Neutrinos: The inscrutable “ghost particles” driving scientists crazy

Somehow, neutrinos went from just another random particle to becoming tiny monsters that require multi-billion-dollar facilities to understand. And there’s just enough mystery surrounding them that we feel compelled to build those facilities since neutrinos might just tear apart the entire particle physics community at the seams.

It started out innocently enough. Nobody asked for or predicted the existence of neutrinos, but there they were in our early particle experiments. Occasionally, heavy atomic nuclei spontaneously—and for no good reason—transform themselves, with either a neutron converting into a proton or vice-versa. As a result of this process, known as beta decay, the nucleus also emits an electron or its antimatter partner, the positron.

There was just one small problem: Nothing added up. The electrons never came out of the nucleus with the same energy; it was a little different every time. Some physicists argued that our conceptions of the conservation of energy only held on average, but that didn’t feel so good to say out loud, so others argued that perhaps there was another, hidden particle participating in the transformations. Something, they argued, had to sap energy away from the electron in a random way to explain this.

Eventually, that little particle got a name, the neutrino, an Italian-ish word meaning “little neutral one.” Whatever the neutrino was, it didn’t carry any electric charge and only participated in the weak nuclear force, so we only saw neutrinos at work in radioactive decay processes. But even with the multitude of decays with energies great and small happening all across the Universe every single second, the elusive nature of neutrinos meant we could only occasionally, rarely, weakly see them.

But see them we did (although it took 25 years), and for a while, we could just pretend that nothing was wrong. The neutrino was just another particle the Universe didn’t strictly need to give us but somehow stubbornly insisted on giving us anyway.

And then we discovered there wasn’t just one neutrino but three of them. For reasons the cosmos has yet to divulge to us, it likes to organize its particles into groups of three, known as generations. Take a nice, stable, regular fundamental particle, like an electron or an up or down quark—those particles represent the first generation. The other two generations share the same properties (like spin and electric charge) but have a heavier mass.

For the electron, we have its generational sibling, the muon, which is just like the electron but 200 times heavier, and the tau, which is also just like the electron but 3,500 times heavier (that’s heavier than a proton). For the down quark, we have its siblings, the “strange” and “bottom” quarks. And we call the heavier versions of the up quark the “charm” and “top” quarks. Why does the Universe do this? Why three generations with these masses? As I said, the cosmos has chosen not to reveal that to us (yet).

So there are three generations of neutrinos, named for the kinds of interactions they participate in. Some nuclear reactions involve only the first generation of particles (which are the most common by far), the up and down quarks, and the electrons. Here, electron-neutrinos are involved. When muons play around, muon-neutrinos come out, too. And no points will be awarded for guessing the name of the neutrinos associated with tau particle interactions.

All this is… fine. Aside from the burning mystery of the existence of particle generations in the first place, it would be a bit greedy for one neutrino to participate in all possible reactions. So it has to share the job with two other generations. It seemed odd, but it all worked.

And then we discovered that neutrinos had mass, and the whole thing blew up.

Neapolitan physics

The first hints of trouble came with observations of neutrinos from the Sun. Our Sun is a giant nuclear reactor, and its core spews out a truly incredible number of neutrinos. For a rough estimate, hold your thumb up toward the Sun (please don’t look directly at the Sun during this experiment). The Sun is pumping out enough neutrinos that approximately 6 billion of them pass through the area of your thumbnail every single second.

No wonder they’ve earned the nickname “the ghost particle.”

However, our early experiments did not find enough neutrinos coming from the Sun. Our detectors only picked up half the number we predicted based on our extensive and well-refined knowledge of nuclear physics. Perhaps we were misunderstanding something fundamental about nuclear physics, but that didn’t feel so good to say aloud, either. So maybe we weren’t understanding something about neutrinos.

For decades, we had assumed that neutrinos have no mass. After all, no experiment had revealed a mass, and there was no good theoretical reason for them to have one. So massless it was.

Until it wasn’t.

