" Contracting the external momentum of one of these particles with the scattering amplitude of the rest of the system gives zero according to the Ward identity.
The propagator for this particle is "negative i times the metric, over momentum squared." Contracting the external momentum of one of these particles with the scattering amplitude of the rest of the system gives zero according to the Ward identity. OneÂ­loop corrections to this particle's propagator lead to vacuum polarization. These (*) spinÂ­one particles have a helicity of either plus or minus hÂ­bar, and they are represented by wavy lines on Feynman diagrams. Virtual ones are exchanged during electronÂ­electron scattering, while real ones are emitted in bremsstrahlung. These particles are the gauge bosons of a U(1) gauge interaction described to great precision by QED. For 10 points, name these massless quanta of light that mediate the electromagnetic force. 
The propagator for this particle is "negative i times the metric, over momentum squared.
photons