Over the years, the conflicting results of neutrino measurements have led some physicists to suggest that there is a "dark area" filled with invisible particles in the universe, which can explain dark matter, cosmic expansion and other puzzling mysteries at the same time In 1993, deep underground at Los Alamos National Laboratory in New Mexico, several lights appeared in a bus sized oil tank, which opened a fascinating exploration story. However, the story is still inconclusive.
Many neutrino physicists feel like walking through a maze, not sure where to go or which clue might lead them astray
At that time, scientists were using the liquid scintillator neutrino detector (LSND) to find the radiation burst produced by neutrinos. Of all known elementary particles, neutrinos are the lightest and most elusive. Bill Lewis, one of the leaders of the experiment, said: "we saw the results we wanted to see, but it also surprised us."
The problem is that they see too much. Theoretical physicists assume that neutrinos may oscillate between different types - called different "flavors" during flight. This hypothesis explains many current astronomical observations. LSND tested this hypothesis and put a beam μ Neutrinos (one of three known neutrinos) aim at the oil tank and calculate the number of electric neutrinos arriving in the oil tank. However, Lewis's team detected much more electrical neutrinos than predicted by neutrino oscillation theory.
Since then, the researchers have designed dozens of neutrino experiments, each on a larger scale than the previous one. Inside the mountains, abandoned mining caves and under the Antarctic ice, physicists have built magnificent experimental facilities for these mysterious particles. However, when these experiments detect neutrinos from various angles, they continue to get contradictory particle behavior images. "Things are getting more and more complicated," Louis said.
"It's a very confusing story. I'd like to call it the 'garden of forked paths'," said Carlos aguiles Delgado, a neutrino physicist at Harvard University. In the 1941 short story of the same name written by Jorge Luis Borges, a famous Argentine writer, time forks out an infinitely possible future. For neutrinos, the conflicting results confuse theoretical physicists, who are not sure which data are trustworthy and which data may lead them astray. Aguiles Delgado said: "like any detective story, sometimes when you see clues, they will lead you in the wrong direction."
In 1993, the liquid scintillator neutrino detector at Los Alamos National Laboratory reported a large number of neutrino detection results, which is very confusing. In the photo, engineer Rick Bolton is in photomultiplier tubes. Once the tank is filled with mineral oil, these photomultiplier tubes can detect the light emitted by neutrino interaction in the tank
The simplest explanation for the LSND anomaly is that there may be a new neutrino, the fourth neutrino - inert neutrino. According to the new rules, inert neutrinos can mix all neutrino types. They will make μ Neutrinos are more likely to oscillate into electric neutrinos in a short distance from the oil tank.
However, over time, the researchers found that the explanation of inert neutrinos did not agree with the results of other experiments. "We found a champion theory, but the problem is that it failed miserably elsewhere," aguiles Delgado said. "We went deep into the forest and now we need to get out."
Physicists were forced to trace the results of previous experiments; They have been thinking over and over again, trying to understand what is behind the chaotic hints and incomplete results. In recent years, they have designed new theories that are more complex than inert neutrinos, but if these theories are correct, physics will completely change - solving neutrino oscillation data anomalies and other major mysteries of physics at the same time. In particular, the new model assumes that additional heavier neutrinos can explain dark matter. At present, we have not observed the dark matter hidden in galaxies. It is estimated that the proportion of dark matter in the composition of the universe is more than four times that of conventional matter.
Recently, Fermi National Accelerator Laboratory near Chicago announced four analysis results of microboone experiment. Based on these results, combined with the latest research results of the icecube Neutrino Observatory in Antarctica, scientists believe that these more complex neutrino theories may be correct, although much effort is needed to demonstrate them.
Aguiles Delgado said that the situation has vaguely changed and new discoveries may be coming.
In 2002, physicist Janet Conrad (currently at MIT) held a detector used in the miniboone experiment. She helped build and led the miniboone experiment
Desperate remedy
In 1930, Wolfgang Pauli put forward the neutrino hypothesis to explain the disappearance of energy in the process of radioactive decay. At that time, he even called the hypothesis "desperate remedy". These hypothetical particles have neither mass nor charge, which makes Pauli doubt whether they can be detected experimentally. "This is something no theorist should do," Pauli wrote in his diary. But in 1956, in an experiment similar to LSND, the existence of neutrinos was confirmed.
