The Quantum Physics of Neutrinos

Cosmic Gall
by poet John Updike

Neutrinos, they are very small.
They have no charge and have no mass
And do not interact at all.
The earth is just a silly ball
To them, through which they simply pass,
Like dustmaids down a drafty hall
Or photons through a sheet of glass.
They snub the most exquisite gas,
Ignore the most substantial wall,
Cold-shoulder steel and sounding brass,
Insult the stallion in his stall,
And, scorning barriers of class,
Infiltrate you and me! Like tall
And painless guillotines, they fall
Down through our heads into the grass.
At night, they enter at Nepal
And pierce the lover and his lass
From underneath the bed—you call
It wonderful; I call it crass.

Poem for a Neutrino
by Liz Ozburn

Particle accelerator buried deep in the earth
A cavern tunneled in a circle
Beneath buffalo and prairies
Metal machines measuring cooling electric magnetic heat
Flinging you around and around
Whipping you around again and again
Pulling you apart
Pain stripping you from yourself
Paring you down
To a sliver so small
No one knew you could be so small
Too small for hollow
You are a ghost of yourself
Your heart beats
Still
Then suddenly they grasp you —
Torn tiny soul
And fling you in a straight line through the earth
No time to shiver
Blue green brown earth space
Dropping the unseeable you
Into a vat to accept
Your hot energy

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YOUTUBE REFERENCES - 

The Physics of Neutrinos

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Wikipedia - Neutrino

Click here

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https://neutrinos.fnal.gov/types/flavor/

What’s a neutrino?

Credit: Symmetry Magazine / Sandbox Studio, Chicago

A neutrino is a particle! It’s one of the so-called fundamental particles, which means it isn’t made of any smaller pieces, at least that we know of. Neutrinos are members of the same group as the most famous fundamental particle, the electron (which is powering the device you’re reading this on right now). But while electrons have a negative charge, neutrinos have no charge at all.

Neutrinos are also incredibly small and light. They have some mass, but not much. They are the lightest of all the subatomic particles that have mass. They’re also extremely common—in fact, they’re the most abundant massive particle in the universe. Neutrinos come from all kinds of different sources and are often the product of heavy particles turning into lighter ones, a process called “decay.”

These little particles have an interesting history. First predicted in 1930, they weren’t discovered in experiments until 1956, and scientists thought they were massless until even later. While we keep learning more about neutrinos, with new answers come new mysteries.

Neutrinos are also tricky to study. The only ways they interact is through gravity and the weak force, which is, well, weak. This weak force is important only at very short distances, which means tiny neutrinos can skirt through the atoms of massive objects without interacting. Most neutrinos will pass through Earth without interacting at all. To increase the odds of seeing them, scientists build huge detectors and create intense sources of neutrinos.

Physicist Enrico Fermi popularized the name “neutrino”, which is Italian for “little neutral one.” Neutrinos are denoted by the Greek symbol ν, or nu (pronounced “new”). But not all neutrinos are the same. They come in different types and can be thought of in terms of flavors, masses, and energies. Some are antimatter versions. There may even be some yet undiscovered kinds of neutrinos!

Ten quick facts about neutrinos

1. Trillions of the harmless particle stream through you every second, night or day.
2. They are the second most abundant particle in the universe (after particles of light called photons).
3. Neutrinos rarely interact with anything—a lightyear of lead would stop only about half of the neutrinos coming from the sun.
4. About 15 billion neutrinos from the Big Bang are in the average room.
5. Neutrinos interact only through two of the four known forces: the weak force and gravity.
6. So far, scientists have discovered three flavors of neutrinos: electron (νe), muon (νμ), and tau (ντ).
7. They oscillate, or change flavor, as they travel.
8. Their masses are very tiny, but not yet known.
9. Their speed is very close to the speed of light, but also not known exactly.
10. They could be the reason that matter exists in the universe.

Neutrino flavors

Perhaps the most important thing to know about neutrinos is that they come in three types, or flavors:

• electron neutrino (νe)

• muon neutrino (νμ)

• tau neutrino (ντ)

Each flavor of neutrino is considered a fundamental particle, or one of the basic building blocks of our universe that can’t be broken down into any smaller pieces. They are associated with three similarly named fundamental particles, the electron, muon, and tau. When a neutrino (finally!) interacts, its partner particle often shows up. That helps scientists identify what flavor neutrino the particle was before it interacted. Scientists never actually see the neutrino itself; instead, they see the other particles that are made when a neutrino interacts in a detector.

