Quantum properties of heavy-fermion pairs at a lepton collider with polarised beams

TL;DR

This paper explores the quantum properties of heavy-fermion pairs produced in polarized lepton collisions, revealing how beam polarization enhances the detection of non-standard interactions and provides new insights into top-quark physics.

Contribution

It introduces a detailed analysis of spin correlations and quantum measures in heavy-fermion pair production, including new analytic expressions and the impact of beam polarization on sensitivity to new physics.

Findings

Beam polarization enriches spin configuration analysis.

Quantum observables improve sensitivity to non-standard interactions.

Enhanced constraints on new physics scales from top-quark studies.

Abstract

We investigate the quantum properties of heavy-fermion pairs, such as $t\bar t$ or $\tau^+\tau^-$, produced in lepton-lepton collisions with polarised beams. Focusing on spin correlations, entanglement, Bell-inequality violation, and quantum-information-theoretic measures such as purity and magic, we analyse how beam polarisation shapes the structure of the spin-density matrix. We derive analytic expressions for a wide range of helicity configurations, including both Standard Model contributions and generic new-physics effects parametrised by scalar, vector, and tensor four-fermion operators within an effective field theory framework. We show that beam polarisation unlocks a substantially richer set of spin configurations and significantly enhances sensitivity to non-standard interactions. As a phenomenological application, we study $t\bar t$ production at a future linear collider and demonstrate that quantum observables provide a comprehensive and complementary probe of top-quark interactions and stronger constraints on the scale of new physics.