In QED, electrically charged are coupled by a uncharged, massless vector boson called the photon (and identified as a quantum of light).

By adding three additional, massive vector bosons (the $Z^0$ and $W^{\pm}$) which couple to the weak hyper-charge ($T_3 - q \ \sin^2 \theta_W$ in which $q$ is electric charge and $T_3$ is the third component of the weak isospin), the theory can be extended to cover the weak nuclear force as well.

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