Odd Physics Off the Diagonal: Constraining CP-violating SMEFT with Quantum Tomography

TL;DR

This paper introduces a novel quantum tomography method to better constrain CP-violating effects in SMEFT, surpassing traditional angular observables by reconstructing the spin density matrix of diboson systems.

Contribution

It presents a new approach using quantum tomography to reconstruct the spin density matrix, enabling improved sensitivity to CP-violating effects in SMEFT beyond interference-based methods.

Findings

Enhanced sensitivity to CP-odd and CP-even SMEFT contributions.

Ability to detect pure quadratic NP terms.

Outperforms traditional angular observables in constraining CP violation.

Abstract

New sources of charge-parity (CP) violation beyond those described in the Standard Model (SM) are required to explain the observed matter--antimatter asymmetry of the Universe. The Standard Model Effective Field Theory (SMEFT) provides a framework to introduce additional electroweak sources of CP-odd physics in a model-independent manner. However, these CP-violating signatures are mostly degenerate to CP-even SMEFT operators in polarisation-blind observables, distinguishable only in the SM-New Physics (NP) interference using the azimuthal decay angle. Using Quantum Tomography techniques, we present a new approach to constraining these NP effects. Reconstructing the spin density matrix (SDM) of a diboson system, we go beyond `interference resurrection' to exploit the full signature of the Beyond-SM physics, including the pure quadratic NP terms. We show that this approach provides superior simultaneous sensitivity to characteristic features of CP-even and CP-odd contributions, including effects not fully captured by traditional angular observables.