Before EWSB you have four massless bosons: W+, W−, W0 and B0 The W0 and B0 mix to give the photon and Z0 The W+, W− and Z0 gain mass and the photon is left ma

Before EWSB you have four massless bosons: W+, W−, W0 and B0 The W0 and B0 mix to give the photon and Z0 The W+, W− and Z0 gain mass and the photon is left massless