino sources and sources of inaccuracy in the detectors. Even scientists who believe neutrinos oscillate and have mass skepticism about LSND’s figures since the inferred oscillation rate exceeded the rate predicted by solar and atmospheric neutrinos. The solar and atmospheric data revealed neutrinos fluctuate between just three known neutrino types; adding a fourth, the sterile neutrino — so named because it must not feel the force that binds electron, muon, and tau neutrinos to atoms, allowing them to be detected — better suited the LSND data.

SNO, Super-K, and KamLAND, a sequence of definitive neutrino oscillation experiments conducted in the late 1990s and early 2000s, significantly validated the three-neutrino oscillation hypothesis, earning several of the researchers involved a Nobel Prize. The sterile fourth neutrino lingered in the shadows.

Janet Conrad, a physicist at MIT, is seen in 2002 carrying a detector similar to those used in the MiniBooNE experiment, which she helped create and conduct.

The Anomaly Chasers

The Anomaly Chasers are a group of people that investigate unusual events.
Anomalies frequently arise in tests and then vanish after additional analysis, so many scientists dismiss them at first. Janet Conrad, a professor at the Massachusetts Institute of Technology and a “proud anomaly seeker,” feeds on oddities. “We’re a sloppy bunch.” We don’t mind if the place is a shambles. In fact, we like it,” she recently stated on Zoom.

Most particle scientists were working on colliders when Conrad finished her doctorate in 1993, smashing particles together in the hopes of conjuring new ones from the wreckage. Beautiful, all-encompassing theories like supersymmetry, which predicts a complete set of mirror-image particles for all the ones in the Standard Model, were popular; neutrino oscillations, on the other hand, were not. LSND’s finding piqued Conrad’s interest, so he decided to investigate it more. She stated, “I want nature to speak to me; I don’t want to tell nature what to do.”

Conrad and her anomaly-minded colleagues descended down into the LSND detector in the late 1990s and carefully removed over 1,000 of its amber-colored sensors, washed away the heavy oil, and put them in a new neutrino detector dubbed MiniBooNE, a three-story-tall sphere housed at Fermilab. “We had these yoga mats on the scaffolding where you could lie down and stare skyward,” she explained. “It appeared as though the cosmos was made up of small amber moons.” Oh, that was so lovely.”

From 2002 until 2019, this beefed-up version of LSND gathered data. MiniBooNE began to find a similar, abnormal neutrino oscillation rate five years into its lengthy run, implying that the LSND result wasn’t a fluke and that an additional lightweight neutrino may exist after all.

While MiniBooNE was running, though, additional trials began. Each looked into how varied neutrino transit lengths and energy affected their shape-shifting. Their findings appeared to support the three-neutrino hypothesis, contradicting both LSND and MiniBooNE.

The Sterile Neutrino has died

Anomaly hunters had reached a fork in the road, with signs pointing in opposing ways. Three neutrinos were found to have more evidence than four neutrinos. The Planck space telescope then delivered another blow to sterile neutrinos.

By detecting feeble radiation from that period, known as the cosmic microwave background, Planck was able to create an astonishingly comprehensive image of the universe as it looked not long after the Big Bang in 2013. Cosmologists were able to put their beliefs about the early universe to the test thanks to Planck’s image of this primordial light.

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Neutrinos would have been extremely energetic in the early cosmos, impacting how rapidly the universe expanded. Researchers were able to determine how many types of neutrinos inhabited the nascent universe by deducing the expansion rate from Planck’s cosmic microwave background data. According to the statistics, there were three distinct categories. According to Joachim Kopp, a theoretical physicist at CERN, these and other cosmic findings “very clearly ruled out the possibility of a fourth neutrino species” — at least, the simple, lightweight, sterile one theorists had envisaged.

