On the sensitivity of the D parameter to new physics

TL;DR

This paper uses effective field theory to analyze the D correlation parameter in nuclear beta decay, showing that potential new physics effects are tightly constrained by existing experimental data and that significant deviations are unlikely without fine tuning.

Contribution

It provides a comprehensive classification of CP-violating EFT scenarios affecting the D parameter and assesses their compatibility with current experimental bounds.

Findings

Shifts larger than 10^{-5} in D are in tension with data.

Electric dipole moments and LHC bounds strongly constrain new physics.

Fine tuning is required to evade these constraints.

Abstract

Measurements of angular correlations in nuclear beta decay are important tests of the Standard Model (SM). Among those, the so-called D correlation parameter occupies a particular place because it is odd under time reversal, and because the experimental sensitivity is at the $10^{-4}$ level, with plans of further improvement in the near future. Using effective field theory~(EFT) techniques, we reassess its potential to discover or constrain new physics beyond the SM. We provide a comprehensive classification of CP-violating EFT scenarios which generate a shift of the D parameter away from the SM prediction. We show that, in each scenario, a shift larger than $10^{-5}$ is in serious tension with the existing experimental data, where bounds coming from electric dipole moments and LHC observables play a decisive role. The tension can only be avoided by fine tuning of the parameters in the UV completion of the EFT. We illustrate this using examples of leptoquark UV completions. Finally, we comment on the possibility to probe CP-conserving new physics via the D parameter.