Quarks and leptons

AQA Physics for Section 1 - Particles & Radiation

What are cosmic rays?

High-energy particles from space (mainly protons or small nuclei)

What happens when cosmic rays enter the atmosphere?

They collide with atoms and produce showers of particles and antiparticles

What is a muon?

• Negatively charged particle

• A lepton

• Rest mass of 200x the electron

What are pions?

• Positively / neutrally / negatively charged particles

• A meson

• Rest mass between a muon and proton

What are kaons?

• Positively / neutrally / negatively charged particles

• A meson

• Rest mass between a pion and proton

How are particles like the pion or kaon produced?

In high-energy collisions (cosmic rays / particle accelerators)

Why are kaons called ‘strange’?

• Produced via the strong interaction

• Decay via the weak interaction

Describe Kaon decay, Pion decay, and Muon decay

• Kaon → pions / muon + neutrino

• Pion → muon + neutrino

• Muon → electron + neutrino

What is always conserved in particle decays?

• Energy

• Momentum

• Charge

• Lepton number

• Baryon number

• Strangeness (in strong interactions)

State the rest mass order (low → high)

electron < muon < pion < kaon < proton

How are particles classified?

• Matter and antimatter

• → Leptons

• → Hadrons → Baryons + Mesons

What is the difference between baryons and mesons?

• Baryons → hadrons that decay into protons (e.g. protons / neutrons)

• Mesons → hadrons that do NOT decay into protons (e.g. pions, kaons)

Name some Leptons and Hadrons

• Leptons: electrons, muon, neutrino

• Hadrons: proton, neutron, pion, kaon

What is conserved in particle collision?

• Total energy = rest energy + kinetic energy

• Charge and momentum

What is special about neutrino interactions?

• Electron neutrinos → produce electrons

• Muon neutrinos → produce muons

What is the correct muon decay equation?

μ^- → e^- + (anti)V_e + V_μ

What extra rule applies to leptons in interactions?

Lepton types must be conserved separately:

• electron family

• muon family

Give an example of a neutrino interaction (with neutron)

V_e + n → p + e^-

How are strange particles created?

Always in pairs (strangeness is conserved)

What is a meson made of? What is a baryon made of?

• Meson: One quark + one antiquark

• Baryon: Three quarks

What is the quark composition of a proton and a neutron?

• Proton: uud

• Neutron: dud

What quark change occurs in Beta minus decay?

Down → Up

What quark change occurs in Beta plus decay?

Down → Up

What is the only stable baryon?

Proton

What happens to a free neutron?

It decays into a proton + electron + electron antineutrino (beta- decay)

What happens when a quark meets its antiquark?

They annihilate each other