Classification of Particles

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Hadrons

Hadrons (e.g. protons and neutrons) are particles that feel the strong nuclear force. Hadrons are made of quarks and can be split into two categories: baryons or mesons.

Baryons

Baryons are hadrons that contain three quarks.
Protons and neutrons are both baryons.
- There are other types of baryons which can be found in cosmic rays, but you don't need to know about these at A Level.
The proton is the only stable baryon.
- All other baryons will eventually decay into a proton.

Antibaryons

The antiparticles of baryons are called antibaryons.
- Examples include antiprotons and antineutrons.

Baryon number

Baryon number, B, describes the number of baryons present.
- Each baryon has a baryon number B = 1.
- Antibaryons have a baryon number B = -1.
B is a quantum number.
- This means that it can only be an integer (i.e. you can't have B = 0.5).
B must always be conserved.
- Like energy and momentum, baryon number must be conserved in all interactions.

Mesons

Mesons are hadrons that contain a quark and an anti-quark.
Examples include pions and kaons.
- There are also many other types of mesons, but you don't need to know about them at A Level.
Mesons can be found in cosmic rays or particle accelerators.
Mesons have a baryon number B = 0 because they're not baryons!

Pions vs kaons

Pions are the exchange particle of the strong nuclear force between nucleons.
- There are three versions, each with a different electric charge: π⁺, π⁰ and π^-.
Kaons are more massive and often decay into pions. They come in many types including K⁺, K⁰ and K^-.

Leptons

Leptons are fundamental particles and so they are not made of any smaller particles. Leptons do NOT feel the strong nuclear force.

Electrons and muons

The most common type of lepton is the electron, e^-.
There are other types of leptons, such as the muon, μ^-:
- Muons are negatively charged and heavier than electrons.
- Muons are unstable and always decay into electrons.
- Muons are normally found in cosmic rays.

Neutrinos

Neutrinos are also leptons.
- Electrons and muons each have their own neutrino: the electron neutrino, ν_e, and the muon neutrino, ν_μ.
Neutrinos are (almost) massless particles with zero charge, so they don't do much.
- Just as well, because there are 100 trillion neutrinos passing through your body every second from cosmic rays!

Lepton numbers

The lepton number describes the number of leptons.
You get a different lepton number for each type of lepton.
- Electron lepton number, L_e, is 1 for electrons and electron neutrinos.
- Muon lepton number, L_μ, is 1 for muons and muon neutrinos.
Lepton numbers are quantum numbers, meaning they only come in integer numbers.
Each lepton number must be conserved individually.
Antileptons also exist. These have opposite charges and lepton numbers.

Strangeness

Strangeness is a fundamental property of matter, like mass, charge or baryon number.

Quantum number

Strangeness is a quantum number so can only take integer values.
The strange quark, s, has a strangeness of -1.
The antistrange quark, s, has a strangeness of +1.
Particles made from strange quarks (such as kaons) have an associated strangeness.

Conserved in the strong interaction

Unlike other quantum numbers, strangeness is only conserved in the strong interaction.
Strange particles, such as kaons, are produced via the strong interaction but decay via the weak interaction.
- This is why kaons are always produced in pairs (K⁺ and K^-).
- The strangeness of each particle (-1 and +1) cancel out.
In weak interactions, such as kaon decay, strangeness can change by -1, 0 or +1.