Quantum entanglement describes a connection between particles that share a common origin. In an entangled state, the properties of the particles cannot be described independently, even when they are separated by substantial distances or time. Measuring a property of one particle can therefore reveal information about its partner. Albert Einstein famously referred to this aspect of quantum mechanics as “spooky action at a distance.” The effect is a cornerstone of proposed quantum technologies, including processors that manipulate multiple qubits together and communication systems designed to use quantum states to protect information.

The new measurement focused on a specific particle-physics process. When a Higgs boson decays, it can briefly produce two Z bosons, each of which subsequently decays into a pair of electrons or muons. The researchers examined the angular relationships between the visible particles left behind by those decays. Because the Z bosons exist for only a fraction of a second, their properties cannot be measured directly with a detector. Instead, the ATLAS collaboration reconstructed the directions in which the decay products were emitted and used those angles to infer information about the spins of the original Z bosons.

That reconstruction allowed the team to test whether the two parent Z bosons possessed the joint quantum state required for entanglement. The analysis produced strong evidence that they did. The experiment therefore demonstrates that a quantum connection between the decay products is present in the selected Higgs decay channel. It is not simply a comparison of separate Z bosons, nor does it rely on their spatial separation after leaving the collision point. The test concerns correlations encoded in the spins of two particles that originated together in a highly energetic collision.

The setting is among the most demanding in which entanglement has been sought. Protons were accelerated to 99.99 percent of the speed of light and brought together in collisions with energies of 13 trillion electron volts. Their collisions can create the Higgs boson, which is itself short-lived and decays almost immediately. The subsequent Z bosons are also fleeting, but they leave detectable electrons or muons whose momentum and emission angles can be measured with high precision by the ATLAS detector. This chain of short-lived intermediate particles links the quantum state under examination to observable final-state particles.

Oxford’s Alan Barr, a co-author of the study and a professor in the Department of Physics, had previously proposed that particle colliders could investigate entanglement at energies far beyond those accessible in more conventional laboratory experiments. He also contributed to the construction of the Large Hadron Collider and recognized that its collecting system could serve purposes beyond the discovery of new particles. His earlier ideas informed a 2023 ATLAS experiment that demonstrated entanglement between pairs of top quarks, the heaviest known elementary particles. The Z-boson result builds on that work by examining a different pair of particles and a different production process.

Barr said the new finding showed how robust the effect can be. “We’re used to thinking of entanglement as something delicate, seen in laboratory experiments with single photons,” he said. “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.” He added that the result was a reminder that the rules of quantum mechanics underlying proposed quantum computers operate at the extreme energies reached by the collider as well as in carefully controlled laboratories.

The study also illustrates an effort to bring tools and concepts from quantum information science into particle physics. Collider physicists are using quantum-state measurements to extract information that may be difficult to recognize through conventional event analyses alone. The release said this cross-pollination could support more sensitive methods for analyzing collider data and potentially reveal subtle effects associated with physics beyond the current understanding of the universe. That possibility remains a prospect of the research program rather than a demonstrated result of the present measurement, which concerns the observation of entanglement itself.

At Oxford, Barr co-leads an interdisciplinary project examining the foundations of quantum mechanics at high energies. The project combines empirical measurements with questions about what those measurements imply for the fundamental nature of reality. Oxford philosopher Chris Timpson, a project co-principal investigator, 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 involvement of both physics and philosophy reflects the unusually broad interpretation of the work, which treats a collider result as both a precise particle-physics measurement and an opportunity to probe the conceptual structure of quantum theory.

Oxford researchers are also contributing to the ongoing upgrade of the ATLAS detector, including work on modules for its pixel system. The detector upgrade, together with the planned High-Luminosity Large Hadron Collider, is intended to provide larger data sets and improved capabilities for studying quantum phenomena in collider events. The present paper is a peer-reviewed measurement rather than a demonstration of a practical quantum-computing or communication system, and it does not report entanglement between arbitrary Z bosons. Its scope is the specified Higgs-boson decay channel studied by ATLAS. Even so, by finding a strong quantum signature among particles created at 13 teraelectron-volts, the experiment establishes that entanglement remains observable in an exceptionally energetic and fleeting environment.

Subject of Research: Experimental evidence of quantum entanglement between Z-boson pairs produced in Higgs boson decays at the ATLAS experiment

Article Title: Oxford physicists help uncover ‘spooky’ quantum effect in the Large Hadron Collider

Article References: Oxford physicists help uncover ‘spooky’ quantum effect in the Large Hadron Collider. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: Quantum entanglement, ATLAS experiment, Z bosons, Higgs boson, Large Hadron Collider, CERN, Quantum information, Particle physics, Quantum foundations, Oxford, physicists, help

Quantum black holes are a speculative consequence of combining general relativity and quantum mechanics. Under the scenario investigated by the CMS researchers, sufficiently energetic proton collisions could compress energy into a region small enough for spacetime to form a tiny black hole if hypothetical extra spatial dimensions exist. Such an object would not be a stable, macroscopic black hole. Instead, it would be expected to decay almost immediately into particles. Physicists could therefore search for its existence indirectly by identifying the pattern of those decay products, rather than by observing a black hole directly. The idea was proposed decades ago, but early ATLAS and CMS searches used comparatively small datasets and found no evidence for the predicted signal.

