Nuclear physicists have found a new way to see the shape and details inside atomic nuclei using the Relativistic Heavy Ion Collider (RHIC), a particle collider at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory. The method relies on particles of light surrounding gold ions accelerated by a collider and a new type of quantum entanglement never seen before.
Through a series of quantum fluctuations, particles of light (aka photons) interact with gluons — glue-like particles that bind quarks to protons and neutrons in atomic nuclei. These interactions create an intermediate particle that rapidly decays into two “π mesons” (π) with different charges.By measuring the velocity and angle of these π+ and π– When particles hit RHIC’s STAR detector, scientists can backtrack to obtain key information about the photons — and use that information to map the arrangement of gluons within atomic nuclei with greater precision than ever before.
“This technique is similar to the way doctors use positron emission tomography (PET scans) to look inside the brain and other body parts,” said former Brookhaven Laboratory physicist James Daniel Brandenburg, who joined Ohio State University in The STAR Collaborative Member will become a University Assistant Professor in January 2023. “But in this case we’re talking about femtometer – a billionth of a meter – the size of a single proton. “
Even more surprising, the STAR physicists said, was the observation of a whole new kind of quantum interference that made their measurements possible.
“We measured two outgoing particles and it was clear that they had different charges — they were different particles — but the interference patterns we saw suggested that the particles were entangled or synchronized with each other, even though they were distinguishable particles,” Brookhaven said. Said the physicist and STAR collaborator Xu Zhangbu.
The discovery could have applications far beyond the lofty goal of mapping the building blocks of matter.
For example, many scientists, including those awarded the 2022 Nobel Prize in Physics, are seeking to harness entanglement—the “consciousness” and interaction of physically separated particles. One goal is to create more powerful communication tools and computers than exist. But to date, most other observations of entanglement, including the recent demonstration of laser interference at different wavelengths, have been between photons or electrons of the same kind.
“This is the first experimental observation of entanglement between different particles,” Brandenburg said.
This work was just published in scientific progress.
illuminate gluon
RHIC operates as a DOE Office of Science User Facility where physicists study the innermost building blocks of nuclear matter — the quarks and gluons that make up protons and neutrons. They do this by moving the nuclei of heavy atoms, such as gold, together in opposite directions around a collider at nearly the speed of light. The strength of these collisions between atomic nuclei (also known as ions) can “melt” the boundary between individual protons and neutrons, so scientists can study the quarks and gluons that existed in the early universe — before protons and neutrons were formed.
But nuclear physicists also want to know how quarks and gluons behave in atomic nuclei that exist today—to better understand the forces that hold these building blocks together.
A recent discovery using photon “clouds” surrounding RHIC speeding ions suggests a way to use these light particles to glimpse inside atomic nuclei. If two gold ions pass in close proximity to each other without colliding, photons surrounding one ion can probe the internal structure of the other ion.
“In earlier work, we demonstrated that these photons are polarized, with their electric field radiating outward from the center of the ion. Now we use this tool, polarized light, to efficiently image high-energy nuclei,” Xu said.
Quantum interference observed between π+ and π– In the newly analyzed data, the polarization direction of the photons can be measured very precisely. This in turn allows the physicists to observe the gluon distribution along the direction of the photon’s motion and perpendicular to it.
It turns out that 2D imaging is very important.
“In all past measurements, we didn’t know the polarization direction and measured the average density of gluons — as a function of the distance from the center of the nucleus,” Brandenburg said. “That’s a one-dimensional image.”
These measurements both made the nucleus appear too large compared to predictions from theoretical models and measurements of the distribution of charges in the nucleus.
“With this 2D imaging technique, we were able to solve the 20-year-old mystery of why this happened,” Brandenburg said.
The new measurements show that the momentum and energy of the photon itself are intertwined with that of the gluon. Measuring only along the direction of the photon (or not knowing what that direction is) will result in images distorted by these photon effects. But lateral measurements avoid photon ambiguity.
“Now that we can take a picture, we can really differentiate the gluon density at a given angle with radii,” Brandenburg said. “These images are so precise that we can even start to see the difference between where the protons are and where the neutrons are inside these large nuclei.”
The scientists say the new picture is in qualitative agreement with theoretical predictions using the distribution of gluons, as well as measurements of the distribution of charges within atomic nuclei.
Measurement Details
To understand how physicists make these two-dimensional measurements, let’s go back to the particles created by photon-gluon interactions.It’s called rho, and it decays very quickly — less than four one billionth One Second – Enter π+ and π–The sum of the momentums of the two pions gives physicists the momentum of the parent particle, rho — as well as information including the gluon distribution and photon blurring effects.
extract only Gluon distribution, the scientists measured the angle between the paths of π+ or π– and the locus of rho. The closer this angle is to 90 degrees, the less blur you will get from the photon detector. By tracking pions from rho particles moving at a range of angles and energies, scientists can map the distribution of gluons throughout the nucleus.
The quantum singularity that now makes measurement possible – proving π+ and π– The particles hitting the STAR detectors are produced by interference patterns created by the entanglement of these two different, oppositely charged particles.
Remember that all the particles we’re talking about exist not only as physical objects, but also as waves. Like ripples on the surface of a pond radiating outward as they hit rocks, the mathematical “wave functions” that describe the peaks and troughs of particle waves can reinforce or cancel each other out.
When the photons surrounding the two near-failing high-speed ions interact with the gluons inside the nucleus, it’s as if these interactions actually produce two rho particles, one for each nucleus. Decays to π with each rho+ and π–, the wave function of a negative pion from one rho decay interferes with the wave function of a negative pion from the other. When the enhanced wave function hits the STAR detector, the detector sees a π–. The same thing happens to the wave functions of two positively charged muons, the detector sees a π+.
“Interference occurs between two wave functions of the same particle, but there is no entanglement between two different particles – π+ and π– — that kind of interference won’t happen,” says STAR collaborator Wangmei Zha of the University of Science and Technology of China, one of the original proponents of this explanation. “That’s the weirdness of quantum mechanics!
Can the rhos be simply entangled? Scientists say no. The origin distance of the rho particle wave function is 20 times the distance they can travel in their short lifetime, so they cannot interact until they decay to π+ and π–. But the wave function of π+ and π– From each rho decay, the quantum information of its parent particle is preserved; their peaks and troughs are in phase, “mutually perceiving”, despite the fact that the detectors are meters apart.
“If π+ and π– Without entanglement, two π+ (or π–) wave function will have a random phase without any detectable interference effects,” said STAR collaborator Chi Yang from Shandong University in China, who also helped lead the analysis of this result. Correlated directional polarization — or being able to make these precise measurements. “
Future measurements using heavier particles and different lifetimes at the RHIC — and at the Electron-Ion Collider (EIC) built at Brookhaven — will probe the more detailed distribution of gluons within atomic nuclei and test other possible quantum interference scenarios.
The work was funded by the DOE Office of Science, the National Science Foundation and a range of international agencies detailed in the published paper. The STAR team used computing at Brookhaven Laboratory’s RHIC and ATLAS Computing Facility/Center for Scientific Data and Computing, and the National Energy Research Scientific Computing Center (NERSC), a DOE Office of Science user facility at Lawrence Berkeley National Laboratory. resources — and the Open Science Grid Consortium.
