This article appears in the June 2024 issue of Front Vision, an educational Chinese-language magazine for kids. It is reproduced here with permission.
Catching Ghost Particles: The story of neutrinos
by Kathryn Hulick
Zip! Zing! Right now, teeny-tiny particles are pelting your body. Every second, 100 trillion of them zoom through you. But they don’t hurt. In fact, they usually don’t disturb even a single atom or molecule on the way through. These particles can pass through all of planet Earth without hitting anything!
What are these sneaky little things? They are called neutrinos. They have no electric charge. They come in three varieties, or “flavors,” called electron, muon, and tau. Oddly, a neutrino switches between all three flavors as it travels, a trait called oscillation. Neutrino flavors have different masses that are all extremely close to zero. Neutrinos are “really, really, super light,” says Kate Scholberg. She’s a physicist at Duke University in North Carolina. An electron is the lightest particle in an atom of typical matter. But neutrinos make them seem heavy. Scholberg says, “If an electron is an elephant, a neutrino is lighter than a mouse.”
Almost all the neutrinos zipping through Earth come from the Sun. But neutrinos come from all kinds of different sources, including black holes merging or a star exploding in a supernova. A banana even emits neutrinos – they pop out when potassium atoms go through radioactive decay. The standard cosmological model predicts that neutrinos left over from the Big Bang still pepper the universe.
Neutrinos are everywhere! There are far more of them in the universe than any other particles that have mass.
The Ghost Particle
Austrian physicist Wolfgang Pauli predicted that neutrinos should exist back in 1930, long before anyone caught one. A few years later in 1934, Italian-American physicist Enrico Fermi gave the particle its name. “Neutrino” means “little neutral one” in Italian.
These physicists came up with the idea of the neutrino to solve a problem. Energy seemed to go missing during beta decay, a process that happens inside some types of radioactive atoms. In one type of beta decay, called beta minus, a neutron turns into a proton and an electron. According to the law of conservation of energy, the energy of the electron should always equal the difference between the energies of the neutron and the proton. But when physicists measured these electrons, they had all sorts of energies, and all of them were smaller than predicted. It seemed as if some of the energy had disappeared! Some physicists wondered if the law of conservation of energy maybe didn’t apply to such tiny particles. But Pauli proposed that a new, electrically neutral particle had carried away the missing energy.
Physicists realized that neutrinos, if they existed, would be especially difficult to detect. Some even thought this could be impossible. “There is no practically possible way of observing the neutrino,” wrote Hans Bethe and Rudolf Peierls in 1934. The problem was that a particle has to interact with something for us to be able to observe it. You see things with your eyes, for example, because light hits the things you’re looking at. Scientific instruments can capture other kinds of evidence, such as scattered particles.
Most of the time, though, neutrinos are essentially invisible—nothing hits them and they don’t hit anything. “They are like ghosts,” says Scholberg. A common nickname for the neutrino is “the ghost particle.” Very rarely, though, a neutrino will happen to hit another particle. This is such a rare occurrence that in your entire lifetime, a neutrino will smack into an atom in your body just a handful of times. Could scientists find a way to catch this rare event happening?
In 1956, physicists Frederick Reines and Clyde Cowan Jr. finally succeeded. They were working on Project Poltergeist, an effort at Los Alamos National Laboratory to try to catch neutrinos. A poltergeist is another name for a ghost. The thing that finally helped reveal neutrinos’ existence was a brand new technology: the nuclear reactor, invented in 1942. A nuclear reactor gives off huge numbers of neutrinos. And the more neutrinos there are, the greater the chance is that a few of them will hit something in a detector. Reines realized that setting up a neutrino detector in a nuclear power plant just might work. So that’s what he and Cowan did. In 1956, they sent a telegram to Pauli. It read: “We are happy to inform you that we have definitively detected neutrinos.” Reines went on to earn the 1995 Nobel Prize in Physics for the discovery.
A Cube Beneath Antarctica
Today, scientists keep an eye out for neutrinos at around a dozen different observatories. These capture evidence of natural neutrinos produced in space. The largest neutrino observatory in the world, IceCube, began operating in 2011 in Antarctica. “It’s mind-bogglingly huge,” says Scholberg.
