Classification of Particles
Physics: AQA A Level, topic 2.1.10
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, 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.
Test yourself on Classification of Particles
What is the baryon number of a proton?
Which of these statements is true of muons?
Which of these particles are leptons?
What is the most common type of lepton?
Which of these quantities are fundamental properties of matter?