Little Neutral One

Way back in the 1930's, there was something strange about radioactivity. We knew it came in three types, called alpha, beta and gamma radiation.

Alpha is made up of helium nuclei (two protons and two neutrons, which paper can easily stop), Beta is electrons, which can be stopped by foil, and gamma is electromagnetic waves, which need lead to stop them .

The problem was Beta, because the electrons were emitted at lots of different energies. This was trouble because we could not work out why. The answer lay in two different directions.

Either the conservation of energy was being violated (unthinkable), OR the energy was being carried away by some new particle not previously seen.

Enrico Fermi reasoned it must be very light, and electrically neutral. He called it the "neutrino", meaning "little neutral one".

Years later, it was actually detected, and we now know that gadzillions of them pass through you every second.

But they are so unfriendly, they pass through everything.

To illustrate how elusive they are, physicists use a term call "mean free path", which is a measure of how far something goes before it hits anything, an "average" distance.

For example, the mean free path of an electron in a metal is about a centimeter. For an alpha particle in air, it is about 4-5 centimeters. Beta particles manage about 15 centimeters.

It is a measure of how unfriendly neutrinos are when it is realised that their mean free path IN LEAD is about three light years.

That is, they will, on average, travel through up to three light years of lead (the densest stable element) before hitting any of it.

There was for a while, a huge debate about their mass. They were thought, just like the photon, to be massless, but the problem was their helicity. Helicity is simply put, spin, but there are two ways to spin, clockwise and anticlockwise.

Massless particles travel at the speed of light, and if neutrinos were indeed massless, they never stopped and should always spin either one way or the other. Never both.

We discovered both helicities, which simply means, they would have to have stopped and started again.

Logically, this therefore meant that they HAD to have mass.

Sure enough, years later it was given an upper limit. The lightest particle we knew of up till then was the electron.

The neutrino almost does not exist, as it is 512,000 times lighter than even the electron.

Nothing messes with the neutrino.