The analysis yielded strong evidence that they did. The result extends a 2023 ATLAS measurement that demonstrated entanglement between pairs of top quarks, the heaviest known elementary particle.

Why is a philosopher on the team?

Study co-author Professor Alan Barr, of Oxford's Department of Physics, was among the first scientists to propose using particle colliders to study entanglement at unprecedented energies. He helped build the LHC decades ago.

Barr said: "We're used to thinking of entanglement as something delicate, seen in laboratory experiments with single photons. Finding it alive and well among particles as heavy and short-lived as Z bosons, created in some of the most violent collisions we can produce on Earth, shows just how fundamental and robust this quantum effect really is."

"It's a nice reminder that the same strange rules of quantum mechanics that may one day power quantum computers are at work everywhere in nature, even at the extreme energies of the Large Hadron Collider," he added.

At Oxford, Barr co-leads an interdisciplinary project that pairs particle physicists with philosophers of physics to examine what these results say about the foundations of quantum mechanics.

Project co-PI Professor Chris Timpson, of Oxford's Faculty of Philosophy, said: "Entanglement is both the most promising and the most puzzling aspect of quantum reality; these collider experiments detecting entanglement present a new frontier in investigations of the foundations of quantum mechanics."

The work reflects a wider push to bring quantum-information tools into high-energy physics. Borrowing analytical techniques from quantum computing, researchers hope to sharpen searches for unknown particles or subtle effects that do not fit current theory.

What comes next at the LHC?

Oxford researchers are building components for an upgrade to the ATLAS pixel detector. Combined with the High-Luminosity LHC upgrade now under way, the new hardware is expected to produce vastly larger data sets and let physicists investigate quantum phenomena with greater precision.

"This measurement demonstrates the scientific power of the ATLAS collaboration and the unique capabilities of CERN's Large Hadron Collider," said Professor Daniela Bortoletto, of Oxford's Department of Physics and the UK coordinator for the upgraded ATLAS pixel system.

"Oxford researchers have played a leading role in developing these new approaches to studying quantum phenomena at the highest energies, and we are proud to contribute to an international effort that is opening new ways to explore the fundamental laws of nature," she added.

How solid is the evidence?

The ATLAS team describes its findings as "strong evidence" rather than a definitive discovery, and statistical uncertainties remain. The techniques used to reconstruct spin correlations also rely on detailed modeling of how Z bosons decay inside the detector, so the conclusions depend partly on those simulations.

The full study, "Measurements of Z-boson pair entanglement in decays of Higgs bosons at the ATLAS experiment," appears in Physical Review Letters (DOI: 10.1103/y1nh-1b82).