However, when physicists detected neutrinos from the sun, one of the natural sources of such particles, and found that their number was less than half that predicted by the theoretical model of stellar nuclear reaction, the situation soon fell into chaos. By the 1990s, the strange behavior of neutrinos became increasingly obvious. Not only do solar neutrinos seem to disappear mysteriously, but when cosmic rays collide with the upper atmosphere, the neutrinos that fall to the earth also disappear.
Earlier, Italian physicist Bruno pontikov proposed a solution that neutrinos can change their patterns. Like many elementary particles, there are three types of neutrinos: Electric neutrinos μ Neutrinos and τ Neutrinos. Therefore, pontikov believes that neutrinos may change between these types rather than disappear during their movement. For example, some electric neutrinos produced by the sun may become μ Neutrinos, it looks like they've disappeared. Over time, theoretical physicists began to focus on describing how neutrinos oscillate between different types according to energy and propagation distance, so as to match data from the sun and the universe.
However, the concept of neutrino type transformation is difficult for many physicists to accept. Only when these three neutrinos are quantum mechanical mixtures of three different masses, can they work mathematically; In other words, type transformation means that neutrinos must have mass. But the standard model of particle physics - a set of equations describing known elementary particles and forces, which has been verified many times - clearly believes that neutrinos have no mass.
The sun and atmosphere are very complex, so another neutrino source is specially selected in the LSND experiment to find clearer evidence of neutrino type transformation. Researchers soon made progress. "We find a possible evidence almost every week," Louis said. In 1995, the New York Times published a report on the front page about how the experiment searched for neutrinos with type changes.
Critics of the LSND experiment point out that the detector may bring errors, and the neutrino source in nature may also produce interference. Even those scientists who support the idea of neutrino oscillation do not believe the LSND data, because the inferred oscillation rate exceeds the implied rate of solar neutrinos and atmospheric neutrinos. Solar and atmospheric data show that neutrinos oscillate only between three known "flavors"; If we add the fourth neutrino - inert neutrino - it is more consistent with the LSND data. Inert neutrinos are named because they do not interact with fundamental forces other than gravity and cannot be detected.
In the late 1990s and early 21st century, the results of a series of neutrino oscillation experiments - Sudbury Neutrino Observatory (SnO), Super-K and kamland - supported the three neutrino oscillation model, and some researchers involved in the experiment won Nobel prizes. On the other hand, the hypothetical fourth neutrino is still covered with a mysterious veil.
Carlos aguiles Delgado, a neutrino physicist at Harvard University, has designed some new theories to explain the complex neutrino measurement data
Chasing the abnormal
Anomalies often appear suddenly in experiments and then disappear in further research, so many researchers ignore them at first. However, Professor Janet Conrad of MIT is a "proud chaser" who attaches great importance to all kinds of abnormal phenomena. "We don't mind chaos. In fact, we like it very much," she said
When Conrad completed his PhD in 1993, most particle physicists were studying colliders, trying to find new particles in debris through particle impact. Some beautiful and all inclusive theories, such as supersymmetry theory, predict that all particles in the standard model have a set of mirror particles; But for the subtle neutrino oscillation, the situation is quite different. Nevertheless, Conrad was interested in the results of the LSND experiment and decided to continue his research. "I want nature to talk to me; I don't want to tell nature what to do," she said.
In the late 1990s, Conrad and colleagues who are also keen on anomalies came to the interior of the LSND detector, carefully extracted more than 1000 amber sensors, wiped off the viscous oil and installed them on a new neutrino detector. The new detector is three stories high and located in Fermilab. It is called "miniboone" by them. "We have yoga mats so you can lie on the scaffold and look up," Conrad said. "It's like a universe of amber moons. Wow, it's so beautiful."
This enhanced version of the LSND experiment collects data from 2002 to 2019. After five years of operation, the miniboone experiment began to observe similar abnormal neutrino oscillation rates, indicating that the result of LSND is not accidental, and there may be an ultra light neutrino.
While the miniboone experiment was carried out, other neutrino experiments also began. These experiments explored different neutrino propagation distances and energies to understand how this affects their shape changes. Their results seem to confirm the three neutrino model, which is contradictory not only to LSND, but also to miniboone.
