First determination of the full spin density matrix in
Using a large dataset of events collected by the BESIII detector, this study presents the first complete determination of the spin density matrix in , revealing a significant transverse polarization of the hyperon and providing evidence for quantum entanglement through the violation of the CHSH Bell inequality.
Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
In the subatomic world, particles are not just tiny billiard balls; they carry a hidden property called "spin," which acts like a built-in compass needle pointing in a specific direction. When these particles are created in high-energy collisions, their spins can become linked in a way that defies our everyday intuition, a phenomenon known as quantum entanglement. This connection means that measuring the spin of one particle instantly reveals the state of its partner, no matter how far apart they are. Scientists have long studied this behavior with light particles and atoms, but observing it in heavier, complex particles made of quarks has remained a significant challenge. Understanding how these massive particles are born and how their spins interact offers a rare glimpse into the fundamental forces that hold matter together and tests the very limits of how the universe operates.
A team of researchers using the BESIII detector at a particle accelerator in China has now taken a major step forward by capturing a complete picture of this behavior in a specific type of particle decay. They analyzed hundreds of millions of events where a heavy particle called the broke apart to form a pair of lighter particles: a lambda baryon and its antimatter twin, the anti-lambda. By tracking the paths of the debris left behind when these particles decayed, the scientists were able to reconstruct the full "spin density matrix." This is essentially a detailed map that describes not only how the particles were spinning when they were created but also how their spins were correlated with one another. It is the first time such a complete description has been achieved for this specific reaction, providing a unified view of both the mechanics of their creation and the quantum nature of their connection.
The study relied on a massive dataset containing 448.1 million of these particle events. The researchers focused on the angles at which the decay products flew out, using a sophisticated five-dimensional analysis to extract two key numbers that describe the production process. They found that the angular distribution parameter, which dictates how the particles are scattered, is 0.82. More importantly, they measured a relative phase difference between the electric and magnetic properties of the interaction to be 20.6 degrees. This non-zero phase implies a specific type of motion: it suggests that the lambda particles are not just spinning randomly but are actually polarized sideways, moving with a maximum sideways spin value of 0.074. This sideways polarization was detected with a statistical certainty of 6.3 standard deviations, meaning the chance of this result being a random fluke is virtually zero.
Beyond simply measuring how the particles spin, this complete map allowed the team to test the famous Bell inequalities, which are mathematical limits that separate the predictable world of classical physics from the strange world of quantum mechanics. If the particles behaved according to classical rules, their correlations would stay below a certain threshold. However, the researchers calculated an average value for the Bell nonlocality measure of 1.266, which clearly exceeds the classical limit. This violation of the Clauser-Horne-Shimony-Holt inequality was observed with a significance of 3.7 standard deviations, providing strong evidence that these heavy baryons are indeed entangled in a way that cannot be explained by classical physics. The results confirm that the quantum link between the particle and its antiparticle survives the violent process of their creation and decay.
The findings also shed light on why different types of similar particles behave differently. While the lambda particle shows a positive spin pattern in this decay, a similar particle called the sigma-zero shows a negative pattern in other experiments. The researchers suggest this difference arises from the internal arrangement of the quarks inside them, specifically how the quarks pair up. By comparing their new results with previous measurements from other particle decays, the team extracted details about the forces at play, such as the ratio of different wave interactions and the effective size of the interaction region. These measurements help refine our understanding of the strong nuclear force and the internal structure of matter, turning the decay into a unique laboratory for studying the deep connection between how particles are made and how they remain quantum mechanically linked.
Drowning in papers in your field?
Get daily digests of the most novel papers matching your research keywords — with technical summaries, in your language.