[Just as visible light and x-rays are used to study biosamples,
cells, and atoms, high-energy photons can be used to probe gluons
inside of atomic nuclei.]
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NEW TYPE OF ENTANGLEMENT LETS SCIENTISTS ‘SEE’ INSIDE NUCLEI  
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Brookhaven National Laboratory
January 15, 2023
Phys.org
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_ Just as visible light and x-rays are used to study biosamples,
cells, and atoms, high-energy photons can be used to probe gluons
inside of atomic nuclei. _

The house-size STAR detector at the Relativistic Heavy Ion Collider
(RHIC) acts like a giant 3D digital camera to track particles emerging
from particle collisions at the center of the detector. , Brookhaven
National Laboratory

 

Nuclear physicists have found a new way to use the Relativistic Heavy
Ion Collider (RHIC)—a particle collider at the U.S. Department of
Energy's (DOE) Brookhaven National Laboratory—to see the shape and
details inside atomic nuclei. The method relies on particles of light
that surround gold ions as they speed around the collider and a new
type of quantum entanglement that's never been seen before.

Through a series of quantum fluctuations, the particles of light
(a.k.a. photons) interact with gluons—gluelike particles that hold
quarks together within the protons and neutrons of nuclei. Those
interactions produce an intermediate particle that quickly decays into
two differently charged "pions" (π). By measuring the velocity and
angles at which these π+ and π- particles strike RHIC's STAR
detector, the scientists can backtrack to get crucial information
about the photon—and use that to map out the arrangement of gluons
within the nucleus with higher precision than ever before.

"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," said former Brookhaven Lab physicist James Daniel
Brandenburg, a member of the STAR collaboration who joined The Ohio
State University as an assistant professor in January 2023. "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."

Even more amazing, the STAR physicists say, is the observation of an
entirely new kind of quantum interference that makes their
measurements possible.

"We measure two outgoing particles and clearly their charges are
different—they are different particles—but we see interference
patterns that indicate these particles are entangled, or in sync with
one another, even though they are distinguishable particles," said
Brookhaven physicist and STAR collaborator Zhangbu Xu.

That discovery may have applications well beyond the lofty goal of
mapping out the building blocks of matter.

For example, many scientists, including those awarded the 2022 Nobel
Prize in Physics
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seeking to harness entanglement—a kind of "awareness" and
interaction of physically separated particles. One goal is to create
significantly more powerful communication tools and computers than
exist today. But most other observations of entanglement to date,
including a recent demonstration of interference
[[link removed]] of
lasers with different wavelengths, have been between photons or
identical electrons.

"This is the first-ever experimental observation of entanglement
between dissimilar particles," Brandenburg said.

The work is described in a paper just published in _Science
Advances_.

Daniel Brandenburg and Zhangbu Xu at the STAR detector of the
Relativistic Heavy Ion Collider (RHIC). Credit: Brookhaven National
Laboratory

Shining a light on gluons

RHIC operates as a DOE Office of Science user facility where
physicists can study the innermost building blocks of nuclear
matter—the quarks and gluons that make up protons and neutrons. They
do this by smashing together the nuclei of heavy atoms such as gold
traveling in opposite directions
[[link removed]] around the collider at
close to the speed of light. The intensity of these collisions between
nuclei (also called ions) can "melt" the boundaries between individual
protons and neutrons so scientists can study the quarks and gluons as
they existed in the very early universe—before protons and neutrons
formed.

But nuclear physicists
[[link removed]] also want to know how
quarks and gluons behave within atomic nuclei
[[link removed]] as they exist today—to
better understand the force that holds these building blocks together.

A recent discovery
[[link removed]] using
"clouds" of photons that surround RHIC's speeding ions suggests a way
to use these particles of light to get a glimpse inside the nuclei. If
two gold ions [[link removed]] pass one another
very closely without colliding, the photons surrounding one ion can
probe the internal structure of the other.

Left: Scientists use the STAR detector to study gluon distributions by
tracking pairs of positive (blue) and negative (magenta) pions (π).
These π pairs come from the decay of a rho particle (purple, ρ0) —
generated by interactions between photons surrounding one speeding
gold ion and the gluons within another passing by very closely without
colliding. The closer the angle (Φ) between either π and the rho's
trajectory is to 90 degrees, the clearer the view scientists get of
the gluon distribution. Right/inset: The measured π+ and π-
particles experience a new type of quantum entanglement. Here's the
evidence: When the nuclei pass one another, it's as if two rho
particles (purple) are generated, one in each nucleus (gold) at a
distance of 20 femtometers. As each rho decays, the wavefunctions of
the negative pions from each rho decay interfere and reinforce one
another, while the wavefunctions of the positive pions from each decay
do the same, resulting in one π+ and one π- wavefunction (a.k.a.
particle) striking the detector. These reinforcing patterns would not
be possible if the π+ and π- were not entangled. Credit: Brookhaven
National Laboratory

"In that earlier work, we demonstrated that those photons are
polarized, with their electric field radiating outward from the center
of the ion. And now we use that tool, the polarized light, to
effectively image the nuclei at high energy," Xu said.