The ice cube neutrino detector found high-energy neutrinos passing through the ice layer on the surface of Antarctica. The data collected in the original computer installed in the laboratory on the ground
"Death" of inert neutrinos
The pursuer of the anomaly has come to a fork in the road, and the road sign points in the opposite direction. The evidence supporting the existence of three neutrinos exceeds the evidence supporting the existence of four neutrinos. Then the Planck Space Telescope gave another blow to the inert neutrino hypothesis.
In 2013, the Planck space telescope detected weak radiation from the so-called cosmic microwave background, taking an incredibly detailed picture of the universe shortly after the big bang. The detailed description of the cosmic microwave background radiation enables cosmologists to deeply test their theories about the early universe.
In the early universe, neutrinos were very active, so they strongly affected the rate of cosmic expansion. Using the cosmic microwave background data detected by the Planck space telescope, the researchers calculated the expansion rate of the universe, and then estimated how many kinds of neutrinos filled the young universe. The data show that there were three kinds of neutrinos at that time. Joachim Kopp, a theoretical physicist at CERN, said the discovery and other cosmological observations "quite certainly exclude the existence of a fourth neutrino" - at least the inert neutrino that theoretical physicists have considered.
By 2018, everyone thought it was a foregone conclusion. At a neutrino physics conference in Heidelberg, Germany, Michel MarToni stood in a magnificent auditorium and announced the "death" of inert neutrinos. Aguiles Delgado recalled: "he said, 'if you don't know it's over, you should know it now.'"
MarToni's speech sounded an alarm to neutrino theorists, who need some new ideas. "The way forward is impassable," aguiles Delgado said back to Borges's metaphor. "Where should we go now?"
He and his colleagues began to re-examine the basic assumptions of inert neutrino theory. "In physics, we always use this Occam razor method, right? We start with the simplest assumption, here is a new particle, but we don't do anything except oscillatory behavior," he said. "This may be a stupid assumption."
Dark area
Over the past three years, neutrino physicists are increasingly considering the possibility of adding multiple neutrinos, which may interact through their own mysterious forces. This "Dark Sector" composed of invisible particles may have complex interrelationships, similar to (but independent of) the interrelationships of electrons, quarks and other standard model particles. "This dark area is very rich and complex, which is entirely possible," said Matthews Horst, a theoretical physicist at the Institute of circular theoretical physics in Waterloo, Canada
Adding mysterious forces to the model can suppress the number of neutrinos produced in the early universe, thus avoiding the challenges posed by the observations of the Planck telescope. At the same time, a dark area with so many characteristics can also fill many loopholes in our understanding. Since the 1990s, physicists have wondered whether the mass of dark matter neutrinos in galaxies can be explained. They quickly concluded that none of the three known neutrinos had sufficient mass, but if there was a larger family of neutrinos - including some heavier neutrinos - it might be able to explain dark matter.
An invisible but rich dark area? This idea is not new, but in these models, the number of neutrinos has become more and more. This study puts the different problems of dark matter and neutrino anomalies into the same category. "It's a convergence," said aguiles Delgado
A rich and complex dark region helps explain why today's universe seems to expand faster than expected - a phenomenon known as the "Hubble tension" - and why galaxies do not gather as much as expected if dark matter consists of a single inert particle. "Changing the physical properties of dark matter does have an impact on such cosmic tension," said Christina kresch, an astrophysicist at Princeton University in the United States
These models resonate with the old view. For example, as early as a few decades ago, it was assumed that there were some very heavy neutrinos, which could explain the puzzling small mass of three known neutrinos (in the "seesaw mechanism", the mass of known light neutrinos and heavy neutrinos may be inversely proportional). The decay of heavy neutrinos after the Big Bang is considered to be the possible reason why there is much more matter than antimatter in the universe today. "A lot of people, including myself, are studying this connection," Joachim Kopp said.
Earlier this year, aguiles Delgado, Conrad and several collaborators proposed a dark region model containing three different mass heavy neutrinos. Their findings will soon be published in Physical Review D. The model explains the data of LSND and miniboone by reconciling the decay of a heavy neutrino and the oscillation of a light neutrino; At the same time, the model also leaves space for explaining the origin of neutrino mass, the asymmetry of matter antimatter in the universe, and dark matter.
When designing the new model, researchers chasing anomalies took into account a defect of the miniboone experiment: it could not distinguish between the signals generated by electric neutrinos and those generated by the decay of some particles. This leads to a possibility: in addition to light neutrinos oscillating between different types, heavy neutrinos may also decay inside the detector, which explains why the detector obtains rich signals.
