Particle Lab: Particle Physics Simulation & Feynman Diagrams
Build particles, explore interactions and construct Feynman diagrams in this free particle physics simulation for A-Level Physics.
Drag labelled particle arrows and exchange lines into a diagram. Check charge, baryon number and lepton numbers at each vertex, then follow the white highlights through the completed interaction.
A particle physics simulation for A-Level learning
This particle physics simulation connects quark structure, particle interactions and conservation laws. Use Particle Lab for independent revision, a classroom demonstration or a short investigation. It supports the core ideas studied across AQA, Edexcel, OCR A and OCR B. The lab identifies extension diagrams separately; check the specification for your own course.
Explore first, then predict and build
The lab opens in Feynman Studio with Explore selected and the first diagram playing. Pause or move the progress slider to inspect it. Choose another interaction to compare the incoming particles, outgoing particles and exchange line.
Next, select Predict & build. Incoming particles are supplied. Drag each outgoing particle or exchange line onto the active question mark. A correct answer stays visible and advances the investigation. An incorrect answer gives feedback while preserving your progress. Open Why? to inspect the conservation balance for that step.
Practise with guides or connect your own diagram
Build a diagram provides faint guides for labelled line pieces. Free build lets you arrange the interaction vertices and connect pieces between ports. Select a piece and tap two ports, or drag it onto a starting port and select the destination. Tap a placed line label to reverse its arrow. In Free build, focus a line label and press Delete to remove it.
Seven investigations cover hadrons, weak interactions, strange particles, the strong interaction, Feynman diagrams, conservation challenges and an interaction workshop. Desktop users can drag pieces; selecting a piece and tapping the target offers an alternative on touch screens.
How to read and build Feynman diagrams
Read time upwards
Incoming particles appear below the interaction region and outgoing particles above it. Each vertex represents an interaction. An internal line connects vertices and represents a virtual particle. Its label and line style identify the exchange shown in this contribution to the process.
Check every vertex
Add electric charge, baryon number and lepton numbers entering a vertex. Compare them with the totals leaving it. Then check the complete reaction. A balanced charge alone does not establish that a process occurs: the interaction must provide the required coupling, and energy and momentum must also be conserved.
At each vertex: total Q entering = total Q leaving
Antiparticle arrows and virtual particles
A fermion arrow on an antiparticle line points against the chosen time direction. It encodes particle-flow convention rather than motion into the past. The arrow on a charged exchange line also has to be read with its particle label and direction.
Do not describe virtual particles as borrowing energy. Energy and momentum are conserved at vertices. An internal line is not constrained to have the energy–momentum relation of a free, on-shell particle.
What do the white dots show?
The moving highlights guide you through the notation. They are not measured particle trajectories or a physical recording of an interaction. Once every prediction is correct, the highlights trace the completed diagram while all accepted elements remain visible.
Quarks, antiquarks and hadrons
Build a baryon or a meson
A conventional baryon contains three valence quarks. An antibaryon contains three valence antiquarks. A conventional meson contains a quark and an antiquark. These are the valence patterns used in the lab; real hadrons also contain gluons and sea quark–antiquark contributions.
The u quark has charge +⅔e; the d and s quarks have charge −⅓e. Each quark has baryon number +⅓, while an antiquark has baryon number −⅓ and opposite additive quantum numbers. An antiparticle has the same rest mass as its partner and is not necessarily negatively charged.
Proton: uud → Q = +1e, B = 1 · Neutron: udd → Q = 0, B = 1
Why can two quarks fail the hadron test?
Two quarks have total baryon number ⅔. That is neither the three-quark baryon pattern nor the quark–antiquark meson pattern. Use the numerical ledger to explain why a proposed combination fails the conventional-hadron exercise instead of judging its appearance.
Strong, weak and electromagnetic interactions
Strong interactions preserve quark flavour
Gluons mediate the fundamental strong interaction between colour charges. A strong interaction does not change a quark from one flavour to another. At nucleon level, pion exchange provides an effective description of the residual strong force; it is distinct from the fundamental gluon description.
Weak interactions can change flavour
In beta-minus decay, a down quark changes into an up quark, producing an electron and an electron antineutrino through a virtual W⁻ exchange. Charged-current weak interactions act on quarks and leptons. Neutral-current weak interactions use Z⁰ exchange and are shown as an extension.
Strangeness and lepton families
Strong and electromagnetic interactions conserve net strangeness. Weak interactions can change it. In the reactions modelled here, electron-family and muon-family lepton numbers are conserved separately. Distinguish a neutrino from its antineutrino and keep the flavour subscript with the symbol.
Electromagnetic exchange
A virtual photon can transfer four-momentum between charged particles. Compare electron–electron and electron–proton scattering with weak interactions to see which identities stay the same. Some reactions require more than one Feynman diagram to describe their full probability.
Worked example: beta-minus decay
d → u + W⁻ followed by W⁻ → e⁻ + ν̄ₑ
Find the exchange charge
At the first vertex, the incoming down quark has charge −⅓e and the outgoing up quark has charge +⅔e. Therefore, the exchange must carry −1e: −⅓ = ⅔ + (−1). Baryon number balances because ⅓ = ⅓ + 0.
Choose the correct neutrino
At the second vertex, the W⁻ has electron lepton number zero. The electron has electron lepton number +1, so the other outgoing particle must contribute −1. It is an electron antineutrino. Electric charge also balances: −1 = −1 + 0.
For the whole neutron decay, n → p + e⁻ + ν̄ₑ. The other two valence quarks are spectators. The W is virtual; a real W boson is not produced by a free neutron.
Try these investigations
- Build beta-minus decay at quark level. Explain the W charge before placing it.
- Compare beta-plus decay and electron capture. Which particles enter each vertex?
- Reverse an antiparticle arrow and ask the checker to explain the error.
- Compare photon, charged-pion and gluon exchange. Which quantities and particle identities stay unchanged?
- Complete a conservation challenge and explain why passing charge conservation alone is insufficient.
Continue your A-Level Physics revision
Use A-Level Physics questions by topic or the all-board Physics question engine for exam practice. Connect particle interactions with Binding Energy Lab, or browse all free physics simulations.
For reference, consult the AQA particles and radiation specification, CERN’s Standard Model overview and the Particle Data Group. If this particle physics simulation helps, share it with a classmate or colleague.