What the electroweak unification achieved — Glashow, Salam, and Weinberg showed EM and weak force are one

Which gauge symmetry underlies the unification of the electromagnetic, weak, and strong forces at high energies, and how does it manifest in the Standard Model?

SU(2)xSU(3) gauge symmetry unifies forces in the Standard Model, manifesting as weak and strong interactions

How the Higgs mechanism works — spontaneous breaking of electroweak symmetry gives W and Z bosons their mass

Higgs field's non-zero vacuum expectation value breaks symmetry, endowing W and Z bosons with mass

What QCD (quantum chromodynamics) describes — the strong force between quarks mediated by gluons

QCD describes the strong force interactions between quarks via gluon exchange

How many types of fundamental forces are represented in the Standard Model of particle physics and which gauge bosons mediate each of these forces?

Four fundamental forces: electromagnetic (photon), weak (W and Z bosons), strong (gluons), gravity (graviton, not yet confirmed)

What the hierarchy problem asks — why is gravity so much weaker than the other three forces

The hierarchy problem questions the vast difference in strength between gravity and other fundamental forces

What gauge symmetry is — local phase transformations that leave physics invariant, the basis of all fundamental forces

Gauge symmetry: Local phase transformations preserving physics, underlying fundamental forces