QED Feynman Lab

Processes and one-loop examples appear immediately; valid edits update directly. Enter exact centre-of-mass energy and scattering angle or use the angle slider. Reset restores the default process inputs. Display contains vertex dots and optional fermion-line highlighting. Only the raw energy/angle strings and control selections are retained, including unfinished inputs; diagrams and computed values are rebuilt. Builder drawings, undo history, active section and highlighting playback are not retained.

Explore QED topologies, not particle trajectories or calculated amplitudes. In the process view, incoming momenta are p₁ and p₂; outgoing momenta are p₃ and p₄. Solid lines carry fermions, wavy lines photons. Antifermion arrows oppose their momentum direction. θ is the centre-of-mass angle between p₁ and p₃.

The on-shell invariants use s=(p₁+p₂)², t=(p₁−p₃)² and u=(p₁−p₄)², with s+t+u=Σmᵢ² (c=1). Both initial and final mass thresholds apply. Electron and muon masses are rounded to 0.000511 and 0.10566 GeV. γ* in the bremsstrahlung subdiagram is virtual, so that view has no on-shell kinematics.

Each interaction contributes one coupling e: the displayed factor eᵛ counts vertices, not a cross section or a relative loop correction. The loop gallery shows representative topologies, not the complete gauge-invariant sum.

Builder: place legs and vertices with the mouse or Add node. Select source then target in momentum direction; the line species comes from an external leg, or from Internal line between vertices. Undo reverses edits. Validation checks degree, flavor, charge flow, external legs and connectedness. It does not evaluate spinors, integrals, kinematics or cancellations. Highlighting only emphasizes fermion lines; reduced-motion preferences are respected.

References: Tong: QED rules and scattering; PDG: two-body kinematics.

Automatically added nodes form a grid that reflows when the node count changes. Click-placed nodes keep their relative positions.