The scientific motivation extends beyond the black-hole search itself. Quantum field theory, represented by the Standard Model, describes particles and interactions with exceptional success, while general relativity describes gravity on large scales. Combining them into one theory remains a central challenge because gravity becomes especially important where particles are extremely small and energetic. One longstanding puzzle is the hierarchy problem: gravity appears far weaker than the other fundamental forces at the scales familiar to human experience. Some extra-dimensional models address that disparity by allowing gravity to become stronger at short distances, potentially bringing the Planck energy scale closer to the reach of particle accelerators. The absence of new particles at the LHC has therefore made null searches especially important for narrowing the range of theories that remain viable.

The CMS team analyzed collision data in two complementary ways. The first used sphericity, an event-shape variable intended to capture the roughly spherical pattern expected from the decay of a newly formed quantum black hole. The second focused on the total energy carried by the particles produced in a collision, since a black-hole decay would be expected to generate a highly energetic final state. Both approaches were used to define regions of data in which a signal would most likely appear. The study did not find the characteristic behavior predicted by the models, producing an exclusion limit rather than evidence for a new particle. The published result constrains the considered scenarios up to a black-hole mass of about 12 tera-electron volts, while the conclusion about extra dimensions remains conditional on the parameters of the theories tested.

For the new analysis, UCSB particle theorist Nathaniel Craig and collaborators used a phase-space distance method that had not previously been applied to particle-physics data. Phase space represents an event through a multidimensional set of properties, including the directions, energies and momenta of its particles. Rather than examining only one variable, the method measures how individual observed events differ from one another and from reference signal and background distributions. Those distances are condensed into a single Support Vector Machine score, with larger values indicating events that more closely resemble the desired signal. The researchers reported that phase-space distance performed better than sphericity in their comparison.

That result carries a practical advantage beyond this particular analysis. The method is supervised, meaning that the researchers can inspect the mathematical basis of the classification rather than relying on an unexplained black-box output. A higher score is therefore not merely an abstract indicator generated by a machine-learning system; it is tied to an interpretable measure of event geometry. Zhang noted that theories generally predict a range of possible locations for a new particle rather than one exact answer. Each search can eliminate part of that range, while the structure of the data can guide the design of subsequent searches. The same phase-space technique may be transferable to searches for other exotic particles, unusual interactions and previously unknown processes, although the present study does not demonstrate that it will improve every future analysis.

One limitation is that a null result constrains models only to the extent that their predictions resemble the event patterns the analysis can resolve. The exclusion also depends on theoretical assumptions about extra dimensions, black-hole decay and the relevant mass and energy ranges. It does not establish that microscopic black holes are impossible at the LHC, that extra dimensions do not exist, or that quantum gravity can never be studied with particle collisions. Steven Giddings, a UCSB physicist who works on the interface between quantum mechanics and gravity, said that without extra dimensions, producing even the smallest, microgram-scale black holes would require collisions with roughly a million billion times more energy than the LHC currently provides. The exclusion obtained in the present study is consequently specific to accessible energies and to the models for which it was designed.

The same paper documented a search for sphalerons, which are not proposed as ordinary particles but as unstable configurations of particle fields. Like quantum black holes in the models considered, sphaleron processes would be expected to generate approximately spherical energy distributions. Their detection could also bear on the matter–antimatter asymmetry: the standard cosmological account of the Big Bang produces matter and antimatter in equal amounts, whereas the observed universe contains an imbalance that remains unexplained. The researchers did not detect the hypothetical sphaleron processes and consequently constrained the fraction of interactions capable of causing such transitions. These limits, like the black-hole results, depend on the models and analysis assumptions and are not a general prohibition of the underlying physics.

Tamas Vami, a researcher in the CMS experiment, said the search illustrates how experimental data can guide attempts to unify the fundamental forces. A detection could have opened a direct experimental window onto quantum gravity, but the absence of a signal still yields measurable knowledge by excluding regions of theoretical parameter space. Zhang described the result as an exclusion limit: if the sought object existed with the properties assumed by the tested models, the analysis would have detected it, and the fact that it did not rules out that scenario. Future runs with the upgraded High Luminosity Large Hadron Collider will provide additional data for investigating possible new physics, including processes that might illuminate the early universe. The immediate conclusion is narrower but still significant: quantum black holes and sphalerons were not observed in the CMS dataset, and the phase-space distance method provided a more discriminating search tool for evaluating their expected signatures.

Subject of Research: A CMS search for quantum black holes and sphalerons using phase-space distance and supervised machine learning

Article Title: Physicists extend the search for quantum black holes at the LHC

Article References: Physicists extend the search for quantum black holes at the LHC. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: quantum black holes, Large Hadron Collider, CMS experiment, extra dimensions, quantum gravity, phase-space distance, machine learning, sphalerons, particle searches, hierarchy problem, Physicists, extend