CP violation at ATLAS in effective field theory

Abstract

CP violation beyond the Standard Model (SM) is a crucial missing piece for explaining the observed matter-antimatter symmetry in the Universe. Recently, the ATLAS experiment at the Large Hadron Collider has 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 multi-boson 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 vector-like 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.