The neutrino puzzle

This facility is part of a particle emergence experiment Electron neutrino to the axis of the neutrino particle beam in the main injector of Permilev, Fermi National Accelerator Laboratory in Batavia, Illinois (experiment NOvANuMI Off-Axis Electron-Neutrino Appearance). A similar but larger detector lies 800 kilometers away, in Minnesota, where it captures neutrino particles that have passed through that detector and through all the earth that separates them. The NOυA project, which has been operating since 2014, is the neutrino experiment whose distance it covers is the largest in the world, but it lays the foundations for something much bigger - "the neutrino experiment in the depths of the earth" (DUNE - Deep Underground Neutrino Experiment). This experiment will begin at Premil, where a particle accelerator will accelerate protons and slam them into graphite to create a beam of neutrino particles. These particles will then fly through 1,300 kilometers of soil, from Illinois to South Dakota. The additional 500 kilometers they will have to travel will increase the likelihood that the neutrino particles will exhibit some of their unique strange behavior.

DUNE is the most ambitious particle physics experiment planned on US soil since the Superconducting Accelerator (SSC), a project from the 90s that ultimately did not come to fruition. According to the plan, the DUNE project, which costs 20 billion dollars, will start operating in the next decade and should operate for at least 1.5 years. But not only the Americans are excited about it - 20 researchers from 1,000 countries are participating in the project, and more are on the way. This experiment will be the largest neutrino experiment in the world. It will also mark the first time that CERN, Europe's main laboratory for particle physics, will invest in a project outside the continent. Just as the Large Hadron Collider (LHC) discovered the celebrated Higgs boson in 30, revealing the existence of an invisible field that fills the cosmos, scientists hope that DUNE can use neutrinos to understand the universe at a deeper level. "We want to do for the neutrinos what the LHC did for the Higgs," says one of DUNE's speakers, Mark Thomson, an energetic Englishman at the University of Cambridge who is helping to promote the project. "We believe we are on the brink of the next great revolution in particle physics."

Neutrinos raise such pretentious hopes because they are the first particles to escape the so-called "Standard Model," the best description physicists have of nature's fundamental particles and the laws that govern them. The Standard Model, which explains the behavior of every other known particle with astonishing accuracy, predicts that neutrinos have no mass. And this is indeed what scientists believed until about 15 years ago, when experiments conducted in Canada and Japan revealed that neutrino particles actually have an extremely small amount of mass. However, it seems that the neutrino particles do not get their mass the way other particles get it. Instead, their weight seems to come through what is called new physics - some particle, some force or some phenomenon that scientists have yet to discover.

Over the past few years, neutrinos have increasingly looked like a promising bridge to the future of physics, as other attempts to score this front have failed. So far, the LHC has failed to produce any particle not predicted by the Standard Model. Experiments designed to reveal the particles that make up dark matter, that invisible substance that is the main component of the cosmos, have also come up empty-handed. "We know that the Standard Model is incomplete - there are other things going on, but we don't know what they are," says Permil neutrino physicist Steven Park. "There are people who have chosen to entrust the fate of their careers to the hands of the LHC. And others bet on the neutrino particles.”

A weighty mystery

The day after my visit to NOυA's cave, I found myself sitting in an empty office on the third floor of Rathburn-Wilson Hall, the main building of Fermilab. Park, who works here alongside theoretician Andre de Gobia of Northwestern University, says he chose this room for our meeting because it was once the office of Leon Lederman, the former director of Permilev, who retired and developed a way to create a beam of neutrino particles using a particle accelerator. This work, which laid the foundations for the DUNE project, revealed the existence of one of the three known types of neutrinos in 1962 and later earned Lederman a Nobel Prize. Park and de Gobia admit that although the field has advanced immeasurably since Lederman's day, scientists are still confused. "The thing about neutrinos is that the more we understand, the more questions there are," says Park. "They are very inclusive particles."

Park, born in New Zealand, was drawn to neutrino research shortly after arriving in the US for advanced degree studies in the 70s. In the following decades, the neutrino particles lost the name they got as boring and massless particles. "There were revolutions there, one after another," he says. "The question is, do we have more revolutions waiting for us in the future?" He and De Gobia are willing to bet that they do. "We have only begun to measure the properties of neutrino particles at a level approaching the measurements made on other particles," says de Gobia. "We don't know their masses, there could be new [types of neutrinos], the neutrino particles might talk to particles that don't talk to anyone else."

DUNE will focus on the strange tendency of neutrino particles to switch identities, a process called oscillation. These particles have three types of identities, called "flavors": electron neutrinos, ion neutrinos and teutonic neutrinos. The researchers can differentiate between them because when they react with atoms that are inside detectors, they produce different end products - an electron neutrino produces electrons, a muon neutrino produces muons, and a tau neutrino produces tau particles (the muon and tau are the electron's heavier cousins). As strange as it sounds, these three flavors are interchangeable. A single particle may leave Fermilov as a ion neutrino and arrive in South Dakota as an electron neutrino. Or rather as neutrinos and tauoni. As far as physicists know, neutrinos are the only particles that exhibit this strange behavior of identity conversion.

