25 Days of Particles: Day 4

Electron Neutrino

Classification: lepton, fermion
Fundamental: yes
Family: first
Mass: < 0.000002 MeV
Interactions: Weak, Gravity (barely)
Spin: 1/2

Once upon a time in 1930, Wolfgang Pauli was studying beta decay and discovered a problem. In beta decay, a nucleus suddenly acquires an extra proton and spits out an electron (we now know this is from a neutron decaying, but the neutron hadn't been discovered quite yet). But Pauli noticed a problem with this decay. If you start with a stationary nucleus and it suddenly fires an electron off in one direction, something's got to recoil in the other direction. This is required by the laws of conversation of momentum and conversation of energy and conservation of angular momentum. Stuff's got to recoil.

The problem was that there wasn't any recoil.

Or rather, that there wasn't enough, and to complicate matters further the missing amount of recoil wasn't always the same. Still, physicists are strongly attached to their conservation laws, and Pauli made the logical guess that something was recoiling, but that he couldn't detect it. That meant the particle had to be neutral. He dubbed this hypothetical particle the neutron. We can probably blame Pauli for starting the trend that continues to this day of naming particles way, way before we ever actually see them in experiments.

Of course, at about the same time, a much more massive neutral particle was also getting discovered, and it also got named the neutron. Enrico Fermi resolved the issue by naming the smaller of the two the neutrino.



To be exact, it was the electron neutrino, the little buddy of the electron. The electron neutrino is electrically neutral and extremely light. In fact, experiments have yet to do more than put an upper bound on the neutrino mass; we can't say what the mass is, only what it must be smaller than. So while the neutrino can theoretically interact via gravity, it's so light that nobody would notice, and it doesn't interact via the electromagnetic or strong forces. It's primary way of interacting with the world is through the weak force. The electron neutrino and the electron form a doublet for the weak force; if an electron is involved in such a reaction, an electron neutrino must be as well. They always come together.

Of course, the weak force is called weak because it is a weak force. It doesn't pull on matter much. Neutrinos are too light to decay into anything, so when a neutrino gets created, it's going to hang around for a while. Again, no one will really notice, but the neutrinos are there. The sun and every other star produces a huge flux of neutrinos that cascade through the earth and through us every day. Eat a banana and you'll produce another few hundred over the course of the day as the radioactive potassium in the banana decays in you. But no one notices, because neutrinos don't interact with anything. It's estimated fully half of the normal matter in the universe is made up of neutrinos, zipping around and doing not much else. They just not that sociable.