To build IceCube, engineers drilled 86 boreholes over two kilometers deep into the ice. These holes form a hexagonal pattern that’s about a kilometer across in length and width. Each borehole contains a string studded with light sensors, electronics, and other equipment. In total, IceCube has 5,160 separate sensors, called digital optical modules (DOMs), keeping watch for neutrino collisions, almost like security cameras. These collisions are still very rare, and that’s why IceCube is so huge. The more space a neutrino detector watches, the greater the chance is that it will catch some rare collisions. In its first 12 years of operation, IceCube detected hundreds of thousands of neutrinos—mostly coming from the Sun.
IceCube can’t see the neutrinos themselves. Instead, its sensors notice the mess a neutrino makes when it bashes into a molecule of ice. “The [neutrino] sort of splatters a nucleus of an atom and you get a whole bunch of particles coming out in a big shower—that’s what we can see,” explains Scholberg.
Normally, nothing can move faster than light. But light itself slows down as it moves through ice and some other materials. The particles that burst out after a neutrino collision move more quickly than light does inside ice. This causes something called Cherenkov radiation. “It’s kind of like a shock wave of light,” says Scholberg. And seeing it in the IceCube detector is a sure sign that a neutrino passed through. The pattern of the Cherenkov radiation reveals the direction a neutrino came from, its energy, and sometimes what flavor it was.
Interestingly, IceCube and other neutrino detectors don’t always look up toward the sky. They can also look through the center of the Earth, towards space on the other side. That’s because most neutrinos zip through the bulk of Earth all the way to the detector. Since most of the neutrinos a detector catches come from the sun, it can make a picture of the sun from underground, says Scholberg.
When something exciting happens out in space, like two black holes merging, or a star exploding in a supernova, a huge burst of neutrinos stream outward. When the neutrinos from one of these events pass through Earth, says Scholberg, they make the entire ice glow for a few seconds. The glow isn’t visible to human eyes, but the sensors pick it up. Neutrino observatories wait and watch for these special events.
Neutrinos can act as an early warning system for a supernova. That’s because they begin streaming outward from such an event before light does. Supernovas occur in the Milky Way about once every ten to fifty years. In 1987, supernova SN1987A triggered three separate neutrino detectors.
Scientists have plans for several exciting new neutrino observatories. To build DUNE, engineers are excavating an area the size of eight soccer fields around two kilometers below the surface of South Dakota in the US. When DUNE is complete, those caverns will contain four tanks of liquid argon, each the size of a seven-story building.
P-One isn’t under construction yet. But scientists hope to build this neutrino detector at the bottom of the Pacific Ocean. Strings of detectors anchored to the sea floor will reach upward, swaying in the water like strands of kelp and watching for neutrino collisions. China is planning a similar detector named TRIDENT that would be anchored at the bottom of the South China Sea.
The Most Ghostly of All
IceCube and all the other neutrino detectors people have built or plan to build only capture high-energy neutrinos. These neutrinos move at close to the speed of light. However, not all neutrinos are that speedy. Over time, a neutrino slows down and loses energy. “The lower the energy of the neutrino, the less likely it is to interact. And they’re already practically not interacting at all,” says Scholberg.
Physicists don’t know how to build a detector that might have a chance of catching these most-ghostly of all ghost particles. But they are working hard to figure out how this might be done. That’s because the most sluggish, low-energy neutrinos of all are especially important to our understanding of the universe. These neutrinos, the Cosmic Neutrino Background (CNB), are the ones produced during the universe’s birth in the Big Bang.
The neutrinos that observatories can capture have energy in the mega-electron-volt range or higher. Cosmic neutrinos are predicted to have a wimpy 168 micro-electron-volts of energy. That’s billions of times less energetic, and billions of times less likely to interact with anything. These neutrinos should be spread out evenly all over the universe, with around 113 cosmic neutrinos and anti-neutrinos of each flavor in every cubic centimeter, a space around the size of the tip of a finger. Detecting them directly is “the holy grail of neutrino physics,” says Scholberg. It’s a goal everyone hopes to achieve someday.
Scientists have already detected the Cosmic Microwave Background, which is light left over from the birth of the universe. However, light didn’t start shining until the universe was already around 379,000 years old.