When physicists discovered this tendency of neutrino particles to change shape a decade and a half ago, a stubborn mystery was solved. In the 60s, when scientists began studying neutrino particles streaming from the Sun, they measured only about a third of the output predicted by theory. The oscillation phenomenon explained why: the missing two-thirds changed from electron neutrinos to ion neutrinos or tawny neutrinos on their way to Earth. However, the instruments were aimed to see only electron neutrino particles. Although this discovery solved this problem, which was called the solar neutrino problem, it revealed a new puzzle: according to the theory, the only way the neutrino particles could change flavor was if they had mass—something the Standard Model did not predict.

The reason scientists know that neutrinos must have mass is a puzzling consequence of quantum theory. In order for neutrinos to switch flavors, each flavor must be made of several different "mass states". It may be strange, but there seems to be no distinct mass for any flavor of neutrino; Instead, the flavors are a mixture of three possible masses. (If that sounds strange, blame quantum mechanics, which tells us that particles are not absolute entities but uncertain nebulae of probabilities.) As a neutrino flies through space, the particles associated with each mass state move at a slightly different rate from each other, as a result of Einstein's special theory of relativity which stated that its speed of a particle moving at a speed close to the speed of light depends on its mass. The explanation is that over time, it is this difference that causes the mass mixture of each neutrino to change, so that a particle that starts out as, for example, a muionic neutrino, defined by its exact mass mixture, can become an electron neutrino or a tauonic neutrino.

The scientists still do not know what the exact neutrino mass states are - they only know that they are different and that they are not zero. However, by counting the number of neutrino particles that oscillate during the journey from Illinois to South Dakota, DUNE will try to determine how the different neutrino states relate to each other. According to the theory, it is possible that the order of the three possible neutrino masses is such that two of them are very small and one is large, or alternatively, two of the masses are large and one is smaller. The first of the two options is called the normal hierarchy, while the second organization is called the reverse hierarchy. DUNE should be able to distinguish between the two hierarchies because the explanation is that the material inside the Earth can affect neutrino oscillations; If the normal hierarchy is correct, the relationships between the three tastes that scientists expect to discover should be different than if the reverse hierarchy is correct. "By shooting neutrinos through matter, you can determine this difference very easily, and the farther you shoot the neutrinos, the clearer the signal will be," says Thomson. "It's a physical piece that DUNE is definitely going to crack in a few years."

the mass source

Once the researchers knew the mass order of the neutrino particles, they could approach the bigger question - how neutrino particles acquire their mass. Most particles, such as the quarks that make up the protons and neutrons inside atoms, acquire their mass through interaction with the Higgs field; This field, which fills all space, is linked to the Higgs boson discovered at the LHC. However, the Higgs mechanism only works on particles that appear in both the right and left versions, a fundamental difference related to the direction of their spin in relation to the direction of their movement. So far, neutrino particles have only been observed left-handed. If they gain mass from the Higgs field, then right-handed neutrinos must also be present. However, right-handed neutrino particles have never been observed, and this raises the possibility that if they are real, they do not react with any of the forces and particles in nature - and such a feature sounds implausible to some physicists. Moreover, if the Higgs field does act on neutrinos, then theorists expect them to have masses similar to those of the other known particles. However, neutrino particles are unbelievably light. Whatever their mass states are, they are smaller than a hundred thousand times the mass of the electron, which is minuscule in any case. "Very few people believe that it is the Higgs mechanism that gives mass to neutrino particles," says Fermilab director Nigel Lockyer. "Apparently there is a completely different mechanism, and therefore there should be other particles involved in how this happens."

One of the possibilities, which excites physicists, is that neutrino particles may be Myorna particles - that is, particles that are their own antiparticles. (This is possible because neutrino particles have no electric charge, and the difference in charge is what differentiates a particle from its antiparticle counterpart.) Theorists believe that Myorna particles have a way to gain mass without involving the Higgs field - perhaps through interactions with a new field that has yet to be discovered. The math behind this scenario also requires the existence of a very heavy group of yet-to-be-discovered neutrino particles; These particles will have a mass up to 1012 times greater than the mass of some of the heaviest particles known, and in a sense they will balance the light neutrino particles. The prospect of discovering a new scale of masses works wonders for particle physicists. "Historically, we have always progressed by studying nature at different scales," says de Gobia. And if some new field gives mass to the neutrino particles, it may affect other particles as well. "If nature knows how to do this with neutrino particles, where else does it do it?" Lockyer speculates. "Theorists ask: Could the dark matter be the mass of Myorna?"

