Check what an interaction conserves.

The six quark flavours are up, down, charm, strange, top and bottom. Leptons are electron, muon, tau and their three neutrinos. Quarks experience the strong interaction; leptons do not. Baryons contain three quarks and mesons a quark–antiquark pair. A proton is uud and a neutron udd.

Photons mediate electromagnetic interactions, gluons the strong interaction, and W and Z bosons the weak interaction. Antiparticles have the same rest mass but opposite additive quantum numbers. Track charge, baryon number and lepton number at every interaction: beta-minus neutron decay needs an electron antineutrino to balance the emitted electron’s lepton number. A $W^{-}$ connects the down-to-up quark change to electron and antineutrino production.

INTERACTIVE MODEL

Build a three-quark baryon

Number of up quarks2

READ THE GRAPH

Explore the relationship

Up charge +2/3 e; down charge −1/3 e. A three-quark baryon uses only the point where up + down = 3.

Total charge / e

Tap or focus a plotted point to read its values.

View plotted data as a table

Explore the relationship
Series	Number of down quarks	Total charge / e
Total charge / e	0	1.33333
Total charge / e	0.05	1.31667
Total charge / e	0.1	1.3
Total charge / e	0.15	1.28333
Total charge / e	0.2	1.26667
Total charge / e	0.25	1.25
Total charge / e	0.3	1.23333
Total charge / e	0.35	1.21667
Total charge / e	0.4	1.2
Total charge / e	0.45	1.18333
Total charge / e	0.5	1.16667
Total charge / e	0.55	1.15
Total charge / e	0.6	1.13333
Total charge / e	0.65	1.11667
Total charge / e	0.7	1.1
Total charge / e	0.75	1.08333
Total charge / e	0.8	1.06667
Total charge / e	0.85	1.05
Total charge / e	0.9	1.03333
Total charge / e	0.95	1.01667
Total charge / e	1	1
Total charge / e	1.05	0.983333
Total charge / e	1.1	0.966667
Total charge / e	1.15	0.95
Total charge / e	1.2	0.933333
Total charge / e	1.25	0.916667
Total charge / e	1.3	0.9
Total charge / e	1.35	0.883333
Total charge / e	1.4	0.866667
Total charge / e	1.45	0.85
Total charge / e	1.5	0.833333
Total charge / e	1.55	0.816667
Total charge / e	1.6	0.8
Total charge / e	1.65	0.783333
Total charge / e	1.7	0.766667
Total charge / e	1.75	0.75
Total charge / e	1.8	0.733333
Total charge / e	1.85	0.716667
Total charge / e	1.9	0.7
Total charge / e	1.95	0.683333
Total charge / e	2	0.666667
Total charge / e	2.05	0.65
Total charge / e	2.1	0.633333
Total charge / e	2.15	0.616667
Total charge / e	2.2	0.6
Total charge / e	2.25	0.583333
Total charge / e	2.3	0.566667
Total charge / e	2.35	0.55
Total charge / e	2.4	0.533333
Total charge / e	2.45	0.516667
Total charge / e	2.5	0.5
Total charge / e	2.55	0.483333
Total charge / e	2.6	0.466667
Total charge / e	2.65	0.45
Total charge / e	2.7	0.433333
Total charge / e	2.75	0.416667
Total charge / e	2.8	0.4
Total charge / e	2.85	0.383333
Total charge / e	2.9	0.366667
Total charge / e	2.95	0.35
Total charge / e	3	0.333333

Probe along the horizontal axis0Total charge / e: 1.3333

Explore: Choose two up quarks and one down quark. Compare total charge with udd.

B=\tfrac13(n_q-n_{\bar q})\qquad L=n_l-n_{\bar l}\qquad n\to p+e^-+\bar\nu_e

WORKED EXAMPLE

Determine the charge of uud.

Up quarks each carry + $\frac{2}{3}$ e; down carries $-\frac{1}{3}$ e.
$\frac{2}{3}$ + $\frac{2}{3}$ − $\frac{1}{3}$ = +1 e, the proton’s charge.

Assumed knowledge

Signed fractions and particle notation.

Learning checkpoints & source

YOUR LEARNING CHECKPOINT

Classify quarks, leptons, baryons and mesons.
Conserve charge, baryon number and lepton number.

QCAA Physics · Unit 4 · The Standard Model ↗

READY TO TRY IT?

Check what an interaction conserves.

Build a three-quark baryon

Explore the relationship

Determine the charge of uud.

Put the idea to work.