According to the Standard Cosmological Model, the newborn Universe had an extraordinarily high temperature and density, so all particles collided frequently – even neutrinos. As the Universe’s temperature cooled, it began expanding rapidly, so density also dropped suddenly. This happened about one second after the Universe’s birth. At this point, most particles were still colliding a lot because of electromagnetism and the strong nuclear force. But neutrinos only feel the weak nuclear force, which wasn’t strong enough to keep them trapped. So they have been floating freely everywhere in the Universe ever since. This is the Cosmic Neutrino Background.
If scientists could measure the CNB, it could reveal whether the Standard Cosmological Model’s version of the universe’s birth is correct or needs some rethinking.
Mysterious Mass
One of the biggest unsolved problems in physics involves the neutrino. Most particles, like electrons and protons, interact with something called the Higgs field. This field prevents them from moving at the speed of light, slowing them down and giving them a feature we call mass. The more a particle interacts with the field, the slower it goes and the more massive it is. But neutrinos don’t seem to interact with this field at all. So they shouldn’t have any mass. But they clearly do. So where does it come from? Physicists have yet to solve this mystery. But they are narrowing down what the different neutrino masses might be.
In 2024, researchers working on the KATRIN experiment, located at the Karlsruhe Institute of Technology in Germany, announced an upper limit on the masses of neutrinos. The particles must be lighter than 0.8 eV/c2. “Hopefully a positive neutrino mass measurement is just around the corner,” said Joseph Formaggio, a physicist at MIT in Cambridge, Massachussetts who is part of KATRIN.
Neutrinos may be super-light. But since they have mass, that means they contribute to the total mass of the universe. The Standard Model of Cosmology says that the universe contains 5% ordinary matter, 27% dark matter, and 68% dark energy.
Physicists don’t know what most of dark matter might be made of. Fast-moving neutrinos, like the ones coming from the sun or a supernova, are too energetic to make up dark matter, though some like to call them a type of “hot” dark matter. Slow-moving neutrinos, like the ones in the CNB, are considered part of the dark matter, says Scholberg. Most likely, they make up around 0.5% to 1.5% of the universe’s dark matter. Knowing neutrinos’ masses more precisely would help cosmologists better understand how the universe and its galaxies formed and changed over time.
Early on in the formation of the universe, neutrinos likely kept matter from clumping up too tightly. That’s because these ghostly particles could zip through clumps without getting stuck. And sometimes their mass may have been enough to pull bits of matter away from clumps. This could have helped define the locations where galaxies, stars, and other structures began to form.
Why Does Matter Exist?
Neutrinos may also hold the answer to yet another big unsolved mystery: why is there something instead of nothing? Every kind of particle has a twin called an antiparticle. It has the same mass but opposite electrical and magnetic properties. If a particle and its antiparticle meet, they destroy each other. They disappear and become pure energy. According to the Standard Cosmological Model, the Big Bang created equal amounts of matter and antimatter. It seems that these should have all destroyed each other. If that had happened, “the universe would be void,” Stefan Schönert told Quanta magazine. “It would be very, very boring for us, who would not exist.” Schönert is a physicist at the Technical University of Munich and part of the GERDA Collaboration.
Since we’re here, though, that means matter somehow got the upper hand and became dominant. This is an asymmetry that physicists would love to explain. And they have some ideas that involve neutrinos.
One theory holds that in the very early universe, there may have been some very heavy neutrinos hanging around. Some of these neutrinos would have decayed in a cascade of reactions. And this process would eventually produce slightly more matter than anti-matter. (Some heavy neutrinos may still be hanging around, and could account for more of the mysterious dark matter.)
For a neutrino to decay in this way, it likely would have to be its own antiparticle. That means it could be observed as either a neutrino or an antineutrino. Is this actually what happens? Physicists don’t know. “The answer to that question is a huge deal,” says Scholberg.
The GERDA Collaboration is looking for an answer. They are hoping to observe an extremely rare natural process in which two neutrons in the nucleus of an atom decay at the same time. The theory is that they could become two protons and emit two electrons, but no neutrinos. This is called neutrinoless double-beta decay. It can only happen if neutrinos and antineutrinos are really the same kind of particle.
Interestingly, this could also explain the other puzzle – why neutrinos have mass. That’s because a neutrino/antineutrino particle can flip-flop its quantum state. And the rate at which this happens would give it mass.
Physicists may have caught ghost particles. But they have yet to fully understand their mysterious nature. What they learn may lead to a new understanding of our universe.