Long-range parity-violating forces from two-photon exchange

We explore a novel radiative correction to electron-nucleon parity-violating NC interaction that appears via an exchange of two photons in presence of hadronic parity violation (HPV) interactions. While the exchange of a single photon in presence of HPV only gives rise to the purely short-range anapole moment, the two-photon exchange is long-range - indeed longer range than the tree-level Z-boson exchange thus endangering the definition of the weak charge of the nucleon. We demonstrate that upon accounting for general properties of the forward parity-violating Compton amplitude it can be shown that this definition is protected by an exact sum rule that forces the long-range effects vanish in the limit of zero electron energy. For the first time we successfully check the validity of this sum rule explicitly in relativistic ChPT at one-loop level. We estimate the importance of this novel correction in a model containing pions and the Delta-1232 resonance for existing and upcoming experiments, and discuss further possibilities.

Reference: https://arxiv.org/abs/1608.07484

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