The analytic structure of amplitudes on backgrounds from gauge invariance and the infra-red

Understand

Gauge invariance and soft limits can be enough to determine the analytic structure of scattering amplitudes in certain theories.

• This prompts the question of how gauge invariance is connected to analytic structure in more general theories.
• Here we focus on QED in background plane waves.
• We show that imposing gauge invariance introduces new virtuality poles into internal momenta on which amplitudes factorise into a series of terms.