Charge et gravité : un nouveau paramètre κ et la frontière inexplorée du principe d'équivalence ✛ ✦

Résumé original

tel que rédigé par les auteurs, en anglais arXiv:2605.12246v2

We introduce and define the phenomenological parameter $$\kappa$$, defined by $$\Delta a/g = \kappa \, \Delta(q/m)$$, to quantify potential linear coupling between electric charge and gravitational acceleration. A synthesis of existing precision equivalence principle experiments yields the first quantitative estimate of the effective sensitivity to this coupling: $$|\kappa| < 2.1 \times 10^{-4}$$ kg/C at 95\% confidence level. This constraint is approximately eleven orders of magnitude less stringent than corresponding bounds on composition-dependent violations, revealing that the electromagnetic axis remains a largely underexplored frontier in empirical gravity. We connect $$\kappa$$ to established frameworks -- the Standard-Model Extension and the $$TH\epsilon\mu$$ formalism -- showing that it occupies a region of parameter space untouched by existing high-precision tests. An effective field theory analysis shows that dimension-six operators that couple curvature directly to the electromagnetic field strength are suppressed by the minuscule terrestrial spacetime curvature ($$G_N \rho_\oplus \sim 10^{-55}~\text{GeV}^2$$) and are therefore phenomenologically irrelevant. Consequently, a future measurement of $$\kappa$$ at an accessible level would not probe minimal geometric couplings but would signal physics beyond minimal gravitational EFT, such as mediation by light scalar fields as in Einstein-Maxwell-Dilaton theory. We examine the Schiff-Barnhill effect, the primary systematic background for any such measurement, and show how it can be separated from a genuine signal. We outline the necessary experimental strategy, focused on maximizing charge-to-mass ratio differences, to transform this overlooked axis into a targeted probe for new physics.