Neutrinos are strange particles, and scientists were quite surprised to find that the flavor of a neutrino changes as it travels. Imagine purchasing a carton of chocolate ice cream at the store, driving home, and opening it only to find it was vanilla! So you put a scoop of vanilla in your bowl and walk into the other room to eat it, where you are surprised to find it is now strawberry. That’s what happens with neutrinos. A particle might start out as an electron neutrino, but as it moves, it morphs into a muon neutrino or a tau neutrino, changing flavors as it goes. Looking at how neutrinos change as they travel gives scientists valuable information about the ghostly particles.

FemiLab

Neutrinos were originally theorized in 1930 by Wolfgang Pauli as a way to balance out the math (and the energy) in a reaction called beta decay, something that happens in the nucleus of an atom. Scientists witnessed a radioactive decay that emitted a proton (positively charged particle) and a so-called “beta particle” (an electron). But because of the dictates of various laws—the conservation of momentum, conservation of energy, and conservation of angular momentum, or spin—there had be an invisible particle that played a role. Originally called a “neutron,” it was later renamed the neutrino, a little neutral particle that carried away some of the energy, momentum, and spin.

Neutrinos were experimentally discovered in a 1956 reactor experiment by Frederick Reines and Clyde Cowan. Antineutrinos from a nuclear reactor interacted with protons and, through a process called “inverse beta decay,” produced a neutron and a type of antimatter called a positron (a positively charged version of an electron). This antimatter quickly annihilated with regular matter, producing gamma rays. Although Reines and Cowan didn’t know that neutrinos could have antiparticles, or that they had flavors, the electron antineutrino had been discovered—and associated with its eponymous particle, the electron.

The Project Poltergeist team led by Reines (holding sign) and Cowan (far right)
was the first to experimentally detect the neutrino. | Credit: Los Alamos Nat'l Lab.

A few years later, a team of scientists at Brookhaven Laboratory used a beam of protons to create a shower of particles that also undergo a sort of “beta decay,” except they decay mostly into muons (heavy cousins of electrons) and neutrinos. The neutrinos that were produced in the accelerator created muons when they interacted, as contrasted with neutrinos produced in reactors, which made antielectrons. The neutrinos were clearly related to their charged partners. They had discovered muon neutrinos.

Ray Davis stands in the Davis Experiment at the Homestake Gold Mine in

South Dakota | Credit: Anna Davis/Sanford Underground Research Facility

Confusion arrived in 1968 with Ray Davis’s experiment in the Homestake Gold Mine in South Dakota. The project was designed to capture neutrinos coming from the sun, but about a third of the expected solar neutrinos ever arrived. Physicists called this “the solar neutrino problem.” It wasn’t until results from the 1998 Super-Kamiokande experiment in Japan (which looked at atmospheric neutrinos) and the 2001 Sudbury Neutrino Observatory in Canada (which looked at solar neutrinos) that physicists knew neutrinos were changing between flavors as they traveled. The solar neutrino problem was solved once scientists realized neutrinos oscillated into different flavors that the Davis experiment couldn’t detect.

In the 1970s, scientists at the Stanford Linear Accelerator Center discovered the tau particle, an even heavier charged particle similar to the electron. The tau neutrino was theorized but not experimentally discovered until 2000 by the DONUT collaboration at Fermilab.

Byron Lundberg and Regina Rameika stand in front
of the E872 (DONUT) detector. | Credit: Fermilab

Scientists had finally discovered the three neutrino flavors and realized that neutrinos had the unexpected ability to change their flavors as they traveled. From their very conception, neutrinos were assumed by scientists to be massless. However, for neutrinos to change flavor, neutrinos had to possess mass. To this day, the appearance of non-zero neutrino mass is one of the greatest examples of physics beyond the Standard Model and one of the few places that the model fails. Scientists are very interested in solving neutrino mysteries about mass, including how much the little particles weigh and how the three masses relate to one another.

