No abstract

In this work, we propose a CPT-even and Lorentz-violating dimension-five nonminimal coupling between fermionic and gauge fields, involving the CTP-even and Lorentz-violating gauge tensor of the SME. This nonminimal coupling modifies the Dirac equation, whose nonrelativistic regime is governed by a Hamiltonian which induces new effects, such as an electric-Zeeman-like spectrum splitting and an anomalous-like contribution to the electron magnetic moment, between others. Some of these new effects allows to constrain the magnitude of this nonminimal coupling in 1 part in 10 16 .

We propose a new CPT -even and Lorentz-violating nonminimal coupling between fermions and Abelian gauge fields involving the CPT -even tensor (KF ) µναβ of the standard model extension. We thus investigate its effects on the cross section of the electron-positron scattering by analyzing the process e + + e − → µ + + µ − . Such a study was performed for the parity-odd and parity-even nonbirefringent components of the Lorentz-violating (KF ) µναβ tensor. Finally, by using experimental data available in the literature, we have imposed upper bounds as tight as 10 −12 (eV) −1 on the magnitude of the CPT -even and Lorentz-violating parameters while nonminimally coupled.

In this letter we show for the first time that the usual CPT-even gauge term of the standard model extension (SME), in its full structure, can be radiatively generated, in a gauge invariant level, in the context of a modified QED endowed with a dimension-five nonminimal coupling term recently proposed in the literature. As a consequence, the existing upper bounds on the coefficients of the tensor (KF ) can be used improve the bounds on the magnitude of the nonminimal coupling, λ (KσF ) , by the factors 10 5 or 10 25 . The nonminimal coupling also generates higher-order derivative contributions to the gauge field effective action quadratic terms.

In this paper, we investigate an electrodynamics in which the physical modes are coupled to a Lorentz-violating (LV) background by means of a higher-derivative term. We analyze the modes associated with the dispersion relations (DRs) obtained from the poles of the propagator. More specifically, we study Maxwell's electrodynamics modified by a LV operator of mass dimension 6. The modification has the form D βα ∂σF σβ ∂ λ F λα , i.e., it possesses two additional derivatives coupled to a CPT -even tensor D βα that plays the role of the fixed background. We first evaluate the propagator and obtain the dispersion relations of the theory. By doing so, we analyze some configurations of the fixed background and search for sectors where the energy is well-defined and causality is assured. A brief analysis of unitarity is included for particular configurations. Afterwards, we perform the same kind of analysis for a more general dimension-6 model. We conclude that the modes of both Lagrange densities are possibly plagued by physical problems, including causality and unitarity violation, and that signal propagation may become physically meaningful only in the high-momentum regime.