The quantization of symmetry-preserving interactions.
Infrastructure of Interaction.
$A_\mu$ as a required field.
Fields to Particles.
Creation of Gauge Quanta.
The Particle Spectrum.
Photons, Gluons, and Weak Bosons.
The Exchange Principle.
Forces as Quantum Trading.
Standard Model Interactions.
Closing the Gauge Circuit.
Interactions as Structural Necessity.
(Symmetry → Connection → Quantization → Interaction → Closure)
Gauge symmetry:
$$G = U(1)\times SU(2)\times SU(3)$$
$$\dim(G) = 1 + 3 + 8 = 12$$
Generators:
$$[T^a, T^b] = i f^{abc} T^c$$
$$A_\mu = A_\mu^a T^a$$
Covariant derivative:
$$D_\mu = \partial_\mu + i g A_\mu^a T^a$$
$$F_{\mu\nu}^a = \partial_\mu A_\nu^a - \partial_\nu A_\mu^a + g f^{abc} A_\mu^b A_\nu^c$$
Gauge field Lagrangian:
$$\mathcal{L}_{gauge} = -\frac{1}{4} F_{\mu\nu}^a F^{\mu\nu a}$$
$$A_\mu^a(x) \rightarrow \hat{A}_\mu^a(x)$$
Mode expansion:
$$\hat{A}_\mu^a = \sum_{\mathbf{k},\lambda} \left( a^a_{\mathbf{k},\lambda} \epsilon_\mu e^{-ikx} + a^{a\dagger}_{\mathbf{k},\lambda} \epsilon_\mu^* e^{ikx} \right)$$
$$a^{a\dagger}|0\rangle = |1^a\rangle$$
$$\text{Gauge boson} = \text{quantized excitation of } A_\mu^a$$
$$T^a \;\Rightarrow\; A_\mu^a \;\Rightarrow\; \text{Gauge Boson}_a$$
$$\dim(G) = N \Rightarrow N \text{ gauge bosons}$$
$$1 + 3 + 8 = 12$$
From covariant derivative:
$$\mathcal{L}_{int} = g \bar{\psi}\gamma^\mu T^a \psi A_\mu^a$$
Define current:
$$J^\mu_a = \bar{\psi}\gamma^\mu T^a \psi$$
Thus:
$$\mathcal{L}_{int} = g J^\mu_a A_\mu^a$$
Scattering amplitude:
$$\mathcal{M} = J^\mu_a \; D_{\mu\nu}^{ab}(k) \; J^\nu_b$$
Propagator:
$$D_{\mu\nu}^{ab}(k) = \frac{-i \delta^{ab} g_{\mu\nu}}{k^2 + i\epsilon}$$
$$\boxed{ \mathcal{L} = \bar{\psi}(i\gamma^\mu D_\mu - m)\psi -\frac{1}{4}F_{\mu\nu}^a F^{\mu\nu a} }$$
Dirac equation:
$$(i\gamma^\mu D_\mu - m)\psi = 0$$
Yang–Mills equation:
$$D^\mu F_{\mu\nu}^a = g J_\nu^a$$
$$D_\mu J^\mu_a = 0$$
$$G \;\Rightarrow\; T^a \;\Rightarrow\; A_\mu^a \;\Rightarrow\; \hat{A}_\mu^a \;\Rightarrow\; a^\dagger \;\Rightarrow\; \text{Bosons} \;\Rightarrow\; J^\mu A_\mu \;\Rightarrow\; \mathcal{L}$$
$$\text{Local symmetry} \;\Rightarrow\; \text{Gauge connection} \;\Rightarrow\; \text{Quantization} \;\Rightarrow\; \text{Bosons} \;\Rightarrow\; \text{Interaction}$$
$$\boxed{ \text{Gauge symmetry fully determines interaction carriers and interaction structure} }$$
$$\boxed{ \text{Symmetry} \;\rightarrow\; \text{Connection} \;\rightarrow\; \text{Quanta} \;\rightarrow\; \text{Interaction} \;\rightarrow\; \text{Closure} }$$
$$\boxed{ \text{Interactions are not introduced; they are inevitable consequences of symmetry quantization} }$$