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When Dmitri Mendeleev was trying to make sense of elements in 1869, he attempted to order them by how much they weighed. When arranged into the periodic table, it became clear that some elements, even though they had very different masses, reacted chemically in a similar way. Mendeleev and others were then able to understand the underlying structure: the atoms of different elements were actually made up of the same underlying components that came in different configurations. We now know those smaller pieces are protons, neutrons, and electrons.

Around 100 years later, Murray Gell-Mann and Yuval Ne’eman did the same thing, arranging particles that had been discovered in cosmic rays and in accelerators by their masses. Scientists saw again that some particles, although they had very different masses, could react in similar ways. The search for the underlying components of these particles (protons, neutrons, and their heavier counterparts) led Gell-Mann and George Zweig to propose quarks, which we now know as fundamental building blocks of matter.

The Standard Model of physics lays out the building blocks of matter:
quarks, leptons, force carriers, and the Higgs boson. Credit: Fermilab

The physics version of the periodic table is now known as the Standard Model, but scientists don’t know if there are smaller underlying particles. What they do know is that there seem to be three different generations of quarks and three different generations of charged leptons, the group that contains electron-like particles and neutrinos. It could just be a coincidence that both quarks and leptons have three generations, but the weak interactions of quarks look a lot like the weak interactions of leptons: just as a heavy quark can decay into a lighter quark, a heavy lepton can decay into a lighter lepton.

Electrons were discovered in 1897, and their heavier cousin, the muon, was discovered in cosmic rays in 1936. The heaviest version, the tau, was not discovered until 1975.

Quarks come in different flavors, and so do the leptons. But the neutrinos don’t get their flavor from how heavy they are. Instead, their flavor is determined by how heavy their charged lepton partner is when the neutrino is created (or from how heavy the charged lepton is that gets produced when the neutrino interacts).

Scientists think that quarks and leptons might be related because of their connection through the weak force. For example, a particle made up only of the lightest quarks (say, an up and an antidown quark) can decay into a pair of leptons: a muon and a muon-flavored neutrino. On the other hand, a particle made up of one or more heavier quarks can decay into lighter quarks or into leptons (or into both).

The study of these transitions between quarks or between neutrinos is called flavor physics. However, calling them both by the same name doesn’t do justice to how different the probabilities are. While there is a huge amount of mixing between neutrino flavors, a quark in one family will mostly change into a quark that’s in the same family if it’s lighter. Only a few percent of the time will it change to a quark that is one generation away, and only one out of a thousand will change to a quark that is two generations away.

Credit: Symmetry Magazine / Sandbox Studio, Chicago

Antineutrinos

Antimatter sounds like something cooked up for a science fiction story, but it is as real as you are. Matter is built up of protons, electrons, and neutrons, each of which has a mass and a charge (either positive, negative, or neutral). Antimatter particles look almost like their matter twins: They have the same masses, but they have opposite charges.

Credit: Symmetry Magazine/Sandbox Studio, Chicago

For example, the electron has a negative electric charge, and the positron (an antielectron) has a positive charge. An antiproton is a negatively charged proton. Antimatter particles such as antiprotons and positrons can get together to form antiatoms the same way protons and electrons form atoms. However, most of what we see in the universe is made of matter rather than antimatter. Scientists aren’t sure where all of the antimatter is, but hope experiments like the Deep Underground Neutrino Experiment will shed light on this issue in the near future. It is also possible to create antiatoms in a laboratory and study them, though this is very difficult to do. When matter and antimatter meet, they annihilate in a fiery burst of light.

An antineutrino is thus simply an “opposite version” of a neutrino. But if one of the main ways matter and antimatter are opposites is charge, then what does it mean that neutrinos are neutral? Does that mean neutrinos and antineutrinos are the same thing, only differing in the particles (positrons or electrons) produced along with them? Scientists aren’t sure. There are many experiments under way or proposed to discover whether that’s the case.

For now, scientists think of the three neutrinos (electron, muon, and tau neutrinos) and the three antineutrinos (electron, muon, and tau antineutrinos) as distinct particles.

Chasing Neutrons

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Scientists are interested in antineutrinos for both practical and theoretical reasons. On the practical side, antineutrinos are produced in prodigious amounts in nuclear reactors, and these antineutrinos can be used to precisely monitor the reactor core. On the other hand, scientists want to study antineutrino oscillations and find out if neutrinos and their antimatter siblings behave in unexpectedly different ways.

