Probing Quantum Entanglement in Higgs to ZZ to Four Lepton Processes in pp Collisions with the ATLAS Experiment

As the only fundamental scalar particle in the Standard Model, the Higgs boson provides a unique opportunity to explore the quantum properties of fundamental interactions. We investigate the quantum correlations between the two Z bosons produced in Higgs decay to four leptons, using data recorded by the ATLAS detector at the LHC. The analysis exploits lepton angular distributions to probe elements of the spin density matrix that describes the ZZ* system, with the goal of assessing whether the system exhibits quantum entanglement. Results based on the full Run-2 and partial Run-3 ATLAS data sets are also interpreted in the context of different polarization hypotheses. This study represents a novel approach to probing quantum entanglement between fundamental particles in a bipartite qutrit system at the highest energies.