Neutrinos

I'd say neutrinos are my favourite particles.

We love you too.

Basically...

Neutrinos are "little neutral ones". Literally (from Latin I believe), Neutrinos are extremely light, uncharged particles that has fascinating properties and interactions.

As fermions, neutrinos have half-integer spins. They interact with the weak interaction only, but maybe they interact with gravity too but their mass is just so small (blow 0.4 eV/c^2). There are 3 flavours of neutrinos (each corresponding to a type of lepton, including the electron, and neutrinos have their antiparticle counterparts as well.

The discovery of the neutrino

Physicists were super annoyed by "missing energies" and "weird momentums" present in beta -ve decays. Wolfgang Pauli then used the beta decay processes Enrico Fermi disvovered to predict the presence of an additional particle - neutrinos.

See, theoretically, following what people used to believe, beta decay only emit the electron (the proton stays in place). As the decay has a fixed energy, this means that the electron should, theoretically, always have the same kinetic energy.

But here's what happened:

The kinetic energies of electrons create a spectrum. What's more, the calculated value falls above the mean kinetic energy, meaning there were energy "missing". However, when we add a neutrino, it compensates for the "missing energy" of the electron.

The linear momentum of the decay products also doesn't make sense. But when a neutrino is added, linear momentum becomes conserved.

Interactions

As neutrinos are involved in the weak interaction, they participate in interactions involving the weak force.

Beta +ve decay:

Proton -> neutron + position + electron-neutrino

Baryon number: 1 -> 1 + 0 + 0

Lepton number: 0 -> 0 + -1 + 1

Beta -ve decay:

Neutron -> proton + electron + anti-electron-neutrino

Baryon number: 1 -> 1 + 0 + 0

Lepton number: 0 -> 0 + 1 + -1

Pi meson decay (or with antiparticle version):

Pi-minus -> muon + anti-muon-neutrino

Baryon number: 0 -> 0 + 0

Lepton number: 0 -> 1 + -1

Tau particle decay (or with antiparticle version):

Tau particle -> pi-minus + tau-neutrino

Baryon number: 0 -> 0 + 0

Lepton number: 1 -> 0 + 1

Neutrinos can also induce reactions such as radioactive decay!

What's different about neutrinos and anti-neutrinos?

Normally, particles and antiparticles differ such that they have opposite charge. But what about neutrinos? They have 0 charge… so how do we know which one is an antiparticle?

Neutrinos and antineutrinos differ in their handedness (or chirality for more poshness). Point both of your thumbs to the direction of motion. Curl your other fingers up. If the spin of the (anti)neutrino corresponds to your fingers on your left hand, it's a neutrino! If the spin on the (anti)neutrino corresponds to your fingers on your right hand, it's an anti-neutrino!

Note that spin doesn't actually mean the particle is rotating like a planet (here it's just drawn for illustration purposes). Spin just represents a spin angular momentum that is associated with these elementary particles. Spin is an intrinsic property of (anti)neutrinos.

Parity symmetry

The universe has parity symmetry: if you look at a particle and its mirror image, then you should not be able to distinguish the particle from its mirror image. The laws of physics in real life and in the mirror image should be the same, so a phenomena that works in real life should work when it is mirrored as well. However, this is not conserved by neutrinos and their handedness. Imagine when neutrinos and anti-neutrinos are reflected in the mirror. Their handedness change!

This means we would be able to distinguish when a phenomena in a mirror: if we see a beta-minus decay happening that emits a left-handed neutrino, we would know it's in a mirror!

Why does neutrinos violate parity symmetry? No idea.

References: https://neutrinos.fnal.gov/mysteries/mass/

What are Neutrinos? (Neutrino Hypothesis, Properties, Handedness)