The new experimental results support this model. The microboone experiment of Fermilab is the follow-up of the miniboone experiment, which has been reconfigured to correct the defects. The results of the experiment will be published in Physical Review Letters, indicating that inert neutrinos themselves cannot explain the abnormal phenomenon of miniboone. The experimental results may imply the possibility that only half of the miniboone events are caused by neutrino oscillations. The microboone team recently reported that the decay of familiar standard model particles almost certainly can not explain the remaining events. Microboone experiment will start the next version of the experiment next year, when it may be able to determine the possibility of heavy particle decay in the dark area inside the detector.
Physicists are also re exploring the original path and examining their dark zone models with existing data. For example, since 2016, the ice cube experimental team has issued a series of statements - more and more confident - that it has found no signs of inert neutrinos passing through the ice. The ice cube is an array of 5000 detectors buried several kilometers under the Antarctic ice. However, a recently released analysis found that if inert neutrinos can decay into other invisible particles, the ice cube data actually favor their existence. The team's full analysis has not yet been published, but the researchers stressed the need to assess this possibility before reaching a conclusion.
Finally, the analysis of all neutrino oscillation experiments also found evidence supporting the decay of inert neutrinos.
Bold assertions about the existence of large numbers of invisible particles require bold evidence, but not everyone can be persuaded. "I've been betting that all anomalies won't happen," said Goran senjanovic of the University of Munich in Germany As one of the proponents of the neutrino mass seesaw model, senjanovich said that we should not assume more and more particles to explain the unexpected results in the experiment. The "most important" should be guided by the existing theory and make only the smallest changes outside the highly successful standard model.
However, in the "garden with bifurcated paths", the assumption of pursuing minimalism and simplicity is often wrong. Standard model predicts electric neutrinos μ Neutrinos and τ Neutrinos are massless, but later studies have shown that they are not. Theoretical physicists once thought that if these neutrinos had mass, they must have enough mass to explain dark matter - but the result was also negative. Maybe we need to expand the standard model more finely. Physicists such as Conrad also stressed that finding clues from abnormal phenomena can bring benefits.
Out of the maze
The challenge now is how to enter the imaginary dark area. Pauli once lamented that proposing undetectable particles is something that no theoretical physicist should do; Fortunately, physicists may be able to "hear" the whispers of the dark world through three familiar neutrinos. "Neutrinos are essentially dark particles," said Neil Weiner, a particle physicist at New York University. "They have the ability to interact and mix with other dark particles, which no other particle can do in the standard model."
The upcoming new neutrino experiment may open the door to the dark region. After microboone, Fermilab's sbnd and Icarus experiments will be launched soon, and neutrino oscillations will be detected at multiple distances and energies to help scientists better understand the complete modes of these oscillations. At the same time, Fermilab's Dune experiment will have higher sensitivity and may detect heavier dark particles. Conrad said that in the "static decay" experiment, careful observation of the process of neutrinos gushing from radioactive sources such as lithium-8 will provide another view to explain the current chaotic results.
The ice cube neutrino detector also offers an unusual vantage point. The experiment can detect high-energy neutrinos produced when cosmic rays collide with the earth's atmosphere. These neutrinos may interact with particles inside the ice cube, scatter and deform into strange, massive neutrinos - researchers suspect that these neutrinos will decay inside miniboone. Matthews Horst said that if the ice cube saw this scattering and heavy neutrino decay from a distance, this "double whammy" feature "would be very strong evidence of the existence of a new particle".
These possibilities make the dark area "more than just a bedtime story," Weiner said. However, even if the dark areas exist and take the common neutrinos as the medium, there is no guarantee that their connection is strong enough to reveal the hidden things. "Any reasonable experiment may not be able to obtain heavy neutrinos at all," said Josh Spitz, a physicist at the University of Michigan
Of course, every neutrino anomaly from LSND may have an ordinary explanation. "Maybe all these anomalies are wrong, maybe they're just very unlucky. They all seem to be related to each other," Conrad said. "That would be a manifestation of the extreme cruelty of nature."
As for aguiles Delgado, he is optimistic that he will eventually get out of the maze. "Science is phased, and then it may suddenly break," he said. "I'm accumulating clues and continuing to ask. Some information is more reliable than others; you have to judge for yourself." (Ren Tian)