The Story of the Neutrino

"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 piece of glass." -John Updike

It was so much fun talking about neutrinos that I thought I'd take the time to tell you what all the fuss is about.

Let's go back -- way back -- to the late 1920s. Not only did we know that everything on Earth was made out of atoms, we knew that atoms were made out of atomic nuclei, which were positively charged, massive, and tiny, and electrons, which were negatively charged, much less massive, and also tiny.
We even had some idea of how the energy levels worked (above) thanks to Niels Bohr, and how radioactive decay worked (below), thanks to Marie Curie, among others.
There was a problem, however, with some of these radioactive decays. Specifically, the ones that underwent Beta Decay in the image above. What's the problem? It looked like the conservation of energy didn't work for them!

Hang on, you say. I know what the conservation of energy says: that energy can't be created or destroyed, but can be converted from one form into another! Even mass, if Einstein's got it right, is just a form of energy. So what do I mean that the conservation of energy didn't work? Let's give you one of the simplest examples: tritium. Nearly all of the hydrogen that exists in the Universe is the simplest atom you can imagine: one proton for the nucleus, and one electron orbiting it. But I can make a hydrogen atom with a proton and a neutron for the nucleus, called deuterium, or I can make it with a proton and two neutrons, called tritium. And while plain, no-neutron hydrogen and rare, one-neutron deuterium are stable, tritium, with its two neutrons, will decay after about 12 years! Big deal, you say. All you have to do is -- if energy and momentum are conserved -- measure the mass of tritium, of Helium-3 and the beta particle (electron), and then you'll know how much Kinetic Energy the electron ought to have. And that should be easy to measure; it should come out with the same energy every time! How much energy? Just a little bit over 18 keV. That a teeny, tiny fraction of them have around 18 keV of energy, almost up to the expected amount, but not quite. But the vast majority of these emitted beta particles have much, much lower energies. How do you fix this problem? Well, Niels Bohr, if you asked him, put forth the idea that maybe energy wasn't really conserved for all decays. Maybe, whenever you have beta decay, you lose just a little bit of energy, and the stuff you start with has more overall energy than the stuff you wind up with.But if you asked Wolfgang Pauli (left, in the above photo, with Heisenberg and Fermi), he had a bold new idea. Maybe, he postulated, energy and momentum really were conserved, but we just weren't seeing all the particles.

Maybe, in addition to the helium-3 nucleus and the electron, there was a little, uncharged, neutral particle that was carrying away the missing energy and momentum. He named it "neutrino", for "little neutral guy", and this was in 1930. Nuclear and particle physics developed a lot over the 1930s, 1940s, and early 1950s with the discovery of the neutron, the advent of the atomic bomb, the Hydrogen bomb, and the construction of the first nuclear power plants. Well, here's the thing. Nuclear power plants work by taking radioactive Uranium rods, sticking them in water, where -- in addition to being enriched to make fissionable materials -- the Uranium radioactively decays, through a great chain of events, eventually culminating in stable lead. Along the way, every time there's a beta decay, an atom produces one of these theoretical neutrinos (well, technically antineutrinos) of moderately high energies. But here's the deal, and this is interesting. If I can take tritium, and have it radioactively decay to form helium-3, an electron, and an antineutrino, I can make the reverse happen!

What's the reverse? Take one of these high-energy antineutrinos and smack it into helium-3! What do I get out? An atom of tritium -- which is hard to find -- and a positron, which is easy to find! Because matter is full of electrons, the anti-particle of positrons. When an electron and a positron collide, they produce two photons of the same energy that the mass of an electron (or positron) has: 511 keV!

Sure, other things make these photons, too. That's why you run the experiment twice: once near the nuclear power plant (which gives you the antineutrinos), and you get something like 500 reactions, and once far away from the nuclear power plant, and you only get something like 200! And that's how you discover the neutrino! Despite being proposed in 1930, it wasn't found until 1956, by Fred Reines and Clyde Cowan, who used Cadmium-Chloride instead of Helium-3, but the underlying physics is the same.

Not only did Pauli win a Nobel Prize for this, Enrico Fermi, who worked out the reaction rates and cross-sections for neutrinos, also won one, and so did Reines and Cowan, who used Fermi's work to design their experiment!

And that's all it takes to discover a new particle! And an experiment like this demonstrates, beyond a shadow of a doubt, that neutrinos exist. Finally, you, too, know how to prove that neutrinos exist!