An antineutrino is the antiparticle partner of the neutrino, meaning that the antineutrino has the same mass but opposite “charge” of the neutrino. Although neutrinos are electromagnetically neutral (they have no electric charge and no magnetic moment), they may carry another kind of charge: lepton number. These are defining features that can distinguish a particle from an antiparticle (along with properties such as helicity).

Family lepton numbers are assigned to the three families of leptons, which are easily remembered by their flavors. The electron and electron neutrino (and their antiparticles) are one set, the muon and muon neutrino are another, and the tau and tau neutrino make up the third. In the electron flavor, lepton number is described in terms of electron number; electrons and electron neutrinos get a value of 1, positrons and electron antineutrinos get a value of -1, and all the other leptons (associated with muons or taus) have a value of 0, because they have no electron flavor. The same happens in the muon flavor with a muon number: muons and muon neutrinos have the number 1, their antiparticles are -1, and everything else has a muon number of 0. Apply the same pattern for tau and the tau neutrino!

This example decay shows a muon transforming into a muon neutrino, an electron, and an electron antineutrino. Lepton number is conserved. Credit: The Particle Adventure/ Lawrence Berkeley National Laboratory

Scientists consider the total lepton number to be conserved if the summed family lepton flavor numbers before a reaction remain unchanged after a reaction. It’s a method of balancing the equations describing the reactions, and it’s a good predictor of whether scientists should expect a certain process to occur. So far, scientists have not observed violation of total lepton number conservation: they always see the appropriate numbers and types of neutrinos and antineutrinos being produced via the weak interaction. However, if neutrinos and antineutrinos are actually the same particle, the lepton number would not be conserved. Neutrinos may not have revealed the full story yet.

The fact that neutrinos oscillate from one flavor to another implies that family lepton flavor is not conserved. And if, for example, neutrinos and antineutrinos oscillate from one flavor to another at different rates, this would imply a violation of the so called charge-parity (CP) symmetry. That would be particularly exciting, because CP symmetry violation is a necessary requirement for going from a “neutral” universe (equal parts of matter and antimatter) to the matter-dominated universe we live in. This remains one of the greatest puzzles that particle physicists are trying to unravel.

Sterile neutrinos

Sterile neutrinos are a special kind of neutrino that has been proposed to explain some unexpected experimental results, but they have not been definitively discovered. Scientists are looking hard for them in many different experiments.

While the standard electron, muon, and tau neutrinos (and antineutrinos) interact with matter through two forces (the weak force and gravity), scientists think sterile neutrinos might interact only through gravity. This would make them even harder to spot than the tricky “regular” neutrinos. Gravity is the weakest of all the forces, and neutrinos are very light—so they don’t give gravity much to work with. Finding slight signals amidst the chaos of the universe will be tough, but not impossible.

While they know of the three flavors of neutrinos, scientists aren’t sure how many kinds of sterile neutrinos there might be. Is there just one to add in, or perhaps a parallel three? Or maybe there are even more!

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Particles (including the wacky neutrino) have properties called spin and helicity. The particles don’t literally spin like a top, but this is still a good way of thinking about it. Helicity refers to how the spin relates to the movement of the particle, and it’s analogous to the idea of someone being left handed or right handed.

Hold out your hands and make two fists. The way your fingers curl represents a particle’s spin, and your thumb points in the direction of travel. These are right-handed and left-handed particles, and they’re important because one of nature’s four forces—the weak force—does not treat them equally. The weak force strongly prefers to interact with left-handed particles.

So far, scientists have found only left-handed neutrinos. But if there are right-handed neutrinos, they could be the predicted sterile neutrinos. Because the weak force would ignore them, sterile (right-handed) neutrinos would interact only through gravity, making them borderline invisible.

One way to discover these secretive particles involves oscillation. Some experiments have seen an excess neutrino oscillation where theory predicted they shouldn’t be. And some experiments have seen neutrinos appearing or disappearing over much shorter distances than the experiments on neutrinos from more distant locations, such as the atmosphere or sun. If neutrinos oscillate into this fourth kind of neutrino, that could explain the rapid changes and the anomalies seen in experiments. Much more data is needed before anything can be decided definitively.

