Quietly churning at the heart of every atom in the universe are a swirling wind of particles that physics longs to understand.
No probe, no microscope, and no X-ray machine can hope to understand the chaotic blurring of quantum gears spinning inside an atom, leaving physicists to theorize as best they can based on the wreckage of high-speed collisions inside particle colliders.
The researchers now have a new tool that already gives them a small glimpse into the protons and neutrons that make up the nuclei of atoms, one that relies on the entanglement of particles that is produced when gold atoms quickly pass each other.
Using the powerful Relativistic Heavy Ion Collider (RHIC) at the US Department of Energy’s Brookhaven National Laboratory, scientists have shown how minute details about the arrangement of gold protons and neutrons can be extracted using a type of quantum interference never before seen in an experiment. .
“This technology is similar to the way doctors use PET scans (positron emission tomography) to see what’s going on inside the brain and other parts of the body,” says physicist James Daniel Brandenburg, a former Brookhaven researcher who is now a member of the STAR collaboration.
“But in this case, we’re talking about mapping features to the femtometer scale — fourths of a millionth of a meter — the size of an individual proton.”
In textbook terms, the anatomy of a proton can be described as a trio of basic building blocks called quarks bound together by the exchange of a force-carrying particle called a gluon.
Had we zoomed in and observed this collaboration first hand, we wouldn’t have seen anything so remarkable. Particles and antiparticles merge in and out of existence in a frothy foam of statistical madness, where the rules of particle distribution are inconsistent.
Putting constraints on the motions and momentum of quarks and gluons takes some clever thinking, but hard evidence is what physicists really want.
Unfortunately, simply shining a light on the proton will not result in a shot of its moving parts. Photons and gluons play by completely different rules, which means they are effectively invisible to each other.
However, there is a loophole. Light waves, saturated with enough energy, can sometimes generate pairs of particles that sit on the edge of existence before vanishing again, among them quarks and antiquarks.
If this spontaneous emanation occurs within sight of an atom’s nucleus, the vicious flashes of opposing quarks can mix with swirling vortices of gluons and temporarily form a conglomerate known as a rho particle, which in a split second They shatter into a pair of charged particles called pions.
These pairs consist of a positive pion, consisting of an up quark and an antiquark, and a negative pion made of a down quark and an up antiquark.
Tracing the trajectory and characteristics of the pions formed in this way may tell us something about the hornet’s nest in which they were born.
Two years ago, researchers at RHIC discovered that it was possible to use the electromagnetic fields surrounding gold atoms moving at high speeds as a source of photons.
“In this previous work, we showed that these photons are polarized, with their electric field radiating outwards from the center of the ion,” says Brookhaven physicist Zhangbu Xu.
“And now we’re using this tool, polarized light, to efficiently image the nuclei at high energy.”
When two gold atoms barely avoid collapsing as they orbit the collider in opposite directions, photons of light passing through each nucleus can give birth to an rho particle, and thus pairs of charged pions.
Physicists measured pions ejected from passing gold nuclei and showed that they do indeed have opposing charges. An analysis of the particle shower’s wavelike properties showed signs of interference that could be traced back to the polarization of light and hinted at something much less expected.
In typical applied and experimental quantum setups, entanglement is observed between the same types of particles: electrons with electrons, photons with photons, and atoms with atoms.
The interference patterns observed in the analysis of the particles produced in this experiment can only be explained by the entanglement of two mismatched particles—a negatively charged pion with a positively charged pion.
Although far from a theoretical anomaly, it is far from an everyday occurrence in the laboratory, reaching the level of The first experimental observation of entanglement involving dissimilar particles.
By tracking the entangled interference patterns in gold’s nuclei, physicists can derive a two-dimensional picture of the distribution of gluons, providing new insights into the structures of nuclear particles.
“Now we can take a picture where we can really differentiate the gluon density at a certain angle And radius,” says Brandenburg.
“The images are so accurate that we can even begin to see the difference between where the protons are and where the neutrons are placed inside these large nuclei.”
This research has been published in Science advances.
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