Silently churning away at the heart of every atom in
the Universe is a swirling wind of particles that physics yearns to understand.
No probe, no microscope, and no X-ray machine can
hope to make sense of the chaotic blur of quantum cogs whirring inside an atom,
leaving physicists to theorize the best they can based on the debris of high-speed
collisions inside particle colliders.
Researchers now have a new tool that is already
providing them with a small glimpse into the protons and neutrons that form the
nuclei of atoms, one based on the entanglement of particles produced as gold
atoms brush past each other at speed.
Using the powerful Relativistic Heavy Ion Collider
(RHIC) at the US Department of Energy's Brookhaven National Laboratory,
scientists have shown how it's possible to glean precise details on the
arrangement of gold's protons and neutrons using a kind of quantum interference
never before seen in an experiment.
"This technique is similar to the way doctors
use positron emission tomography (PET scans) to see what's happening inside the
brain and other body parts," says physicist James Daniel Brandenburg,
formerly a Brookhaven researcher and now a member of the STAR collaboration.
"But in this case, we're talking about mapping
out features on the scale of femtometers – quadrillionths of a meter – the size
of an individual proton."
In textbook terms, the anatomy of a proton can be
described as a trio of fundamental building blocks called quarks bound together
by the exchange of a force-carrying particle called a gluon.
Were we to zoom in and observe this collaboration
firsthand, we'd see nothing so neat. Particles and antiparticles pop in and out
of existence in a seething foam of statistical madness, where the rules on
particle distribution are anything but consistent.
Putting constraints on the movements and momenta of
quarks and gluons requires some clever thinking, but hard evidence is what
physicists really desire.
Unfortunately, simply shining a light onto a proton
won't result in a snapshot of its moving parts. Photons and gluons play by very
different rules, meaning they are effectively invisible to one another.
There is a loophole, however. Imbued with enough
energy, waves of light can occasionally churn up pairs of particles that sit on
the brink of existence before vanishing again, among which are quarks and
antiquarks.
Should this spontaneous emergence occur within
earshot of an atom's nucleus, the poltergeist flicker of opposing quarks could
mix with the swirling volleys of gluons and temporarily form a conglomerate
known as a rho particle, which in a fraction of a second shatters into a pair
of charged particles called pions.
Those pairs consist of a positive pion, composed of
an up quark and down antiquark, and a negative pion made up of a down quark and
an up antiquark.
Tracing the path and properties of pions formed this
way might tell us something about the hornet's nest it was born in.
A couple of years ago, researchers at RHIC
discovered it was possible to use the electromagnetic fields surrounding gold
atoms moving at high speeds as a source of photons.
"In that earlier work, we demonstrated that
those photons are polarized, with their electric field radiating outward from
the center of the ion," says Brookhaven physicist Zhangbu Xu.
"And now we use that tool, the polarized light,
to effectively image the nuclei at high energy."
When two gold atoms barely avoid crashing as they
circle the collider in opposing directions, the photons of light passing
through each nucleus can give birth to a rho particle and, therefore, pairs of
charged pions.
The physicists measured the pions ejected from the
passing gold nuclei and showed they did indeed have opposing charges. An
analysis of the wave-like properties of the shower of particles showed signs of
interference that could be traced back to the light's polarization and hinted
at something far less expected.
In typical applied and experimental quantum
settings, entanglement is observed between the same kinds of particles:
electrons with electrons, photons with photons, and atoms with atoms.
The patterns of interference observed in the
analysis of the particles produced in this experiment could only be explained
by the entanglement of non-identical particles – a negatively charged pion with
a positively charged pion.
Though far from a theoretical anomaly, it's far from
an everyday occurrence in the laboratory, amounting to the first experimental
observation of entanglement involving dissimilar particles.
Back-tracing the entangled interference patterns to
the gold nuclei, the physicists could tease out a two-dimensional portrait of
its gluon distribution, providing new insights into the structures of nuclear
particles.
"Now we can take a picture where we can really
distinguish the density of gluons at a given angle and radius," says
Brandenburg.
"The images are so precise that we can even
start to see the difference between where the protons are and where the
neutrons are laid out inside these big nuclei."
Reference: Science Advances.
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