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Neutrinos interact through two of the four Standard Model forces: the weak force and gravity. It is this lack of interactions (and their tiny mass) that gives them their ghostly nature. For example, 50 percent of the neutrinos coming from the sun will pass through a light-year of lead without interacting.

But imagine a neutrino, already nearly massless, that does not interact through the weak force. How would scientists even know they were there? This is exactly the challenge with searching for “sterile” neutrinos.

Technicians make delicate adjustments to one of LEP’s thousands of magnets in 1999. Credit: CERN

At the Large Electron-Positron (LEP) collider at CERN, scientists measured the particles that emerged from collisions between electrons and positrons. One particle, the Z boson, is the carrier of the weak force, and how quickly it decays depends strongly on the number of particles that it couples to. By measuring the decays of the Z boson, scientists were able to measure to a very high precision that only three neutrinos couple to the weak force: the electron, muon and tau neutrinos. But there could be any number of extra “sterile” neutrinos that LEP would be unable to see—though scientists would still need to figure out why.

The hints of sterile neutrinos come from a couple of experiments. The Liquid Scintillator Neutrino Detector (LSND) experiment at Los Alamos National Laboratory studied a decay-at-rest beam made of mainly muon neutrinos and found more electron neutrinos than they predicted. This was a similar signature of oscillation that had been seen for the known neutrino flavors, but at a distance and energy combination researchers weren’t expecting. A similar signal at a new location is a hint that an unknown kind of neutrino was hiding behind the scenes.

There is a lot of ongoing work to confirm if this interpretation of the LSND results is correct. So far the results have been inconclusive from these experiments. The MiniBooNE experiment at Fermilab saw hints that could also be interpreted as extra electron neutrinos appearing due to the existence of the sterile neutrinos, but the MINOS experiment, Daya Bay Reactor Neutrino Experiment, and other projects looking for neutrinos disappearing due to sterile neutrinos did not see that signal. The Short Baseline Neutrino program at Fermilab will use three liquid-argon detectors that are currently running or under construction. This suite of projects aims to definitively answer this question by making very high-precision measurements of a muon neutrino beam produced at Fermilab.

The MicroBooNE detector is one of the three short-baseline neutrino detectors at Fermilab. It is hunting sterile neutrinos and testing the liquid-argon technology that will be used for the enormous Deep Underground Neutrino Experiment. Credit: MicroBooNE/Fermilab

While the short-baseline experiments look for light sterile neutrinos carrying a relatively small amount of energy—at the electronvolt scale—there could be different sterile neutrinos at different energies. Sterile neutrinos around 1,000 electronvolts could be related to dark matter or other cosmological issues. And neutrinos around 1013 gigaelectronvolts could be the heavy seesaw neutrinos that help explain the small neutrino masses we see in the known light neutrinos. At just above this scale, 1015 gigaelectronvolts, physicists also start talking about grand unified theories and how different forces relate to one another.

So sterile neutrinos could tie into many different elements of physics. All neutrinos that scientists have seen so far are left-handed, and all antineutrinos are right-handed, but if there were right-handed neutrinos, they could act just like sterile neutrinos. Other particles have both left- and right-handed versions, and right-handed neutrinos are a popular way of adding neutrino masses into the Standard Model.

Neutrino masses

Physicists typically refer to neutrinos by their flavors: electron neutrino, muon neutrino, and tau neutrino. This makes good sense—when neutrinos interact in detectors, they typically produce their signature charged particle, making the neutrino flavor immediately obvious. Electron neutrinos interact to make electrons, muon neutrinos make muons, and tau neutrinos make taus.

But there is another way to think about neutrinos—their mass, or “mass state.” Physicists have named these neutrinos mass 1, mass 2, and mass 3, though they can also be referred to as ν1, ν2, and ν3. One might think that each of the three masses has a different flavor, but the truth makes matters more confusing: the mass state of a neutrino does not precisely match up with the flavor state of a neutrino.

A flavor of a neutrino (such as the electron neutrino) is made of a combination of masses (1, 2, and 3), and a neutrino of a certain mass (such as the lightest neutrino) has a certain probability of interacting in a detector to make a certain flavored charged particle (electron, muon, or tau).

