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

After Chadwick (1932), the atom was understood as a nucleus of protons and neutrons surrounded by electrons. By the 1950s, cosmic ray experiments and early accelerators were producing a confusing menagerie of new particles (pions, kaons, lambdas, hyperons). The Standard Model, developed through the 1960s and 1970s and steadily verified, classifies all observed particles as combinations of a small number of truly elementary constituents.

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

Gauge bosons have spin 1 (spin 2 for the graviton). They are exchanged between fermions to mediate the forces. The Standard Model is a quantum field theory of these particles, with three forces unified within it; gravity is described separately by general relativity and is not part of the Standard Model.

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.

Quarks are never observed in isolation (the property of "confinement"). They appear in two main bound-state types:

Particle accelerators bring beams of electrons, protons or heavier nuclei to high energies and collide them. By $E = m c^2$ (and conservation of energy-momentum), high-energy collisions produce particles that do not exist in ordinary matter. Detectors track the outgoing particles' trajectories and energies to reconstruct what happened.

Despite its success, the Standard Model leaves several open questions:

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.

The forces and their carriers are different concepts. The strong force is what holds quarks together; the gluon is its mediator. Saying "the gluon force" is incorrect.

It is not. The Standard Model contains three forces (strong, weak, electromagnetic) and the Higgs. Gravity is general relativity.