By 2018, everyone had realized the game was over. Michele Maltoni announced the death of the sterile neutrino in a huge auditorium during a neutrino physics conference in Heidelberg, Germany. “He responded, ‘If you didn’t realize it was over, you should know it’s over now,'” Argüelles-Delgado recalled.

For neutrino theorists, Maltoni’s talk served as a wake-up call that they required fresh ideas. Returning to his Borges metaphor, Argüelles-Delgado observed, “The route forward was shattered.” “How are we going to maneuver now?”

He and his colleagues began questioning the assumptions that underpin the concept of a sterile neutrino. “In physics, we usually use the Occam’s razor method, right?” “We started with the most basic premise, which was a single new particle that doesn’t do anything but oscillate,” he explained. “It was probably a mistake to make that assumption.”

The dreaded Dark Sector

Neutrino physicists have been progressively considering the potential of numerous extra neutrinos interacting with each other via their own hidden forces during the last three years. The deep interrelationships of this “dark sector” of unseen particles would be similar to (but distinct from) those of electrons, quarks, and other Standard Model particles. Matheus Hostert, a theoretical physicist at the Perimeter Institute for Theoretical Physics in Waterloo, Canada, remarked, “It’s quite feasible that this dark sector is rich and intricate.”

By lowering the quantity of neutrinos that would have been created in the early cosmos, hidden forces can overcome the challenges posed by the Planck observatory. And a black sector, with so many qualities, may fill in a lot of gaps in our knowledge at once. Since the discovery that neutrinos had mass in the 1990s, physicists have questioned if neutrinos may explain the massive quantity of dark matter that seems to surround galaxies. They quickly decided that the three known neutrinos lack the mass required to do so. They could occur if a broader family of neutrinos exists, including some heavy ones.

The concept of a dark sector that is both unseen and productive isn’t new, but the number of models has expanded. The study combines the diverse topics of dark matter and neutrino anomalies into a single study. Argüelles-Delgado stated, “There has been a convergence.”

If dark matter is a single, inactive particle, a rich, complex dark sector may explain why the present-day universe appears to be expanding faster than predicted — a phenomenon known as the Hubble tension — and why galaxies don’t cluster as much as they should. “Changing the physics of dark matter here would have a significant influence on this sort of cosmic strain,” Princeton University astronomer Christina Kreisch said.

The models are based on previous concepts. For example, the presence of very heavy neutrinos was initially proposed decades ago to explain the three known neutrinos’ puzzlingly modest weights. (The masses of known, lightweight neutrinos and heavy neutrinos might have an inverse connection in a “seesaw mechanism.”) The disintegration of heavy neutrinos minutes after the Big Bang has also been proposed as a possible explanation for why there is so much more matter in the universe than antimatter. “Many individuals, including myself, are investigating such ties,” Kopp added.

Earlier this year, Argüelles-Delgado, Conrad, and numerous colleagues suggested a dark sector model that contains three heavy neutrinos of varying masses, which will be published in Physical Review D soon. Their model incorporates both a heavy neutrino decaying and a lightweight neutrino oscillating to account for the LSND and MiniBooNE data; it also allows leeway to explain the genesis of neutrino mass, the universe’s matter-antimatter imbalance via the seesaw mechanism, and dark matter.

The anomaly chasers came up with the new model after noticing a fault in the MiniBooNE experiment: it can’t tell the difference between signals produced by electron neutrinos and signals produced by certain particle decays. This raised the idea that, in addition to lightweight neutrinos cycling between kinds, heavy neutrinos were decaying within the detector, which would explain for the profusion of signals.

That story is supported by brand-new experimental data. MicroBooNE, a follow-up to MiniBooNE that was rebuilt to remedy the issue, will publish a paper in Physical Review Letters soon claiming that sterile neutrinos alone cannot account for the MiniBooNE oddity. Nonetheless, the findings support the hypothesis that only half of MiniBooNE’s events are caused by neutrino oscillations. MicroBooNE recently announced that the rest of the events are virtually probably not explained by decays of common Standard Model particles. The likelihood of heavy particles from the dark sector dying inside MiniBooNE will be assessed in MicroBooNE’s next version, which will be released next year.

