13 TeV Proton-Proton Collisions Advance Understanding of Top Quark Pair Spin States

The fundamental properties of top quarks, the heaviest known elementary particles, continue to challenge our understanding of particle physics, and researchers are meticulously examining their behaviour to test the Standard Model. The CMS Collaboration at the European Organization for Nuclear Research (CERN), led by scientists including those from the CMS experiment, now presents detailed measurements of the quantum state of top quark pairs created in high-energy proton-proton collisions. These measurements, obtained from data collected between 2016 and 2018, focus on spin correlations and allow physicists to decompose the top quark system into its fundamental quantum states, revealing properties such as purity and entropy. Importantly, the results confirm predictions made by the Standard Model, strengthening our current understanding of these elusive particles and providing a foundation for future, more precise investigations.

The top quark system is reconstructed from events containing electrons or muons and jets, utilising a comprehensive dataset equivalent to 138 fb−1 of integrated luminosity. These measurements, alongside previously published results in the helicity basis, allow scientists to decompose the top quark system into distinct Bell and spin eigenstates across various kinematic regions. The spin correlation coefficients provide valuable insights into the properties of the top quark quantum state, including its purity, von Neumann entropy, and degree of entanglement. These measurements, performed in the beam basis, complement earlier results obtained using the helicity basis, enabling a comprehensive decomposition of the top quark system into Bell and spin eigenstates across different kinematic regions. The research team boosted top quarks and their decay products into defined rest frames to accurately analyse spin correlations. Experiments reveal that the spin correlation matrix in the beam basis is diagonal, with C⊥ = Cxx = Cyy and C∥ = Czz, indicating specific symmetries around the beam axis and between incoming protons.

Analysis of the density matrix, decomposed using the Fano-Bloch formalism, allows characterisation of the quantum state of the top quark pair. Researchers determined the contributions from singlet and triplet spin states, providing insights into the underlying production mechanisms, which are predominantly gluon-gluon fusion, supplemented by approximately 10% quark-antiquark annihilation. Furthermore, the study quantifies the purity of the top quark state, a measure of its mixedness, and calculates the von Neumann entropy, characterising the uncertainty inherent in the quantum state. The results demonstrate the ability to probe the quantum properties of heavy particles and provide a foundation for future investigations into the fundamental nature of matter.

Top Quark Spin Correlations Confirmed Precisely

Measurements of top quark-antiquark systems, produced in high-energy proton-proton collisions, extend current understanding of fundamental particle interactions. These measurements were performed using data collected over several years and represent a significant increase in precision compared to previous studies. The team successfully decomposed the system into its fundamental spin states, both in a commonly used “helicity” basis and, for the first time, in a “beam” basis.

This analysis provides experimental validation of theoretical predictions within the Standard Model of particle physics, confirming the expected behaviour of top quarks. Importantly, the team quantified entanglement, a quantum mechanical phenomenon where particles become linked, within the top quark-antiquark system. Evidence of entanglement was found in both the helicity and beam bases, with a particularly strong signal observed at high energies and specific scattering angles. The researchers also calculated key properties of the system, including purity, von Neumann entropy, and entanglement markers, all consistent with Standard Model expectations.