The reason behind the detection deficit is that neutrinos do something so incredibly, powerfully stupid that it almost defies explanation. It turns out that they can change their generation as they travel. For reasons that are only known to retired particle physicists, instead of calling the different kinds of neutrinos “generations,” we call them “flavors.” So, in that parlance, we can say that neutrinos change flavors.

The Sun primarily produces electron-flavored neutrinos, as the dominant nuclear reactions in the Sun’s core only involve the first generation of particles. And our early experiments on the Earth were tuned to find those same electron-neutrinos.

But as those neutrinos traversed 93 million miles of interplanetary nothingness, some of them changed into muon-neutrinos or tau-neutrinos. Since our detectors could only find electron-neutrinos, the transformed neutrinos sailed on through, explaining the discrepancy between predicted and observed values. (I should note that this flavor-changing nature of neutrinos has been verified by a multitude of follow-up experiments, so there’s no getting around it.)

So what does all this have to do with neutrino mass? It’s a bit hard to describe because literally no other common particle changes flavors like this, so it's a trick of nature we’ve only recently discovered. The problem is that what we call an electron-neutrino, muon-neutrino, or tau-neutrino isn’t, strictly, a neutrino. At least, not completely.

If I hand you an electron, you have… an electron. It has a specific mass. A specific spin. A specific charge. A specific generation (or “flavor” if you will). It just is. It’s an electron. The different properties of an electron that we can measure, like its mass, spin, and charge, always and forever line up with exactly what we call an electron. It’s so uncomplicated that you may be wondering why I’m belaboring the point.

Well, the properties of neutrinos don’t line up like this. They’re weird. When we see an electron-neutrino in an experiment, we’re not seeing a single particle with a single set of properties. Instead we’re seeing a composite particle—a trio of particles that exist in a quantum superposition with each other that all work together to give the appearance of an electron-neutrino.

Experiments have forced us to accept that there are three neutrinos we don’t get to see as individuals. Each of those neutrinos (labeled as m1, m2, and m3) has its own mass, and certain combinations of those three masses of neutrinos give us the appearance of an electron-neutrino, or a muon-neutrino, or a tau neutrino. In the language of quantum mechanics, we say that there are three mass eigenstates of the neutrino that mix together to form the three-flavor eigenstates we observe.

Imagine a brand of ice cream that assigns flavors to different sizes of tubs you can buy at the store. Small tubs are always chocolate, medium tubs are always strawberry, and large tubs are always vanilla. The mass eigenstate (the size of the tub) aligns with the flavor eigenstate (the, uh, flavor). These are the electrons, muons, and tau particles. Easy. Simple. Sane.

But neutrinos are not simple. Every tub is really filled with Neapolitan ice cream, a mix of all kinds. And what’s more, the proportion of ingredients changes as the tub travels from the factory to the store. When you open it up, you take a big scoop and shove it in your mouth. You get a taste of all the varieties, but you end up saying “this is generally strawberry” or “this is vaguely vanilla-flavored,” depending on the ratio.

I recognize that this isn’t the best analogy, but when it comes to neutrinos, we have to play the hand we’re dealt.

This is how we know that neutrinos have mass. The different masses of neutrinos travel at different speeds, with some racing ahead while others lag behind. This forces the traveling neutrino to constantly oscillate among the flavors, depending on what the mix is at the moment.

The problem with terrible neutrinos

Fine, you win, Universe. Neutrinos have mass. But we don’t know what the masses are. After decades of experiments, billions of dollars spent, and Herculean effort poured in from countless dedicated scientists and engineers, here is a summary of the current state of the art in neutrino mass knowledge:

• The sum of all three neutrino masses cannot be more than around 0.1 eV/c2
• The absolute value of the square of the difference between m2 and m1 is 0.000074 eV/c2
• The absolute value of the square of the difference between m2 and m3 is 0.00251 eV/c2

That’s it. We haven’t pinned down the masses of any individual neutrino, and we don’t even know which ones are heavier than the others. When it comes to our ability to collect raw data, neutrinos present a triple threat: they’re incredibly lightweight (even the electron weighs over 5 million times more than all the neutrinos combined), they shift their identity as they travel (and their rate of flavor oscillation changes as they travel through different substances, so there’s no one-size-fits-all solution), and they barely interact with anything in the first place (because the weak force is suitably named).

