LEPTONS AT WORK

So we went over hadrons vaguely last time, concluding that they're composed of particles called quarks/ antiquarks - mesons having a quark and an antiquark, and baryons having three quarks/ three antiquarks. Some carry charge, i.e. (p, Kˉ, K+) and some have no charge, i.e. (n). And if electromagnetic interaction is not involved, they're not charged.

So now we'll get to know leptons. Loopy leptons. Leptons don't interact through the strong interaction and they're lightweights - electrons, muons and neutrinos are fundamental particles with no internal constituents (like quarks). This makes them seem lame and all, but just you wait.

Just like any other particle, there are antiparticles (of an identical mass, with opposite properties) - and when antileptons interact with leptons, we end up with something great: hadrons. If you recall the previous topic, the diagram shows an electron-positron collision, resulting in a showering of hadrons - amazing stuff, right? Leptons can be accelerated to collide and they have an associated neutrino and antineutrino... So we'll move on to a subtopic.

Differentiating Between Neutrino Types

Note: A third family, the 'tau' family, exists, but AQA doesn't care about tau and has therefore excluded them from our lives. Maybe it's for the best. 

I hope you can forever remember that neutrinos are basically a similar variant of the typical electron - but neutrinos carry no charge, hence their being called 'neutrinos'. Shocker. Due to being electrically neutral, neutrinos are not affected by the electromagnetic interaction and only if they have mass do they interact via the gravitational interaction with other massive particles. They tend to pass through ordinary matter easily and are pretty much undetectable, like the Batman. What a life they lead.

• We have the beloved electron neutrino and the muon neutrino to think about now. As mentioned above, we don't care for the tau neutrino.
• Electron neutrinos are far more common in our world, but muon neutrinos are detectable as well. 
• The muon neutrino is associated with the muon (remember, this is the heavier electron), and the electon neutrino with the electron.
• In muon and antimuon decay, neutrinos and antineutrinos create muons when interacting with protons and neutrons - this differs from that of beta decay. 
• One type of antineutrino and neutrino existing would facilitate for the production of electons and and muons of the same number. But we have at least three neutrino-antineutrinos, so whatever. 

Fun fact: The terms 'electron neutrino', 'tau neutrino' (why are these guys here again?) and 'muon neutrino' are referred to as flavors. Yeah, flavors. Now, if you'll excuse me, I think some muon neutrino green tea would be nice. Referring to them as 'types' or 'flavors' is fine by the AQA.

Lepton Numbers and the Rules they Obey

Neutrinos are created through radioactive decay or nuclear reactions (like those happening in the Sun, when cosmic rays hit atoms or in nuclear reactors).

When neutrons change into protons or protons change into neutrons, neutrinos are generated. You should recognize those two occurrences are two forms of beta decay. If you didn’t realize that, then go here. Did you go there? Okay, back to this stuff. Leptons interact through the weak interaction (remember, that's responsible for beta decay) and these guys are elementary particles due to them not breaking down further. 

Lepton decay can be sussed out by observing the changes in lepton number - conservation of charge is involved here, of course. Now check out these standardized rules:

1. "In an interaction between a lepton and a hadron, a neutrino or an antineutrino can change into or from a corresponding charged lepton." (AQA Physics A, Nelson Thornes, 2008)

So if we take a look at an electron neutrino interacting with a neutron, a proton and electron are produced. Check out the conservation of charge:

Initially the lepton number is +1 and then +1 after the change too. This is permitted. This is because the lepton was assigned a +1 charge, the antilepton a -1 charge and any nonlepton a charge of 0. 

But if the products were to be an antiproton and a positron, the lepton numbers change.

This is an abomination. The charges go from +1 to -1 and this defies the conservation of charge. This is not permitted.

2. "In muon decay, the muon changes into a muon neutrino. In addition, an electron and an electron antineutrino are created to conserve charge." (AQA Physics A, Nelson Thornes, 2008)

We started off with a muon changing into an electron, an electron antineutrino and a muon neutrino. Inititally the lepton number is +1 and +1 afterwards too (+1 = +1 -1 +1) so this is permitted. 

But oh no, someone wanted a muon antineutrino in the decay products instead of a regular muon neutrino. That's just crazy. Why? Because the lepton numbers just don't add up. See, initially we have a charge of +1, but then through (= +1 -1 -1) we end up with -1. Not cool. This is not permitted. 

So don't do that. 

The above rules are applied to particular branches of leptons, so take the following into account.

1. The electron branch consisting of electron neutrinos, electrons and their counterparts
2. The lepton number for each branch is conserved for all changes
3. Leptons are in two branches: the muon branch and the electron branch
4. The muon branch consists of muons, muon neutrinos and their counterparts
5. Leptons have a lepton number of +1, antileptons have -1 and nonleptons are 0.

To recapitulate:

• Types of leptons include the electron, positron, muon, antimuon, electron neutrino, electron antineutrino, muon neutrino and muon antineutrino. 
• Charged leptons interact through the electromagnetic interaction. Leptons in general interact through the weak interaction - never the strong interaction.
• Leptons can decay into other leptons through the weak interaction and can be produced or annihilated through particle collision. 
• Nonleptons can never be products of this type of decay.
• The lepton number must be conserved at all times. Yes, even on a Sunday.