Soft Scalars in Effective Field Theory

Understand

We derive a soft theorem for a massless scalar in an effective field theory with generic field content using the geometry of field space.

• This result extends the geometric soft theorem for scalar effective field theories by allowing the massless scalar to couple to other scalars, fermions, and gauge bosons.
• The soft theorem keeps its geometric form, but where the field-space geometry now involves the full field content of the theory.
• As a bonus, we also present novel double soft theorems with fermions, which mimic the geometric structure of the double soft theorem for scalars.