Quantum entanglement and top spin correlations in SMEFT at higher orders

TL;DR

This paper analyzes top quark spin correlations and entanglement at the LHC within the Standard Model Effective Field Theory, incorporating higher-order QCD corrections to improve the accuracy of predictions and guide future measurements.

Contribution

It provides the first NLO analysis of top spin observables in SMEFT, identifying relevant operators and highlighting the importance of differential measurements for constraining new physics.

Findings

Higher-order corrections cause notable numerical differences from LO predictions.

Deviations from the SM are most prominent at large top transverse momentum.

Differential measurements enhance the sensitivity to SMEFT effects.

Abstract

We present the first analysis of top spin polarizations, $t \bar t$ spin correlations, and $t \bar t$ spin entanglement at the LHC in the context of the Standard Model Effective Field Theory, that goes beyond Leading Order QCD accuracy. The complete set of independent dimension-6 operators entering $t \bar t$ production is identified, and their effects on all top spin observables are extracted at linear and quadratic order in $c/\Lambda^2$. By comparing results at LO and NLO, we note that the inclusion of higher orders, while not dramatically changing the picture, often amounts to notable numerical differences, that are not fully captured by LO scale variation. We also find that the expected deviations from the SM have an intricate phase space structure, and show up predominantly at large top $p_T$. For this reason, we advocate for the measurement of spin observables differentially or doubly-differentially in the $t \bar t$ phase-space. We show how the inclusion of present and future top spin measurements will improve global fits to top LHC data, by also addressing the issue of flat directions.