Established physics · Lesson 18 of 34
Read four interactions from their diagrams
The same visual grammar describes different processes; the field labels and allowed vertices tell you what changes.
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Muon decay, photon exchange, gluon exchange, and Higgs decay
Muon decay: weak handoff
A muon excitation can decay as μ⁻ → e⁻ + νμ + anti-νe. In a simple diagram, a muon line reaches a vertex with a muon-neutrino line and an internal W⁻ line. The W⁻ line reaches a second vertex with an electron and electron-antineutrino. The muon excitation ends; new excitations of the other fields are produced. The W line is a virtual particle contribution, not an extra detectable final particle.
Check the charges first
Initial electric charge is −1. Final charges are electron −1, muon neutrino 0, and electron antineutrino 0. The diagram’s first vertex has μ⁻ → νμ + W⁻, and the second has W⁻ → e⁻ + anti-νe. These are useful charge checks; a real rate also requires a quantum calculation and conservation of energy and momentum.
Electron scattering: electromagnetic
Two incoming electrons can scatter into two outgoing electrons. A common leading diagram connects their solid electron lines with an internal wavy line labeled photon (γ). The electrons remain electrons while exchanging energy and momentum through an electromagnetic contribution. More than one diagram contributes for identical electrons; the drawing below teaches one route.
Quark scattering: strong
Two quarks can scatter through an internal curly gluon line. The strong vertex is permitted because quarks carry color. At high enough momentum transfer, a small number of diagrams can be useful. At low energies, color confinement and hadron structure make a few simple lines a poor complete description.
Higgs decay: scalar line
A Higgs boson can decay to a bottom quark and bottom antiquark. A dashed H line meets two solid quark lines at a Yukawa-coupling vertex. The Higgs is one field; a dashed line does not mean four separate Higgs fields. A Higgs need not decay this way every time: several permitted channels compete.
From drawing to probability
Each allowed diagram contributes a quantum amplitude, including strengths, momenta, and phases. Contributions for the same outcome combine; then the squared size gives a probability or rate. That is why a visual map of possible connections is the start of a prediction, not the final numerical prediction.
What these four examples do not say
Lines are not measured trajectories. A virtual internal line need not be a real particle hidden inside another. A possible vertex is not proof that a process happens under any conditions. The diagram is one systematic way to calculate from field rules when the approximation is appropriate.
Each row is an example of an allowed connection. The interactive figures below show the paths and labels, not literal particle tracks.
Takeaway: Line styles help, labels decide, vertices enforce couplings, and the whole diagram contributes to a probability calculation.
Check your understanding
In the muon-decay diagram, what is the internal W⁻ line?
- A contribution connecting two weak vertices
- A fourth final particle that must be detected
- The same field as the muon
Answer and explanation
A contribution connecting two weak vertices
The W contribution joins the two weak vertices within the amplitude.
What final particles are specified in μ⁻ decay?
- Electron, muon neutrino, electron antineutrino
- Electron only
- Muon, photon, gluon
Answer and explanation
Electron, muon neutrino, electron antineutrino
The usual decay channel has e⁻, νμ, and anti-νe.
Two electrons scatter through an internal wavy γ. Which interaction is shown?
- Strong
- Electromagnetic
- Higgs Yukawa
Answer and explanation
Electromagnetic
The photon label γ marks an electromagnetic contribution.
What does a curly gluon line connecting quark vertices signify?
- Strong color interaction
- An electron-neutrino oscillation
- A photon changing its name
Answer and explanation
Strong color interaction
Quarks couple to gluons through color charge.
What can a dashed H line meeting bottom and anti-bottom lines depict?
- Higgs decay to a quark–antiquark pair
- Four independent Higgs fields becoming quarks
- A gluon exchange between electrons
Answer and explanation
Higgs decay to a quark–antiquark pair
The Higgs Yukawa coupling permits H → b + anti-b.
Is one diagram automatically the complete measured probability?
- Yes
- No: amplitudes from relevant routes must be combined
- Only if the lines are colored
Answer and explanation
No: amplitudes from relevant routes must be combined
A diagram provides an amplitude contribution; the physical outcome can receive others.
Use the interactive trainer for demonstrations, the field explorer, and saved checkpoint progress.