Particle physics – quarks, hadrons and conservation laws
PhysicsQuantum & Nuclear PhysicsAges 17–18
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Combine quarks and antiquarks into baryons and mesons, read the charge Q, baryon number B and strangeness S, and see why only colour-neutral combinations exist. The Conservation laws screen tests ready-made or your own reactions and decays for Q, B, lepton numbers L_e and L_μ, S and rest energy, and names the interaction responsible. The Standard Model screen shows the three generations of quarks and leptons, the gauge bosons and the Higgs boson; Collision events builds detector tracks from Z, W and Higgs decays and histograms the measured mass; Four forces compares strength, range and exchange particles.
Lesson: Particle physics: quarks, leptons and the fundamental interactions
What it shows
Protons, neutrons and other hadrons are made of quarks. A baryon has three quarks, a meson a quark and an antiquark; quarks carry colour charge, and only colour-neutral combinations exist as free particles. Each quark has charge +2/3 e or −1/3 e and baryon number 1/3, and a strange quark has strangeness −1, so totals follow by adding. In every reaction charge, baryon number and each lepton number are conserved; strangeness is conserved by the strong and electromagnetic interactions but can change by 1 in weak decays. The model names only the lightest states and uses simple rules of thumb to identify the interaction.
How to use
On Build a hadron, press the quark buttons to put up to three quarks or antiquarks in the bag and watch the totals add up. On Conservation laws, pick a reaction or choose Your own reaction, then use Add particles to and the particle buttons; press Run the reaction. Tap a cell of the Standard Model table for its properties. On Collision events, press Collide or 25 collisions to fill the mass histogram. On Four forces, move Distance r between two protons.
Parameters you can change
- Screen Build a hadron, Conservation laws, Standard Model, Collision events, Four forces
- Reaction or decay Beta-minus decay of the neutron, Beta-plus decay of the proton, Electron capture, Decay of the muon, Decay of the positive pion, Decay of the positive kaon, Decay of the lambda particle, Decay of the neutral pion, Pion and proton make strange particles, Kaon and proton make an omega, Proton–antiproton annihilation, Pion and proton make a negative kaon, Proton decay (hypothetical), Neutron decay without a neutrino, Muon to electron and photon, Your own reaction
- Collision event Z boson to electron and positron, Z boson to two muons, Higgs boson to two photons, W boson to electron and neutrino
- Show colour charge
- Distance between two protons 0.1–100000 fm
Questions to explore
- Why can a combination of two quarks never exist as a free particle?
- Which conservation law rules out the decay of a free proton into a positron and a neutral pion?
- Why does the Ω⁻ baryon decay only through the weak interaction?
