# Asymmetry Observables for Measuring Spin Correlations in Top-Quark Pair Production

## TL;DR

This paper introduces a comprehensive set of asymmetry observables to measure all parameters of the top-quark pair spin density matrix, enhancing the search for new physics beyond the standard model.

## Contribution

It provides a novel, frame-independent method to fully characterize top-quark spin correlations using asymmetry observables, applicable across different production mechanisms.

## Key findings

- Defines observables applicable in any reference frame.
- Enables identification of optimal frames for sensitivity.
- Facilitates testing of correlations among density matrix elements.

## Abstract

Spin correlations between top-quark and anti-top-quark in pair production are sensitive to physics beyond the standard model. A complete set of observables, that can be used to measure all independent parameters of the production spin density matrix, is necessary to search for anomalous interactions in the production dynamics. In this letter, we provide a set of asymmetry observables for measuring all the density matrix elements. These observables are defined regardless of the production mechanism, and can be specified in any desired reference frame, particularly the laboratory frame in which the first evidence of spin correlation was observed experimentally. Our method can be used to find an optimal Lorentz frame in which the sensitivity to the density matrices is high for given production process and experimental environment, or to test possible correlations among the matrix elements.