Quick questions on The Standard Model of particle physics: HSC Physics Module 8

Fermions have spin 1/2 and obey the Pauli exclusion principle. They come in two families.

<!-- Diagram: Standard Model chart of quarks, leptons and gauge bosons | reviewed 2026-07-01 --> <svg class="fig" viewBox="0 0 560 400" role="img" aria-labelledby="sm-t sm-d"> <title id="sm-t">The Standard Model of elementary particles</title> <desc id="sm-d">A grid of labelled boxes. Three columns of quarks (up, charm, top on top row; down, strange, bottom on bottom row) sit on the left. Three columns of leptons (electron neutrino, muon neutrino, tau neutrino on top row; electron, muon, tau on bottom row) sit in the middle. A column of four gauge bosons, photon, gluon, Z, W, sits on the right, with the Higgs boson below it.

The Higgs field, postulated in 1964 and finally detected as the Higgs boson in 2012 at CERN's LHC, is the mechanism by which the $W$, $Z$ and the fundamental fermions acquire their masses. The Higgs is spin 0 (a scalar boson) and is the only known elementary scalar. Its discovery completed the Standard Model as originally formulated.

Feel all three Standard Model forces (strong, electromagnetic, weak). Carry fractional electric charge ($+2/3$ or $-1/3$). Always confined inside composite particles (hadrons).

Do not feel the strong force. Charged leptons feel electromagnetic and weak; neutrinos feel only the weak force (and gravity). Six in three generations:

Binds quarks into protons, neutrons and other hadrons. The residual strong force (mediated by pions, themselves quark-antiquark pairs) binds protons and neutrons into nuclei.

Long range, infinitely so for static fields. Holds electrons in atoms, binds atoms into molecules, and underlies all of chemistry, biology and macroscopic phenomena.

Responsible for beta decay (transmuting a down quark into an up quark, or vice versa). Mediated by the massive $W$ and $Z$ bosons, which limit the range. Combined with electromagnetism into a single "electroweak" theory at high energies.

Predicted to be mediated by the spin-2 graviton, never detected. Described classically by general relativity. Outside the Standard Model.