QED positivity bounds
Lasma Alberte (Theoretical Physics, Blackett Laboratory, Imperial College, London SW7 2AZ, United Kingdom); Claudia de Rham (CERCA, Department of Physics, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, USA, Theoretical Physics, Blackett Laboratory, Imperial College, London SW7 2AZ, United Kingdom); Sumer Jaitly (Theoretical Physics, Blackett Laboratory, Imperial College, London SW7 2AZ, United Kingdom); Andrew J. Tolley (CERCA, Department of Physics, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, USA, Theoretical Physics, Blackett Laboratory, Imperial College, London SW7 2AZ, United Kingdom)
We apply positivity bounds directly to a gauge theory with charged scalars and charged fermions, i.e., QED, minimally coupled to gravity. Assuming that the massless -channel pole may be discarded, we show that the improved positivity bounds are violated unless new physics is introduced at the parametrically low scale , consistent with similar results for scalar field theories, far lower than the scale implied by the weak gravity conjecture. This is sharply contrasted with previous treatments which focus on the application of positivity bounds to the low energy gravitational Euler-Heisenberg effective theory only. We emphasize that the low cutoff is a consequence of applying the positivity bounds under the assumption that the pole may be discarded. We conjecture an alternative resolution that a small amount of negativity, consistent with decoupling limits, is allowed and is not in conflict with standard UV completions, including weakly coupled ones.