Abstract
In this paper we study CP violation in photon self-interactions at low energy. These interactions, mediated by the effective operator FFFtilde{F} , where ( tilde{F} ) F is the (dual) electromagnetic field strength, have yet to be directly probed experimentally. Possible sources for such interactions are weakly coupled light scalars with both scalar and pseudoscalar couplings to photons (for instance, complex Higgs-portal scalars or the relaxion), or new light fermions coupled to photons via dipole operators. We propose a method to isolate the CP-violating contribution to the photon self-interactions using Superconducting Radio-Frequency cavities and vacuum birefringence experiments. In addition, we consider several theoretical and experimental indirect bounds on the scale of new physics associated with the above effective operator, and present projections for the sensitivity of the proposed experiments to this scale. We also discuss the implications of these bounds on the CP-violating couplings of new light particles coupled to photons.
Highlights
Effective CPC and CP violating (CPV) photon self-interactions beyond the Standard Model (BSM) can be indirectly constrained by the measured electronic magnetic dipole moment [10], and by the upper limit on the electronic electric dipole moment [12]
While inspired by the linear Fabry-Perot (FP) cavity of the PVLAS experiment [23], which is essentially insensitive to CP-odd effects, we show that a ring cavity geometry is sensitive to CPC and CPV photon interactions simultaneously, which can be distinguished by a temporal analysis of the signal
We considered the possibility that the photon is subject to CP violating (CPV) self-interactions, encoded in the low-energy effective operator FμνF μνFρσFρσ, with (Fμν)
Summary
At energies below the electron mass, interactions among photons are self-consistently described by an effective Lagrangian involving the photon field only. At energies above the electron mass, the electron must be included in the EFT, and the measurements of its electric and magnetic dipole moments put an indirect constrain on the contributions of new physics to photon self-interactions. Assuming C1 and C2 of order one, and considering the current bounds on |aexp − aSeM|/aSeM 10−9 [11] and |de| < 1.1 × 10−29 e × cm [12], we obtain a rough estimate for the bounds on the BSM contribution to the EFT coefficients as bBSM, cBSM 10−2 bEH , dBSM 10−8 bEH These are indirect bounds on bBSM, cBSM and dBSM that strongly constrain possible new physics heavier than the electron. At such energies the EFT coefficients for photon self-interactions, including their CPV part, have been constrained to be bBSM, cBSM, dBSM 10−10 GeV−4, see [34]
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