DUNE won't directly test whether neutrinos are Myorna particles, but by measuring the mass hierarchy, it will help scientists interpret the results of experiments that do - experiments currently being conducted in Japan, Europe, the US and elsewhere. In addition, DUNE should help shed light on the source of neutrino mass by providing details on how neutrino particles move from one mass combination to another during an oscillation. "We want to do the best possible neutrino oscillation experiment," says De Gobia, "because that's exactly where we know we'll learn something about the masses of neutrino particles."

Anti-matter anti-matter

The in-depth look at the strange properties of these tiny particles could also help solve a cosmic-scale mystery: why the universe is made of matter and not antimatter.

According to the cosmologists' prediction, matter and antimatter should have been present in equal amounts after the Big Bang. Somehow, after most of the matter has ionized with most of the antimatter (for these two cannot come into contact with each other without causing mutual annihilation), a slight balance of matter remains. It is this material that makes up the galaxies, stars and planets we see today.

To explain this asymmetry, scientists are on the lookout for a type of particle that behaves differently from its antimatter counterpart, and there are several clues, including signs observed in other experiments, that point to neutrino particles. DUNE will look for signs of CP (Charge Parity violation) - in other words, evidence that antineutrinos are oscillating from flavor to flavor at a different rate than neutrinos. For example, the theory raises the possibility that DUNE might see antimatter ion neutrinos transform into Electron neutrinos at a rate ranging from half the rate at which material neutrinos undergo this transformation to twice as much—a difference that Park calls "enormous" and which would explain why matter won that primordial battle. , even if it turns out that the two are essentially the same thing—that is, if neutrinos are Majorana particles. In this case, the only thing that would distinguish neutrinos from antineutrinos would be whether they are right-handed or left-handed, with respect to the direction of their spin. Material neutrinos, which are left-handed, will be able to behave differently from antimatter neutrino particles, which will be right-handed.)

DUNE will also be able to determine whether neutrino particles have only three flavors or whether there are several more waiting to be discovered, as some theorists speculate. The additional neutrino flavors will be of the type known as sterile neutrinos, since they do not react at all with normal matter. Experiments conducted in the past, such as the Liquid Scintillator Neutrino Detector at the Los Alamos National Laboratory and the MiniBooNE (Mini Booster Neutrino Experiment) at Permilab, showed inconclusive signs that another type of neutrino creates disturbances in the oscillations, and this suggests that there are sterile neutrino particles and that they are heavier than the three normal types of neutrinos. The researchers hope that DUNE will either confirm or disprove this possibility. "Sterile neutrinos can change the pattern of oscillations we see in DUNE to a significant extent," says Thomson.

Bet on the whole jackpot

To deal with all these conundrums, the scientists designed DUNE to collect much more data, with much higher levels of accuracy, than any neutrino experiment that came before it. In this project, they will use a neutrino beam that is about twice as powerful as the most powerful high-energy neutrino stream that exists today, and it will bombard this beam with a detector 100 times larger than the largest detector of its kind today.

The heart of this experiment will be the remote detector, which will be located at the Sanford Underground Research Facility in the nearby town of Lied, South Dakota. This machine will consist of four detector modules, each as long as an Olympic pool but six times deeper, and will be filled with 17,000 tons of liquid argon. When a neutrino particle hits the nucleus of an argon atom, either in the near detector or in the far detector, it will turn - depending on its taste - into an electron, a muon or a tau particle. Muons will travel through the liquid argon in straight lines, as they kick electrons out of argon atoms, leaving behind a trail of electrons that the detector can see. On the other hand, if the neutrino produces an electron, the process will produce a photon that will produce two electrons, then more photons and so on, in a cascade of new particles. Tau neutrino particles, similarly, will produce tau particles - but only if the initial neutrino had enough energy; Tau particles, whose mass is greater than electrons or muons, need more energy to form. The scientists at CERN will begin testing scaled-down versions of the DUNE remote detector in 2018. "These detectors are a bit like space missions because once they're launched you can't really stop them and take them apart to fix things," says Joseph Lyken, deputy director of Permilev. "Once you get the 17,000 tons of liquid argon in, it's just too hard to get them out."

To succeed, DUNE will have to overcome the budgetary and political hurdles that have stymied large physical projects in the past. In July 2017, scientists and officials held a ground-breaking ceremony at the Sanford Facility that marked the beginning of a massive excavation operation that will last at least three years. Of course, a lot of digging was done for the SSC, which was planned to be even bigger than the LHC. The SSC would have apparently succeeded in discovering the Higgs boson, but it was canceled in 1993 due to prohibitive costs and changing political fashions. "You can look at the past and remember Baal-Maitz, and, oh, what a sad story that was," says Lockyer. "The international nature of DUNE is a huge step forward." Commitments and funding coming from more than one country will help DUNE avoid the SSC's grim fate. "I'm saying it's definitely going to happen," Lockyer says. Then he thought for a moment: "But is it possible that it didn't come true? Yes."