Credit: Symmetry Magazine/Sandbox Studio, Chicago

There are many open questions about the neutrino masses, but scientists do know a few things. They know that the masses of the three neutrinos are small. And they know a bit about how the flavor mixture for each mass neutrino breaks down. Mass 1 leans heavily toward electron flavor; mass 2 is more of an even blend of electron, muon and tau; and mass 3 is mostly muon and tau. And the masses of ν1 and ν2 are close to one another, while ν3 weighs either much more or much less than the other two.

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Mass neutrinos and flavor neutrinos coexist in an interesting way. When a neutrino—let’s say an electron neutrino—is born, it is a quantum mechanical combination (or superposition) of all three mass states. Each particle is also a wave, and each has a slightly different mass, meaning each travels at a slightly different speed. As the electron neutrino travels, the different mass states get out of phase with each other; the lightest state has a slightly larger velocity than the heavier states. This causes the original mixture of mass 1, mass 2, and mass 3—which made up the electron neutrino—to change along the neutrino’s path. When the neutrino interacts again after a certain distance, it will have some probability of interacting as an electron neutrino, but it will also have a probability of interacting as either a muon or tau neutrino. Depending on how out of phase the mass states have become, the probability to interact as a muon or tau neutrino could be much greater than the probability to interact as an electron neutrino.

Here’s another way to imagine it: an epic food fight has broken out inside of an ice cream parlor. Your nearby friend slings a scoop in your direction, and it hits you right in your open mouth. You can easily identify the flavor as chocolate (and not vanilla or strawberry) the same way a detector can taste the incoming neutrino flavor (as an electron and not muon or tau neutrino). But something weird happened with that ice cream. It started in your friend’s hand as chocolate ice cream, and arrived in your mouth as chocolate ice cream, but on the journey in between, the ice cream turned into gold, silver, and copper coins.

These coins have different masses—just like the mass neutrinos, mass 1, mass 2, and mass 3. And because they weigh different amounts, they travel at slightly different speeds. Some of the gold coins start to fall behind a little bit, while the lighter copper ones fly a little further. You were relatively close to your friend when she threw the ice cream, so the coins didn’t separate all that much. Their ratio was basically unchanged when they reached you, and they transformed back into tasty chocolate ice cream once they reached your mouth.

Out of the corner of your eye, you see your friend throw chocolate ice cream on a long journey toward someone far across the store. The ice cream leaves her hand and POP: turns into a blob of coins. Over this long distance, the weight really starts to matter. The silver and copper coins arrive close together, and the gold coins a second later. PLOP. The ice cream lands in the target’s mouth, and he declares it tastes like strawberry. The ratio of coins making up the ice cream is different than when it started—and it turns out this affects the flavor.

If you had a pure mass state neutrino, it would consist of only gold coins, or only silver coins, or only copper coins. Because of the way things get really weird at small scales, these pure mass states would still have a probability of interacting as a certain flavor. That is, even if you had all gold coins, you still wouldn’t know for sure if the ice cream would taste like chocolate, strawberry, or vanilla when you detected it. But there would always be the same probability of tasting a certain flavor given a type of coin. In the same way, each of the pure mass state neutrinos has a different ratio of flavors it is likely to produce.

Neutrino physics is messy and delicious.

As if it weren’t confusing enough to have neutrinos of different flavors, different masses, and different matter (antimatter and regular matter), neutrinos also come in a wide variety of energies.

The energy of a neutrino depends on the process that formed it. Because neutrinos have no charge, there’s no way to use electric fields to accelerate them and give them more energy, the way scientists can do with particles such as protons. More energetic reactions will create more energetic neutrinos. These are great for scientists, because particles with more energy are more likely to interact and leave traces. They’re more likely to be stopped by regular matter and transfer that energy to something else (other particles) that detectors can pick up.

This track from the NOvA neutrino detector at Fermilab shows particles produced by a neutrino interaction. Credit: NOvA collaboration

Low-energy neutrinos, such as those left over from the Big Bang, are very difficult to find because not only are they weakly interacting (like all neutrinos), but they also don’t have much energy to pass on to other particles we can see. Even if they do, that signal is likely to be weak and hard to pick out from all the other interactions shouting over it.

Neutrino energy is typically measured in electronvolts. But there is a big range of neutrino energies. Some have one-millionth of an electronvolt, and some have a quintillion electronvolts (that’s a 1 followed by 18 zeros). That means plenty of neutrinos to explore, and interesting information about the processes that formed those neutrinos.

