Neutrinos: The Supreme Loners of Physics

There’s always so little said about the magnificence of and mystery behind neutrinos. In this article I will attempt to do justice to these particles (and waves, I guess).

Neutrinos are often called ‘Nature’s Ghosts’, and that’s one of the few things in physics that’s not a misnomer. They interact less than any of the other known particles, only interacting via the weak force and gravity. Even though around 650 trillion (thats 650 followed by 12 zeroes!) of them pass through you every second, it took decades to prove that they exist at all.

So if Neutrinos are so hard to detect, how did we discover them? Well I’m glad you asked. In the 1930s, scientists were bothered about a particular type of decay called Beta Decay, which seemed to violate the law of conservation of mass. Beta Decay is a process in which an atomic nucleus emits an electron or a positron which converts either a proton into a neutron or a neutron into a proton, which changes the atom from one element to another (micro alchemy!). The problem was that when the mass of the original nucleus was compared to that of the sum of the daughter nucleus and the emitted particle (either electron or positron), the masses did not equate. After the decay, some mass was missing. Lots of ideas were put forth to explain this, including abandoning the Law of Conservation of Mass in the quantum world. Wolfgang Pauli came up with the idea that, the mass was missing because an unobserved particle was being emitted and carrying the missing mass away with it.

There are four known fundamental forces: the strong force, electromagnetism, weak force and gravity. Neutrinos having no colour or charge and thus interact only through the weak interaction and gravity. The mass of an individual neutrino is so less that for the longest time physicists did not know even if they even had mass or not, and even today we haven’t been able to assign a mass to neutrinos, only a range. Let me try to get across how hard it is to detect a single neutrino.

Let’s say you were allergic to neutrinos and you wanted to get away from as many of them as you could. Neutrinos are produced in nuclear reactions, and the biggest nuclear reactor around is the Sun. About 650 trillions neutrinos from the Sum go through you every second. To have a good chance of blocking half of these neutrinos you would need to get behind a solid block of lead five light years in length. And of course by the time you go five light years away from the sun with your block of lead, its likely you’ll get closer to another neutrino source and all your efforts will be in vain.

So what are neutrinos anyway?

Neutrinos belong to a class of particles known as the leptons. The three charged leptons are:

The muon and tau particles have exactly the same properties as the electron, except they’re much much heavier. For each charged lepton there is a neutrino, and they have been very creatively named:

When a neutrino interacts, its partner particle often shows up. That helps physicists identify what flavour neutrino they’re dealing with. Physicists never actually see the neutrino itself; instead, they see the other particles that are made when a neutrino interacts in a detector.

The reasons why physicists are so intrigued by neutrinos are too many to cover in one tiny article; however, what this article is capable of doing is inspiring you to go and read more about it yourself.

One of the reasons why physicists are interested in neutrinos, is because of their mass. All particles get their intrinsic mass from the Higgs Mechanism (perhaps a topic for another day), but the masses of the known neutrinos are so incredibly low that it made people think that maybe neutrinos get their mass from a different mechanism altogether.

Neutrinos are so hard to observe and study that we don’t even know if they’re their own anti particles. If that was true, it could explain one of the most deeply philosophical questions of physics: Why is there something rather than nothing? Look around you, do you see any antimatter particles? Probability says you don’t. One of the predictions of The Big Bang Theory is that equal number of matter and antimatter particles were created. This is a very exciting prediction as at first sight it seems absolutely false. If neutrinos are Majorana (the technical jargon for a particle being its own antiparticle) then we may get one step closer to explaining the matter-antimatter discrepancy that we see around us.

The reason why neutrinos are so famous (and infamous) is because of a phenomenon called neutrino oscillations. Experiments have revealed that neutrinos change their flavour after propagating some distance. If you hand a particle physicist an electron neutrino, a muon neutrino and a tau neutrino and ask him which one is the electron neutrino, the answer will be a profound, eyeopening “yes”. The reason for this (as always) is quantum mechanics. It turns out that the electron neutrino is all three of the neutrinos you handed the physicist at the same time, but all three of the neutrinos are also muon and tau neutrinos also. You tell them apart by the way in which they oscillate (remember they’re waves too). Electron, muon and tau neutrinos all oscillate (vibrate) in different ways. If in the wild you find a neutrino which is doing something weird, like oscillating both like an electron and tau neutrino, then the neutrino is sort of a mixture os the two and it has some probability of being an electron neutrino or a tau neutrino. After producing, say an electron neutrino, at point A and observing it at point B you will notice that the neutrino will be oscillating with a different frequency and will now have some probability of being either an electron, muon or tau neutrino.