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Physicists are also retracing their steps, comparing their dark sector models to historical evidence. For example, the team behind the IceCube experiment, which consists of 5,000 detectors implanted kilometers deep in the ice under the South Pole, has claimed since 2016 that no sterile neutrinos have passed through the ice, with each assertion becoming more certain than the previous. However, since sterile neutrinos may decay into other, unseen particles, IceCube data actually favors their existence, according to a study published earlier this month. The team’s comprehensive study has yet to be released, and experts emphasize the importance of this review before they can make a definitive statement.

Finally, calculations that take into account all neutrino oscillation experiments uncover evidence for decaying sterile neutrinos.

Bold statements concerning the presence of zillions of unseen particles need bold proof, and not everyone is convinced. “I’ve been betting against all the anomalies,” said Goran Senjanovi, a co-creator of the seesaw model of neutrino mass at Ludwig Maximilian University in Munich. Senjanovi remarked that rather than positing more and more particles to explain experimental surprises, we should “first and foremost” be guided by proven theory, taking just the tiniest steps beyond the extremely effective Standard Model.

Presumptions of minimalism and simplicity, on the other hand, have been proven to be incorrect in the Garden of Forking Paths. The electron, muon, and tau neutrinos should be massless, according to the Standard Model, but they aren’t. Theorists formerly assumed that if these neutrinos have mass, they must have enough to explain dark matter – but they don’t. Perhaps the Standard Model requires a far more extensive expansion. Physicists like Conrad emphasize the importance of looking for insights in anomalies.

Getting Out of the Maze

The problem now is figuring out how to get into the hypothetical dark sector, which is, well, dark. Pauli suggested that no theorist should try to create undetectable particles. Fortunately, scientists may be able to hear the invisible world’s murmurs through the three common neutrinos. “The neutrino is essentially a dark particle,” said New York University particle scientist Neal Weiner. “It can interact and mix with other dark particles in a way that none of the other Standard Model particles can.”

Neutrino experiments in the works might lead to the discovery of a doorway to the dark sector. Following MicroBooNE, Fermilab’s SBND and ICARUS experiments will shortly turn on and study neutrino oscillations at a variety of distances and energies, revealing the whole pattern of the oscillations. Meanwhile, Fermilab’s DUNE experiment will detect heavier dark sector particles. Conrad believes that carefully observing neutrinos pour from radioactive sources like lithium-8 in “decay at rest” tests will provide an alternate perspective on the current muddle of data.

IceCube has a unique view position as well. The experiment can detect very energetic neutrinos that are created when cosmic rays impact with the Earth’s atmosphere. These neutrinos may collide with particles inside IceCube, morphing into the unusual, heavy neutrinos that are thought to decay inside MiniBooNE. This “double bang” signature would be “extremely strong evidence of a new particle” if IceCube detected this scattering followed by the heavy neutrino’s disintegration some distance away, according to Hostert.

According to Weiner, the dark sector is “not simply a bedtime story” because of these potential. Even if the dark sector exists and the familiar neutrinos serve as mediators, there’s no assurance that their connection will be strong enough to expose what’s been hidden. “Any sensible experiment may be absolutely inaccessible to heavy [neutrinos,” said Josh Spitz of the University of Michigan.

It’s also possible that each neutrino anomaly discovered so far, beginning with LSND, has its own banal explanation. “Perhaps they’re all incorrect, and it’s just really unlucky that they all seem to have anything to do with one other,” Conrad speculated. “That would be nature’s cruelest act.”

Argüelles-Delgado, for one, is confident about finally escaping the maze. “Science progresses in phases, and then something simply snaps,” he explained. “I’m gathering information and asking questions.” Some sources of information are more trustworthy than others; you must make your own judgment.”

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