The quantum interference observed between the π+ and π- in the
newly analyzed data makes it possible to measure the photons'
polarization direction very precisely. That in turn lets physicists
look at the gluon distribution both along the direction of the
photon's motion and perpendicular to it.

That two-dimensional imaging turns out to be very important.

"All past measurements, where we didn't know the polarization
direction, measured the density of gluons as an average—as a
function of the distance from the center of the nucleus," Brandenburg
said. "That's a one-dimensional image."

Those measurements all came out making the nucleus look too big when
compared with what was predicted by theoretical models and
measurements of the distribution of charge in the nucleus.

"With this 2D imaging technique, we were able to solve the 20-year
mystery of why this happens," Brandenburg said.

The new measurements show that the momentum and energy of the photons
themselves gets convoluted with that of the gluons. Measuring just
along the photon's direction (or not knowing what that direction is)
results in a picture distorted by these photon effects. But measuring
in the transverse direction avoids the photon blurring.

Brandenburg (front) and Xu stand beside STAR. Credit: Brookhaven
National Laboratory

"Now we can take a picture where we can really distinguish the density
of gluons at a given angle _and_ radius," Brandenburg said. "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."

The new pictures match up qualitatively with the theoretical
predictions using gluon distribution, as well as the measurements of
electric charge distribution within the nuclei, the scientists say.

Details of the measurements

To understand how the physicists make these 2D measurements, let's
step back to the particle generated by the photon-gluon interaction.
It's called a rho, and it decays very quickly—in less than
four _septillionths_ of a second—into the π+ and π-. The sum of
the momenta of those two pions gives physicists the momentum of the
parent rho particle—and information that includes the gluon
distribution and the photon blurring effect.

To extract _just_ the gluon distribution, the scientists measure the
angle between the path of either the π+ or π- and the rho's
trajectory. The closer that angle is to 90 degrees, the less blurring
you get from the photon probe. By tracking pions that come from rho
particles moving at a range of angles and energies, the scientists can
map out the gluon [[link removed]] distribution across
the entire nucleus.

Now for the quantum quirkiness that makes the measurements
possible—the evidence that the π+ and π- particles striking the
STAR detector result from interference patterns produced by the
entanglement of these two dissimilar oppositely charged particles.

Keep in mind that all the particles we are talking about exist not
just as physical objects but also as waves. Like ripples on the
surface of a pond radiating outward when they strike a rock, the
mathematical "wavefunctions" that describe the crests and troughs of
particle waves can interfere to reinforce or cancel one another out.

When the photons surrounding two near-miss speeding ions interact with
gluons inside the nuclei, it's as if those interactions actually
generate two rho particles, one in each nucleus. As each rho decays
into a π+ and π-, the wavefunction of the negative pion from one
rho decay interferes with the wavefunction of the negative pion from
the other. When the reinforced wavefunction strikes the STAR detector,
the detector sees one π-. The same thing happens with the
wavefunctions of the two positively charged pions, and the detector
sees one π+.

"The interference is between two wavefunctions of the identical
particles, but without the entanglement between the two dissimilar
particles—the π+ and π-—this interference would not
materialize," said Wangmei Zha, a STAR collaborator at the University
of Science and Technology of China, and one of the original proponents
of this explanation. "This is the weirdness of quantum mechanics!"

Could the rhos simply be entangled? The scientists say no. The rho
particle wavefunctions originate at a distance 20 times the distance
they could travel within their short lifetime, so they cannot interact
with each other before they decay to π+ and π-. But the
wavefunctions of the π+ and π- from each rho decay retain the
quantum information of their parent particles; their crests and
troughs are in phase, "aware of each other," despite striking the
detector meters apart.

"If the π+ and π- were not entangled, the two π+ (or π-)
wavefunctions would have a random phase, without any detectable
interference effect," said Chi Yang, a STAR collaborator from Shandong
University in China, who also helped lead the analysis for this
result. "We wouldn't see any orientation related to the photon
polarization—or be able to make these precision measurements."

Future measurements at RHIC with heavier particles and different
lifetimes—and at an Electron-Ion Collider
[[link removed]] (EIC) being built at Brookhaven—will
probe more detailed distributions of gluons inside nuclei and test
other possible quantum interference scenarios.

MORE INFORMATION: James Brandenburg, Tomography of ultra-relativistic
nuclei with polarized photon-gluon collisions, _Science
Advances_ (2023). DOI: 10.1126/sciadv.abq3903
[[link removed]]. www.science.org/doi/10.1126/sciadv.abq3903
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_BROOKHAVEN LAB is one of 10 national laboratories overseen and
primarily funded by the U.S. Department of Energy's Office of Science.
Brookhaven Lab is managed for the Office of Science by Brookhaven
Science Associates, a partnership between Stony Brook University and
Battelle, and six core universities: Columbia, Cornell, Harvard,
Massachusetts Institute of Technology, Princeton, and Yale._

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