## Weak Interaction and nuclear decay

There are more particles after the decay than just the proton, there is an electron and a new particle that has not been introduced yet. This new particle is the neutrino (which is italian and means “the small neutral one”). As its name suggests, it is electrically neutral. It has no color charge either, so it does not interact strongly. It only interacts with this new force, the weak force, and because the weak force is so weak, it is extremely difficult to detect. It has been detected, for the first time in 1956, 26 years after it had been predicted. It also differs from the quarks and the electron in that its mass has not been measured, it is so light that there are only upper limits on its mass. The best, up do date limit is about 1/250 of the mass of the electron.

[FIGURE OF THE NEUTRON DECAY, n -> p + nu + e?]

This figure shows what happens in the beta decays, the neutron turns into a proton, an electron and this new particle, the neutrino. For now the interaction is a “black box”, we know what comes in and what comes out, but what happens in between?

## Charged Messengers

The other forces had messenger particles, the photon[ref] for electromagnetism and the gluons[ref] for strong interactions, this is also true for the weak force, and like for the other two forces, the messenger is a boson[ref]. There is an important difference though, the messenger particles for the weak force are very heavy, and there are three of them. Their weight is the reason that this force is so weak. This sounds counter-intuitive, right? We won’t dive into this, so if you want to know more, head over here->[READ MORE?]. The messenger particles are called the W bosons, there are two of them, one with positive electric charge and one with negative charge, and there is also a neutral one, which is called Z and which will be explained later.

What actually happens in this "black box", is that the neutron turns into a proton and a [W-] boson, and the [W-] boson decays into an electron and a neutrino. The W boson is very heavy, its mass is about 80 times that of the proton, and it will only exist for an extremely short time (we cannot explain this with what we have explained so far, but you might want to keep reading until we explain the second and third generations of matter to understand the short lifetime of the W). As we know neither the proton nor the neutron is fundamental, so we can still look closer at the neutron becoming the proton and the [W-] boson. The neutron is made up of one up-quark and two down-quarks, and what happens is that one of the down-quarks turns into an up-quark and the [W-] boson. This is the extent to which the Standard Model is describing this weak interaction, the down-quark turning into an up-quark and a [W-] boson and the decay of the [W-]. These two interactions are “black boxes”, perhaps there is something more fundamental occurring if one just looks closely enough, nobody knows that as of today, again this is for someone (maybe you) to find out!

[FIGURE OF THE ENTIRE INTERACTION?]

So far the matter particles that have been introduced are the electron, the up and down-quarks and the neutrino. The quarks are connected in that they can interact strongly with each other, and they are connected to the electron by the electromagnetic force. But the matter particles are grouped into quarks and leptons, the leptons (so far) being the electron and the neutrino[[, muon, tau and their respective neutrino]]. The weak force really gives the connection that motivates giving the electron and the neutrino this common name, leptons, as it allows electrons to turn into neutrinos and the other way around. It also connects the two types of quarks in the same way, allowing the up-quark to turn into the down-quark as mentioned.

## Neutral messenger and discovery

The third messenger of the weak force is the Z boson, which is about as massive as the W bosons, 90 times as massive as the proton. But unlike the W bosons, it is not charged. Both the Z and the W bosons were discovered at CERN, and their discovery is one of the major accomplishments of both CERN and the Standard Model. It is remarkable because the masses of the particles were predicted, and the Z boson, unlike the W bosons, was not involved in the weak interactions that were known, [but was a consequence of the theory and no experiment had seen any indication of its existence when it was predicted, can you explain this one]. Both the W bosons and the Z boson were discovered in 1983, and the two persons responsible for the experiments, Rubbia and van der Meer, got the Nobel prize for this just one year after the discovery, in 1984.

## Outlook

Is this it? For what we know, yes. What we know by now is sufficient to explain everything that surrounds us, from the atoms and nuclei being formed, quarks glued together, down to things as rare and obscure as nuclear decay and radiation. Is this the whole picture? Not quite. We have not explained antimatter, and then there are some really strange particles turning up in cosmic rays. And, in the end, you might want to learn about the Higgs as well. Read on!