# Constraining new physics in entangled two-qubit systems: top-quark, tau-lepton and photon pairs

**Authors:** Marco Fabbrichesi, Roberto Floreanini, Emidio Gabrielli  
**arXiv:** [2208.11723](https://arxiv.org/abs/2208.11723)  
**Date:** 2022-08-24  
**Summary page:** https://www.summarizepaper.com/en/arxiv-id/2208.11723v1/

## Key points

- Quantum entanglement as a sensitive probe for physics beyond the Standard Model
- Use of concurrence to measure entanglement in processes involving top-quark pairs, tau-lepton pairs, and photon pairs
- Entanglement in production cross sections of top-quark and tau-lepton pairs provides effective constraints on magnetic dipole term in coupling between top quarks and gluons
- Entanglement measurements used to distinguish between CP-even and CP-odd couplings in tau-lepton pair spin states
- Analysis of spin states from Higgs boson decay to discern between CP-even and odd couplings
- Final states in Higgs boson decays remain maximally entangled even with presence of CP-odd couplings, limiting their use for setting bounds on new physics
- Violations of Bell inequalities observed across all four processes studied, with violations in Higgs boson decays into tau leptons or two photons showing promise for probing new physics.

## Summary

In their paper titled "Constraining new physics in entangled two-qubit systems: top-quark, tau-lepton and photon pairs," authors Marco Fabbrichesi, Roberto Floreanini, and Emidio Gabrielli explore the measurement of quantum entanglement as a sensitive probe for physics beyond the Standard Model. They investigate the use of concurrence, a measure of entanglement, in various processes involving top-quark pairs, tau-lepton pairs, and photon pairs. The authors first focus on the production of top-quark pairs at colliders. By examining the spin states of these particles, they aim to constrain the magnetic dipole term in the coupling between top quarks and gluons. The results demonstrate that differences in entanglement in the production cross sections of top-quark and tau-lepton pairs can provide more effective constraints compared to previous methods such as total cross sections or classical correlations. Next, they turn their attention to tau-lepton pair spin states to bound contact interactions. Through entanglement measurements, they aim to distinguish between CP-even and CP-odd couplings. This analysis showcases both the power and limitations of using entanglement as a tool for probing new physics. The authors also analyze spin states from the decay of the Higgs boson. They examine both tau-lepton pair spin states and two-photon spin states to discern between CP-even and odd couplings. Interestingly, they find that even in the presence of CP-odd couplings, final states in Higgs boson decays remain maximally entangled; thus these decays cannot be utilized to set bounds on new physics. Furthermore, they discuss violations of Bell inequalities across all four processes studied; observing violations in Higgs boson decays into tau leptons or two photons is particularly promising for this purpose. Overall, this research demonstrates how measuring quantum entanglement can serve as a powerful and sensitive probe for physics beyond the Standard Model by providing valuable insights into its potential for constraining various couplings and interactions while also highlighting its limitations and challenges associated with this approach.