Neutrinos come in a wide variety of energies. Some of the lowest-energy ones come from the Big Bang, while the most energetic seen thus far have come from extragalactic sources. The neutrino cross section (on the y axis) is a measure of how likely the neutrino is to be stopped by regular matter. The higher energy a neutrino has, the more likely it is to interact. Credit: J.A. Formaggio and G.P. Zeller

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Because neutrinos come in a very broad range of energies, an even broader range of techniques have to be used to see them.

The lowest-energy neutrinos come from just a few seconds after the Big Bang, and it is expected that these neutrinos have only a fraction of an electronvolt of energy. This is less energy than it takes to even knock an electron out of a hydrogen atom, making them incredibly hard to detect, because you need a detector with an even lower threshold. It turns out that materials at room temperature are vibrating with thermal energies that are much higher than these Big Bang neutrinos, so one way to see these lowest-energy neutrinos is to use stiller, colder materials (at cryogenic temperatures) and look for nuclei that receive a small amount of energy seemingly out of the blue. Another proposed method to see these neutrinos is using these low-energy neutrinos to stimulate a beta decay, then searching for an outgoing electron that has just a little more energy than one would expect. The trick, then, is building a detector that can measure tiny differences in electron energies.

This plot shows the sun in neutrinos. The bright yellow at the center means a high concentration of neutrinos from that direction. Credit: Super-Kamiokande Collaboration/Kamioka Observatory, ICRR, Univ. of Tokyo

Neutrinos from the sun come in energies from tens to millions of electronvolts, a result of the many different fusion processes that take place there simultaneously. Scientists now know that most neutrinos from the sun are in the tens to hundreds of electronvolts. Researchers were able to see these only by building an extremely large scintillator detector and making sure there were no radioactive contaminants anywhere nearby.

The first neutrinos from the sun that scientists were able to see were the ones energetic enough to change a chlorine atom into an excited argon atom (by changing a neutron inside a nucleus to a proton). The was Ray Davis’s experiment at the Homestake Gold Mine. By measuring the radioactive decay of that excited argon atom, scientists made the first measurements of neutrinos from the sun.

Once a neutrino is energetic enough to knock an electron out of its orbital, then detectors that are sensitive to electric charges can pick the little particles up. The striking thing about this reaction is that the electron is knocked out of its orbital at exactly the same angle as the incoming neutrino hit with. If a detector can measure that outgoing electron angle and take into account the detector’s relationship to the sun, then you can actually “see” the sun with neutrinos. The detector, whether it’s on Earth’s surface or underground, will see the sun all the time, day or night.

Neutrinos from nuclear reactors have a million times more energy than Big Bang neutrinos, so they can be seen by measuring their interactions with atoms. One key difference is that the neutrinos from reactors are actually antineutrinos, so instead of changing neutrons to protons, they change protons to neutrons—and the neutrons are much harder to detect. The neutrons can be captured by certain particles that then decay and produce photons, particles of light, which can signal that a neutrino was there.

The Deep Underground Neutrino Experiment hosted by Fermilab will use an intense beam of neutrinos with billions of electronvolts of energy. Credit: DUNE/Fermilab

As neutrinos go from a million electronvolts to a billion electronvolts, they can start to transfer more energy to the particles in a detector. At a billion electronvolts, that same process of a neutrino colliding with a nucleus can produce an electron that travels through dozens of centimeters of plastic or a muon that travels through meters of steel. At 10 billion electronvolts, the neutrinos have enough energy to completely break up a nucleus.

Finally, if you need a meter of steel to see a 1-GeV muon, then you need a kilometer of steel to see a 1-TeV muon. The detectors that have seen the highest-energy neutrinos are those that are made with a cubic kilometer of detector material. The question is, how on Earth can you afford a cubic kilometer of detector? You have to use some material that’s already available in large quantities and figure out how to pull a signal out of it. People have made detectors out of both ocean water and the ice in Antarctica to see these highest-energy neutrinos.

The IceCube experiment uses a cubic kilometer of ice in Antarctica as its detector medium. More than 5,000 sensors in the ice look for neutrinos from outer space. Credit: IceCube Collaboration/University of Wisconsin-Madison