Automated Computation of Spin-Density Matrices Enables Collider Physics Analysis with Qubit and Qutrit States

Understanding the spin of particles created in high-energy collisions is crucial for testing the Standard Model of particle physics and searching for new phenomena, and now, Valentin Durupt, Fabio Maltoni, and Olivier Mattelaer, all from Université Catholique de Louvain, have developed a fully automated system to calculate these spin properties with unprecedented efficiency. Their new framework, implemented within the widely used \textsc{MadGraph5_aMC@NLO} software, automatically computes ‘spin-density matrices’ which describe the polarisation of particles produced in collisions. This achievement allows physicists to systematically quantify multi-particle correlations, opening up new avenues for precision measurements at the Large Hadron Collider and providing a powerful tool to probe the fundamental laws of nature. The team validated their system against known processes and then applied it to explore correlations in several LHC final states, demonstrating its potential for future discoveries.

Top Quark Production and Decay Studies

A comprehensive collection of research focuses on the production and decay of top quarks, a fundamental particle in high-energy physics. Studies encompass single top production, the creation of top-antitop pairs, and even the rarer four-top quark production, extending to associated production where top quarks appear alongside other particles like W bosons. A key area of research involves understanding the polarization and correlations of top quarks, including their intrinsic spin alignment. This work details the development and application of Monte Carlo event generators, sophisticated tools used to simulate particle collisions, relying on calculations at various levels of precision, including Next-to-Leading Order, and incorporating models of parton showers.

The research explores connections to precision tests of the Standard Model and emerging concepts in quantum information and entanglement. This research explores the potential for quantum entanglement in top quark production and decay, investigating whether these processes can create entangled states and if entanglement can enhance the precision of measurements. Researchers are exploring if the study of top quarks can provide insights into fundamental questions in quantum information theory and if entanglement can be used as a resource to improve Monte Carlo simulations. Furthermore, the research aims to improve the precision of measurements of top quark properties, such as mass, width, and spin correlations, by developing more accurate Monte Carlo simulations and reducing systematic uncertainties in experimental measurements. This system, integrated within the MG5aMC computational environment, provides a general-purpose tool capable of constructing these matrices for any tree-level scattering process. The framework outputs event-by-event data in a standardized format, facilitating the analysis of quantum observables, and supports diverse final states, including systems with qubits and qutrits, accommodating both polarized and unpolarized initial conditions. Experiments revealed the ability to systematically quantify multi-particle quantum correlations, utilizing a dedicated library of quantum observables, including purity, concurrence, and entanglement of formation for qubits, alongside measures like negativity and “magic” for characterizing quantum entanglement. The framework enables the computation of these measures directly from the produced density matrices, providing a comprehensive toolkit for exploring quantum phenomena in particle collisions.

Automated Spin Density Matrix Calculation Framework

This research presents a fully automated framework for calculating spin-density matrices, which describe the quantum spin state of particles produced in high-energy collisions. By building upon existing tools, the team developed a system capable of constructing these matrices for a wide range of scattering processes at the tree level, outputting the data in a standard file format for further analysis. This represents a significant advance as previous calculations were often process-specific and required substantial manual effort. The implementation supports various particle configurations, including those with multiple qubits and qutrits, and allows for configurable reference frames and initial polarisation states.

The team validated the framework against known results for top quark and vector boson production, demonstrating its accuracy, and then applied it to explore correlations in several LHC final states. This work enables systematic quantification of multi-particle correlations and provides a foundation for studying quantum phenomena in collider physics, potentially offering insights into both Standard Model dynamics and physics beyond it. Future work will likely focus on incorporating higher-order quantum effects to improve the precision of the calculations.