Flipped quantum interference unlocks clearer gluon maps from near-miss nuclear encounters

Spin reveals a sharper way to image nuclei: In near-miss gold-ion collisions at RHIC, photons from one nucleus can briefly transform into particles that probe the gluons inside the other nucleus. This image compares two such probes. Credit: Joanna Pendzick/Brookhaven National Laboratory

Scientists studying particle collisions at the Relativistic Heavy Ion Collider (RHIC) usually capture what happens when atomic nuclei smash into one another at nearly the speed of light. But even when the nuclei don't collide, interesting things can happen. In a new paper just published in Physical Review Letters , members of RHIC's STAR collaboration describe a new way to use near-miss collisions at RHIC to study what's going on inside the nucleus. The approach advances the reach of RHIC, a U.S. Department of Energy (DOE) Office of Science user facility at DOE's Brookhaven National Laboratory, into the next frontier in nuclear physics—a journey into the inner workings of the building blocks of matter.

The technique relies on particles of light, known as photons, that surround the nuclei as they speed around the 2.4-mile (3.9-kilometer) RHIC racetrack. Acting something like the beam of a giant X-ray machine, the photons around one nucleus can interact with particles called gluons inside a nucleus whizzing by in the opposite direction. By tracking the signals produced by those interactions, scientists can map out the distribution of the gluons—the glue-like particles that hold the nucleus together.

"This is an extension of the many ways people have used light to probe hidden structures in our world—from using X-rays to see broken bones and reveal the 3D atomic structures of proteins, to capturing signals from the cosmic microwave background to study the evolution of the universe," said Ashik Ikbal, a STAR collaborator from Kent State University who carried out this work as a major component of his postdoctoral research. "In this case, we're using light to map out features at a scale much smaller than atoms to study the gluons that hold quarks together inside the protons and neutrons of atomic nuclei."

Nuclear physicists are particularly interested in gluons because they appear to play an outsized role in establishing the fundamental properties of protons and neutrons—the building blocks of nearly all the visible matter in our universe. Mapping out gluons is one of the central goals of the Electron-Ion Collider (EIC), a new nuclear physics research machine under construction at Brookhaven Lab that will build on RHIC's infrastructure and science.

At the EIC, virtual photons emitted by electrons will provide the "beams" that scientists use to reveal gluons' arrangements and interactions within protons and nuclei. These new results from RHIC provide a preview of this imaging technique and a way to test its assumptions.

This study strengthens the case for using the spin of particles produced by light to make sharper images of gluons inside gold nuclei. This method will be used at the future Electron-Ion Collider (EIC), where virtual photons (γ*) emitted by electrons (e-) during electron-ion collisions will provide the imaging beam. Credit: Tiffany Bowman/Brookhaven National Laboratory

Using light to create particles and map structures

The particles of light used in this imaging technique at RHIC are something of an artifact. They emerge as a cloud of electromagnetic energy that surrounds the positively charged ions traveling around the circular accelerator at close to the speed of light. When two ions traveling in opposite directions pass very close by one another without colliding, these "shockwaves" of energy can sometimes interact with one another to create new particles of matter and antimatter out of pure energy.

At other times, the photons create new particles by interacting with gluons inside the nuclei. For example, an earlier STAR paper traced photon-gluon interactions that generated particles known as rho mesons. STAR scientists detected those particles by looking for pairs of oppositely charged pions—the "daughters" into which the rhos decay. By tracking the pions' speed and the angles at which they struck the detector, the scientists suggested they could use ripples of interference generated by these quantum-entangled particles to map out gluon distributions within the nuclei.

But because the rho particles decay so quickly, there was uncertainty about the origin of the interference—specifically, whether it was coming from the decaying "daughter" pions or the rho "parents." In addition, the somewhat lightweight rho particles lack the "focus" to map detailed gluon features.

Flipping the interference pattern

This new paper builds on that previous work by tracking the daughters of heavier mesons known as J/psi particles, which are also created in photon-gluon interactions.

"The heavier yet more compact structure of J/psi particles should boost their imaging resolution," said Zebo Tang, a professor from the Uni…