Quantum entanglement and Bell inequality violation at colliders

Abstract: The study of entanglement in particle physics has been gathering pace in the past few years. It is a new field that is providing important results about the possibility of detecting entanglement and testing Bell inequality at colliders for final states as diverse as top-quark, $\tau$-lepton pairs and $\Lambda$-baryons, massive gauge bosons and vector mesons. In this review, after presenting definitions, tools and basic results that are necessary for understanding these developments, we summarize the main findings -- as published by the beginning of year 2024 -- including analyses of experimental data in $B$ meson decays and top-quark pair production. We include a detailed discussion of the results for both qubit and qutrits systems, that is, final states containing spin one-half and spin one particles. Entanglement has also been proposed as a new tool to constrain new particles and fields beyond the Standard Model and we introduce the reader to this promising feature as well.

• The paper demonstrates that collider experiments can reveal quantum entanglement and significant Bell inequality violations, notably in top-quark pair production at high scattering angles.
• It applies precise spin correlation measurements and quantum state tomography across varied particle systems such as Λ baryons and B-meson decays.
• Monte Carlo simulations support its findings, indicating that enhanced detector efficiency at colliders can effectively test physics beyond the Standard Model.

Exploring Quantum Entanglement and Bell Inequality Violation at Colliders

The paper "Quantum entanglement and Bell inequality violation at colliders," authored by Alan J. Barr, Marco Fabbriches, Roberto Floreanini, Emidio Gabrielli, and Luca Marzola, dives into the burgeoning field of entanglement in particle physics, with a specific focus on assessing quantum entanglement and testing Bell inequalities at colliders. It reviews studies on B-meson decays, top-quark pair production, and discusses using entanglement as a tool to probe physics beyond the Standard Model (SM).

Overview of Key Concepts

The paper underscores the core aspects of quantum mechanics such as entanglement and Bell inequality. Entanglement arises from the inseparability of quantum states in systems that have interacted and then separated, yielding non-classical correlations even over large distances. Bell inequalities, derived by physicist J. S. Bell, provide a framework to determine whether observed correlations can be explained through local realism, a classical worldview, or entail nonlocal quantum effects.

Collider Physics and Entanglement

Collider physics has historically leveraged quantum field theory (QFT) primarily for understanding particle interactions and decay processes. However, here, the authors extend this framework by evaluating entanglement and Bell nonlocality in collider-produced systems. This endeavor necessitates precise spin correlation measurements of final states after particle production and decay processes, termed quantum state tomography.

Numerical and Experimental Insights

The analysis includes systems like qubits and qutrits—specifically addressing entangled Λ baryons, top-quark pairs, τ-lepton pairs via Drell-Yan production, Higgs boson decay processes (h → ττ¯, h → γγ), B-meson decays to vector mesons, and diboson production. Each particle system is explored for potential Bell inequality violations, with particular attention given to specific ranges of kinematic variables where such violations might manifest significantly.

For instance, in top-quark pair production at the LHC, significant insights are drawn about the entanglement at maximum thresholds and the Bell inequality violations at high scattering angles. Monte Carlo simulations support these findings, indicating measurable entanglement and potential Bell violations at existing and upcoming LHC data volumes.

Potential Implications and Future Directions

Entanglement in collider experiments opens new avenues for rigorous tests of QFT and provides a new lens to probe deviations from SM predictions. Importantly, entangled states offer enhanced sensitivity to new physics, which may reveal insights into phenomena such as gluon dipole moments or τ-lepton electromagnetic properties. This potential is crucial for advancements in particle physics, providing a reliable probe for SM extensions or undiscovered interactions.

Additionally, overcoming loopholes akin to those historically addressed in low-energy quantum mechanics, such as detection and locality loopholes, is imperative. Current collider setups offer high detector efficiency which could potentially close the detection loophole. However, closing the locality loophole requires careful kinematic vetting to avoid space-like separations.

Conclusion

The research highlights significant theoretical and experimental strides in applying fundamental quantum mechanics principles like entanglement to high-energy physics at colliders. The potential to test Bell inequalities offers a richly layered opportunity to explore beyond-quantum theories and understand deeper the quantum correlations transcending particle processes. These results could spur further analyses and methodology enhancements, signifying an exciting frontier in quantum collider physics.