Single Transverse Spin Asymmetry as a New Probe of SMEFT Dipole Operators

TL;DR

This paper proposes using single transverse spin asymmetries at future electron-positron colliders as a novel method to more effectively probe SMEFT dipole operators, especially for constraining electron dipole couplings and potential CP violation.

Contribution

It introduces a new approach leveraging transverse spin asymmetries to improve constraints on SMEFT dipole operators, overcoming interference limitations in traditional observables.

Findings

Potential to improve constraints on electron dipole couplings by 10-100 times.

Ability to simultaneously constrain real and imaginary parts of Wilson coefficients.

Provides a new avenue to explore CP-violating effects in SMEFT.

Abstract

Electroweak dipole operators in the Standard Model Effective Field Theory (SMEFT) are important indirect probes of quantum effects of new physics beyond the Standard Model (SM), yet they remain poorly constrained by current experimental analyses for lack of interference with the SM amplitudes in constructing cross section observables. In this Letter, we point out that dipole operators flip fermion helicities so are ideally studied through single transverse spin asymmetries. We illustrate this at a future electron-positron collider with transversely polarized beams, where such effect exhibits as azimuthal $\cos\phi$ and $\sin\phi$ distributions which originate from the interference of the electron dipole operators with the SM and are linearly dependent on their Wilson coefficients. This new method can improve the current constraints on the electron dipole couplings by one to two orders of magnitude, without depending on other new physics operators, and can also simultaneously constrain both their real and imaginary parts, offering a new opportunity for probing potential $CP$-violating effects.