Spin-spin correlations in $Λ\barΛ$ pair production at the SPD experiment at NICA

In the subatomic world, particles do not just collide and bounce; they carry an intrinsic property called spin, a form of angular momentum that behaves like a tiny internal compass. When two particles are created together from a single source, their spins can become linked in a way that defies everyday intuition, a phenomenon known as quantum entanglement. For decades, physicists have studied these connections in high-energy collisions, but a new frontier has opened up: understanding how these delicate quantum links survive the chaotic process of turning invisible quarks into visible matter. This transition, called hadronization, is where the fundamental rules of the strong force take over, and it is here that the quantum nature of the initial particles can either be preserved or lost to the surrounding environment. By measuring how the spins of newly formed particles align with one another, scientists can peer into this hidden process, testing whether the strange, non-local connections of quantum mechanics persist even as matter takes shape.

A team of researchers at the Joint Institute for Nuclear Research in Dubna, Russia, has now proposed a detailed plan to explore this question using the upcoming Spin Physics Detector at the NICA collider. Their study focuses on a specific pair of particles: the lambda and anti-lambda hyperons. These particles are unique because they contain a strange quark, and their decay patterns act as a built-in camera, revealing the direction of their spin at the moment they were created. The researchers used powerful computer simulations to predict what would happen if they collided protons and deuterons at energies up to 27 billion electron volts, a range that sits between the lower energies of previous experiments and the extreme energies of the Large Hadron Collider. They found that at these intermediate energies, the conditions are particularly favorable for observing the original spin connections because the chaotic "noise" of the collision is lower, meaning fewer extra particles are produced to dilute the signal.

The core of their investigation involves tracing the origin of the strange quark pairs that eventually become these hyperons. In some cases, these pairs emerge directly from the vacuum of space itself, while in others, they are born from the splitting of high-energy gluons or the collision of other quarks. The team simulated these different production mechanisms using established models of particle physics and discovered that the strength of the spin connection depends heavily on the collision energy. As the energy drops, the contribution from heavier particles that decay into lambda hyperons decreases rapidly. This is a crucial finding because these heavier decays tend to scramble the original spin information. By focusing on lower energies, the detector would be able to see a clearer picture of the initial quantum state, potentially revealing a stronger correlation between the spins of the two hyperons than has been seen at higher energies.

The researchers also examined how the distance between the two particles affects their connection. Previous experiments at much higher energies found that as the separation between particles grew, the spin correlation weakened, suggesting that the quantum link was being broken by the surrounding environment. The new study predicts that at the NICA energies, this effect can be measured with high precision. The simulations indicate that the detector is capable of reconstructing the paths of these particles with sufficient accuracy to distinguish between those produced directly and those that came from the decay of heavier parents. In fact, the team calculated that for every month of data collection, the experiment could gather hundreds of thousands of these specific particle pairs, providing a statistical sample large enough to measure the spin alignment with great precision.

A significant part of the proposal involves using polarized beams, where the spins of the incoming protons or deuterons are aligned in a specific direction before the collision. This feature allows the scientists to test whether the way the particles are produced depends on the initial orientation of the beam. If the spin correlation changes when the beam is polarized, it would point to specific mechanisms involving the strong force that are sensitive to this alignment. If the correlation remains unchanged, it would suggest that the particles are being created through a mechanism that ignores the initial spin direction, such as the direct materialization from the vacuum. This distinction is vital for understanding the fundamental rules that govern how matter is assembled from energy.

The study concludes that the proposed experiment is not only feasible but essential for filling a gap in our understanding of quantum mechanics in high-energy physics. By operating in an energy range that has not been thoroughly explored for this specific type of measurement, the Spin Physics Detector offers a unique opportunity to observe the transition from quantum entanglement to classical behavior. The simulations show that the detector's design is well-suited to capture the necessary data, with reconstruction efficiencies that allow for a robust analysis of the spin correlations. If the predictions hold true, the experiment will provide a stringent test of how quantum information survives the violent process of hadronization, offering a new window into the non-perturbative dynamics of the strong force and the very nature of reality at the smallest scales.