Now why was that so important?

Remember how I said neutrinos have such little mass that for the longest time it was thought that they were massless? It turns out that massless neutrinos cannot oscillate. So if we observe neutrino oscillations, it follows that neutrinos must have mass.

You’re probably sick and tired of me saying that neutrinos react with matter very weakly. But did you ask yourself why? As I have mentioned, they only experience the weak nuclear force and gravity. Their masses are so small that gravity can be ignored safely. In order to interact with other types of matter, particles need to emit carriers of forces: photons for electromagnetism, gluons for the strong nuclear force, the W or Z bosons for the weak nuclear force and the hypothetical graviton for gravity. Let’s take the example of a nucleus of an atom. To interact with this nucleus, a neutrino must emit a W or Z boson. In reality, it’s more of a virtual boson, existing only long enough to exchange energy between the neutrino and the nucleus. The emitted boson gets its energy from, well, nowhere. It cheats energy from the Heisenberg Uncertainty principle, which says that there is a fundamental uncertainty (WHICH HAS NOTHING TO DO WITH OUR MEASUREMENT) about certain pairs of properties: like position and momentum, and in our case energy and time. So, the lesser the time that the emitted virtual boson exists, the more energy it can steal from the uncertainty principle. The catch here is that the bosons of the weak force are massive, unlike the massless photon or gluons. In order to simply exist, the bosons emitted by the neutrinos need to have a lot of energy, and thus the bosons can only exist for a very brief period of time, meaning they can travel very small distances. In order to interact with the nucleus that we mentioned, it basically needs to be inside the nucleus. This happens a stupidly rare number of times, and therefore neutrinos are very hard to detect. This is the reason, by the way, why the weak force is called weak. The weak force is not a force in the traditional sense that you might think, it does not push or pull, instead it’s responsible for changing the flavour of particles and all types of radioactive decay. The only reason why its called weak is because its very rare (in certain interactions, it does not happen rarely at all, but is the first force to act). That’s why I think it should be called the uncommon force.

The final thing that I want to touch upon is some experimental physics. Neutrinos are one of the hardest things to detect in all of physics, figuring out ways to detect them requires creativity, ingenuity and a whole lot of money.

The weakly interacting nature of the neutrino makes them extremely difficult to study, but in order to have a chance at spotting even a single neutrino, a truly enormous number need to reach the detector. In order to do that neutrinos need to be channeled into a really concentrated beam. Because neutrinos react so weakly, they cannot be directly channeled into a beam, doing this requires some ingenious hacks, one of which is:

Accelerate protons around 3 to 4 kilometre circumference ring, using giant electromagnets to around 99.99% the speed of light. Easy right? Smash these protons into a graphite barrier, and as they collide with nuclei they produce all sorts of amazing subatomic particles. More electromagnets are then used to sort the positively charged pions from the others and focus them into a beam. These pions quickly decay into muons and muon neutrinos. This beam then passes through several hundred meters of pure solid rock, which removes the muons from the bream. And viola! You have your very own beam of muon neutrinos. Bon appetite!

After we have the beam of neutrinos, all we have to do is actually detect them. One method is to take a giant tank of liquid argon and shoot around 10 trillion neutrinos at it per second. This huge number of neutrinos ensures that at least a few will interact with the argon atoms and be detected. If a neutrino interacts with an argon atom, it will break it apart and release charged particles (in our example pions and muons), these charged particles will then travel through the liquid argon and knock the electrons free from the atoms. The sides of the detector is charged, so a giant electric field fills the tank. This electric field attracts the free electrons, which allows us to trace the path of the particles. These paths have the potential to teach us basically everything about neutrino oscillations.

This article does not even dip its toes into the vast ocean of physics that is concerned with neutrinos. I can hardly believe that I wrote a whole article about neutrinos without elaborating on sterile neutrinos, the role that neutrinos play is supersymmetric theories, extremely interesting neutrino experiments like DUNE (Deep Underground Neutrino Experiment), ANITA (ANtarctic Impulsive Transient Antenna), The Ice Cube Observatory and the T2K experiment. Those topics I guess, are for you to explore on your own.

DUNE (Deep Underground Neutrino Experiment)

Originally published at on December 15, 2020.