New type of entanglement lets scientists 'see' inside nuclei

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 ...

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.

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. Credit: Brookhaven National Laboratory Read more

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

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 Read more

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