$CP$ violation beyond the Standard Model (SM) is a crucial missing piece for explaining the observed matter-antimatter asymmetry in the Universe. Recently, the ATLAS experiment at the Large Hadron Collider performed an analysis of electroweak $Zjj$ production, thereby excluding the SM locally at 95% confidence level in the measurement of $CP$-sensitive observables. We take the excess interpretation in terms of anomalous gauge-Higgs interactions at face value and discuss further steps that are required to scrutinize its origin. In particular, we discuss the relevance of multiboson production using adapted angular observables to show how they can be used to directly tension the reported $Zjj$ excess in a more comprehensive analysis. To connect the excess to a concrete UV scenario for which the underlying assumptions of the $Zjj$ analysis are valid, we identify vectorlike leptons as a candidate theory consistent with the observed $CP$-odd Wilson coefficient hierarchy observed by ATLAS. We perform a complete one-loop matching calculation to motivate further model-specific and correlated new physics searches. In parallel, we provide estimates of the sensitivity reach of the LHC’s high luminosity phase for this particular scenario of $CP$ violation in light of electroweak precision and Run-2 Higgs data. These provide strong constraints on the model’s $CP$-even low-energy phenomenology, but also inform the size of the $CP$-odd SM deformation indirectly via our model hypothesis.

• Received 6 October 2020
• Accepted 3 February 2021

DOI:https://doi.org/10.1103/PhysRevD.103.055008

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP³.

Published by the American Physical Society