To narrow things down a bit, the first thing we want to know is the sum of the three neutrino masses. Ironically, despite all our time and money spent trying to study them with Earth-based detectors and laboratories, our best constraints come from deep cosmological surveys. We know how much total stuff is in the Universe, and we know how neutrinos behave, so we can figure out what proportion of the mass of the cosmos is in the form of neutrinos and turn that into an estimate of the neutrino mass itself. But if you want neutrinos to make up a decent fraction of all the stuff in the Universe, you start running into issues.

The problem affects the formation of large structures like galaxies and clusters. The issue is that neutrinos are “hot”—they travel at nearly the speed of light. If you soak the early Universe in an abundance of fast-moving neutrinos, structures have a very hard time getting their act together. Add too many neutrinos, and things like galaxies simply can’t form. So we can measure the rate of structure formation and turn that around into a total limit for all neutrino masses combined.

But why do we care so much? What’s the big deal if neutrinos have mass? The big deal is that we have no idea how neutrinos get their mass.

Normal, healthy particles like the electron and the top quark get their masses through something called the Higgs mechanism. The Higgs boson is an omnipresent quantum field that soaks all of space and time and forces all other particles to interact with it. This interaction creates a mass, with the strength of the interaction is connected to the amount of mass that a particular particle gets.

So one possibility is that neutrinos are just like everybody else, and they talk to the Higgs, too. But neutrinos have yet another weird property shared with absolutely nobody else. All the other particles come in right-handed and left-handed varieties (this is a property called chirality, and it’s determined by the direction of a particle’s spin relative to its direction of momentum). Crucially, the Higgs boson, in its machinations to give everything mass, works with left-handed and right-handed varieties of particles.

Yet every single neutrino we have ever observed throughout history has only been left-handed. As far as we can tell, there are no right-handed neutrinos. Zero. Zilch. Nada.

What the Higgs is going on?

Maybe there’s another, fourth neutrino out there that is right-handed and does the dirty work of talking to the Higgs. But if it existed, that’s all it would do; it simply wouldn’t participate in any other interaction.

Maybe neutrinos get their mass through some other trick. There’s a very real possibility that neutrinos are also their own antiparticles. (I know I haven’t introduced antimatter here, so suffice it to say that this would be absolutely bizarre.) But through some very complex and exceedingly technical processes, a particle that is its own antiparticle can acquire a mass.

Maybe.

Whatever the answer is, it’s beyond our current understanding of physics, which is simultaneously exciting and frustrating. There are hundreds of theories out there that could explain the neutrino mass, so the first and most important job is to pin those masses down and see which ideas are right and which are wrong.

Trouble in Neutrinoland

That’s where we’re currently running into trouble. Neutrino experiments don’t come cheap, and they don’t come small. The famed Super-Kamiokande detector in Japan, which detected all of 12 neutrinos from a distant supernova in 1987, uses 50,000 metric tons of ultrapure water to catch a glimpse of the occasional neutrino interaction. The IceCube experiment uses kilometer-long strands of detectors sunk into the Antarctic ice sheet at the South Pole to find its neutrinos, making it one of the most expensive experiments at the most expensive scientific facility in the world. In its 12 years of operation, it has found 100 high-energy neutrinos.

A collaboration led by Fermilab is trying to build DUNE, the Deep Underground Neutrino Experiment. Originally estimated to cost $1.7 billion, the budget has now ballooned to over $3 billion. The project has become so mismanaged that the Department of Energy put its full approval on hold and even took the radical step of declining to automatically renew the University of Chicago’s license to operate the troubled lab (bids are open now if you’re interested and think you can do a better job).

All the while, trillions of ghostly little neutrinos slip through the Earth, a constant stream created by nuclear reactions across the cosmos. These particles hold the keys to new physics, exotic processes, and a deeper understanding of the fundamental workings of nature. But they absolutely refuse to divulge their secrets easily, bedeviling our best attempts to understand them for almost a century.

Honestly, it would be easier if they just never existed.