Neutrinos, beta decay and momentum conservation

I studied neutrinos as part of my Physics masters project, and I have to say, they are pretty cool. I find particle physics as a whole really interesting – there’s a lot of stuff that we just don’t know about yet, and a lot of really massive experiments (like the LHC) to try and discover more.

Neutrinos in particular are a mystery. We know they exist, but we can’t actually see them – we can only see the ‘signature’ particles they produce in certain reactions.

The three flavours of neutrino: electron, muon, and tau.

Who knows: maybe neutrinos actually look like this! Image: particlezoo.net.

But that’s not all that’s weird about neutrinos. They’re also really, really light: much lighter than their linked particles, the electron, muon and tau. They carry no charge, again unlike the particles they’re associated with. In fact, you might wonder why they’re even associated with these particles at all!

It all comes down to one reaction, known as beta-decay. Here, one neutron – one of the particles that makes up an atom’s nucleus, along with the proton – decays into a proton, an electron, and an electron neutrino (that’s the yellow one).

Originally, it was thought that only the proton and electron are produced, and that does make sense: a proton has positive charge, while an electron is negative, so they balance out to make a neutron’s neutral charge. So the reaction would look like this:

n  (neutron, no charge) —>  p (proton, positive charge) +  e (electron, negative charge)

But charge isn’t the only thing that is conserved (i.e., balances out) in reactions. Newton’s first law says that if something is travelling at a constant speed or completely still, the forces on it must be balanced. Since force, energy and momentum are all directly linked:

  • Energy = force x distance
  • Momentum =  force x time = mass x velocity
  • Force = momentum / time = energy / distance   …etc etc

the momentum has to be balanced, like the force, and conserved, like energy always is. But in 1930, Wolfgang Pauli noticed that in beta-decay, the momentum wasn’t conserved. The proton stayed in the nucleus, while the electron left it (the electrons leaving were called “beta-radiation” at the time, hence the name beta-decay). This meant that even though the neutron had no momentum, and the proton had no momentum (neither of them went anywhere) the electron did; and since momentum has to be conserved, this didn’t make sense!

Pauli therefore suggested the existence of a new particle, which would balance out the electron’s momentum by travelling in the opposite direction. It would have to have neutral charge, so that the overall charge still balanced out, and it would have to be really, really light, so it couldn’t be detected or make a noticeable difference to the calculations. So, the neutrino was born!

At the time, Pauli wasn’t too happy with his new particle, as to him, it felt too much like he was introducing it just to make the maths work. (Interestingly, this is still how many particles – including the Higgs boson – are suggested: the maths doesn’t work, so add a new particle to sort it out!) Even though we now know the neutrino exists, over 80 years on, we still don’t know everything about it.

These sneaky particles pass through us every day, coming from all around us, from the far edges of the universe and from the burning depths of our own Sun. Hopefully with time and rapidly improving technology, we’ll be able to use these little spies to answer some of the universe’s biggest questions.

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One comment

  1. Pingback: High-energy neutrinos finally slow down to be counted | Jabberwacky

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