Quarks and leptons are the basic matter particles in the Standard Model of particle physics. They are the building blocks for atoms, radiation processes, and many reactions studied in modern physics. Quarks combine to make protons and neutrons, while leptons include the electron that surrounds the atomic nucleus.
Understanding these particles helps explain why matter has structure and why some particles are stable while others quickly decay.
Quarks and leptons are grouped into three generations, with each generation containing heavier relatives of the particles before it. Ordinary matter is made almost entirely from first-generation particles: up quarks, down quarks, electrons, and electron neutrinos. Heavier generations appear in high-energy collisions and then usually decay into lighter particles.
Each particle has properties such as electric charge, mass, spin, and interaction type that determine how it behaves.
Key Facts
- There are 6 quarks: up, down, charm, strange, top, and bottom.
- There are 6 leptons: electron, muon, tau, electron neutrino, muon neutrino, and tau neutrino.
- Quark charges are +2/3 e for up, charm, and top, and -1/3 e for down, strange, and bottom.
- A proton is made of uud, so its charge is +2/3 e + +2/3 e + -1/3 e = +1 e.
- A neutron is made of udd, so its charge is +2/3 e + -1/3 e + -1/3 e = 0.
- Leptons do not feel the strong nuclear force, while quarks do feel the strong nuclear force.
Vocabulary
- Quark
- A fundamental matter particle that feels the strong nuclear force and combines with other quarks to form particles such as protons and neutrons.
- Lepton
- A fundamental matter particle that does not feel the strong nuclear force, such as an electron, muon, tau, or neutrino.
- Generation
- A grouping of matter particles in the Standard Model where later generations are heavier versions of particles with similar charges and interactions.
- Hadron
- A composite particle made of quarks, including baryons such as protons and neutrons and mesons made of a quark and an antiquark.
- Neutrino
- A very light neutral lepton that interacts only through the weak force and gravity.
Common Mistakes to Avoid
- Calling electrons quarks is wrong because electrons are leptons and do not feel the strong nuclear force.
- Adding quark charges as whole numbers is wrong because quarks have fractional electric charges such as +2/3 e and -1/3 e.
- Thinking all three generations make ordinary atoms is wrong because stable everyday matter is mostly made from first-generation particles.
- Assuming neutrinos have electric charge is wrong because neutrinos are electrically neutral and interact very weakly with matter.
Practice Questions
- 1 Find the total electric charge of a particle made of two up quarks and one down quark. Show the charge addition using fractions of e.
- 2 A neutron is made of one up quark and two down quarks. Calculate its total electric charge in units of e.
- 3 Explain why a muon produced in a high-energy collision is not usually found as a permanent part of ordinary atoms, even though it is similar to an electron.