2.1.10

# Classification of Particles

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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, νe, and the muon neutrino, νμ.
• 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, Le, is 1 for electrons and electron neutrinos.
• Muon lepton number, Lμ, 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.