To better understand the “glue” that holds the constituent parts of matter together, scientists have developed a novel method to “see” into the tiniest atomic nuclei. The findings, which were just published in Physical Review Letters, occur from collisions between photons (light particles) and deuterons, the most basic atomic nuclei (made of just one proton bound to one neutron).
The collisions took place at the Relativistic Heavy Ion Collider (RHIC), a user facility for nuclear physics research operated by the U.S. Department of Energy’s (DOE) Office of Science at DOE’s Brookhaven National Laboratory.
In order to understand the particles and forces that create the observable matter of our universe, scientists from all over the world evaluate data from the subatomic collisions produced by RHIC. The photons behaved something like an x-ray beam in these specific collisions, giving us our first look at the arrangement of gluon particles inside the deuteron.
“The gluon is very mysterious,” said Brookhaven Lab physicist Zhoudunming Tu, who led this project for RHIC’s STAR Collaboration. Gluons, as ‘carriers’ of the strong force, are the glue that binds quarks, the inner building blocks of protons and neutrons. They also hold protons and neutrons together to form atomic nuclei. “We want to study the gluon distribution because it’s one of the keys that bind the quarks together. This measurement of gluon distribution in a deuteron has never been done before.”
The photon-deuteron collisions can also aid in the understanding of this process because they occasionally cause the deuterons to disintegrate.
“Measuring the breakup of the deuteron tells us a lot about the basic mechanisms that hold these particles together in nuclei in general,” said Tu.
The Electron-Ion Collider (EIC), a future nuclear physics research facility under construction at Brookhaven Lab, will have a major focus on research that will be devoted to understanding gluons and their function in nuclear matter.
In order to investigate the gluon distributions inside protons and nuclei as well as the force holding nuclei together, physicists at EIC will use photons produced by electrons. However, Tu, who has been formulating research plans at the EIC, recognized he might be able to find some answers by consulting the data from deuterons studies performed in 2016 by RHIC.
“The motivation for studying the deuteron is because it is simple, but it still has everything a complex nucleus has,” Tu explained. “We want to study the simplest case of a nucleus to understand these dynamics including how they change as you move from a simple proton to the more complex nuclei we’ll study at the EIC.”
The gluon is very mysterious. We want to study the gluon distribution because it’s one of the keys that bind the quarks together. This measurement of gluon distribution in a deuteron has never been done before.
Zhoudunming Tu
So, he started sifting through data collected by STAR from hundreds of millions of collisions in 2016.
“The data were there. Nobody had looked into the deuteron’s gluon distribution until I started when I was a Goldhaber Fellow in 2018. I had just joined Brookhaven, and I found this connection to the EIC.”
Shining the light
RHIC has the ability to accelerate a wide variety of ions with electron-free atomic nuclei. Even faster than the speed of light, it is capable of directing beams of two different types of particles through the twin rings of its 2.4-mile-diameter circular racetrack.
However, it cannot directly accelerate photons. Fast-moving particles with a lot of positive charge do, however, emit their own light as a result of physics, which was previously covered here.
As a result, in 2016, when RHIC was causing high-energy gold ions to collide with deuterons, photon clouds were all around the rapidly moving gold ions.
Tu recognized he could observe photon interactions with deuterons to obtain a glimpse inside by finding “ultra-peripheral collisions” where the deuteron only breezes by a cloud of photons from a gold ion.
The photon’s contact with the gluons inside the deuteron causes the formation of a particle known as J/psi, which is a telltale indicator of those interactions.
“I found 350 J/psi. There are only 350 events out of the hundreds of millions of collisions recorded by the STAR experiment. It is actually a very rare event,” Tu said.
The STAR detector can trace the decay products to determine how much momentum was transferred from the collision, despite the J/psi decaying quickly. The gluon distribution can be inferred by measuring the distribution of momentum transfer across all collisions.
“There is a one-to-one connection between the momentum transfer (the ‘kick’ given to the J/psi) and where the gluon is located in the deuteron,” Tu explained. “On average, gluons inside the very core of the deuteron give a very large momentum kick. Gluons on the periphery give a smaller kick. So, looking at the overall momentum distribution can be used to map out the gluon distribution in the deuteron.”
“The findings from our study have filled a gap in our understanding of gluon dynamics between a free proton and a heavy nucleus,” said Shuai Yang, a STAR collaborator from South China Normal University.
In ultra-peripheral nucleus-nucleus collisions at RHIC and Europe’s Large Hadron Collider (LHC), Yang has been a pioneer in the use of light created by quickly moving ions to probe the characteristics of nuclear matter. “This work builds a bridge connecting particle physics and nuclear physics,” he said.
Another leading contributor, William Schmidke of Brookhaven Lab, said, “In fact we have been studying this process for many years. But this is the first result that tells us the gluon dynamics for both individual nucleons (the collective term for protons and neutrons) and the nucleus in the same system.”
Studying deuteron breakup
Each photon-gluon collision produces a J/psi particle as well as a momentum kick that deflects the deuteron or splits that basic nucleus into a proton and neutron. Understanding the breaking process helps us understand the force produced by gluons that holds nuclei together.
The positively charged proton in a breakdown curves away in the RHIC accelerator’s magnetic field. However, the neutral neutron continues to travel forward. STAR has a detector placed 18 meters from its center directly along the beamline at one end in order to catch these “spectator neutrons.”
“This process is very simple,” Tu noted. “Only one J/psi gets produced in the center of STAR. The only other particles that can be created are from this deuteron breakup. So, any time you get a neutron, you know this is coming from the deuteron breakup. The STAR detector can unambiguously measure this process at high energy.”
Understanding the function of gluons in the interaction between protons and neutrons can be accomplished by measuring the breakup process’ relationship to a J/psi particle created by gluon interaction. There may be discrepancies between that understanding and what is known by scientists concerning those interactions at low energies.
“At high energy, the photon ‘sees’ almost nothing but gluons inside the deuteron,” Tu said. “After the gluons ‘kick’ the J/psi particle, how this ‘kick’ leads to a breakup is very likely related to the gluon dynamics between the proton and neutron. The advantage of this measurement is that we can experimentally identify the gluon-dominated channel and the nuclear breakup at the same time.”
Tu further adds that “spectator tagging,” the term for measuring neutrons produced by nuclear fusion, is a comprehensive and practical approach that will undoubtedly be used at upcoming EICs.
But at the EIC, “the instrumentation will be much better and will have more coverage,” he explained. “We’ll be able to further improve the precision of gluon spatial distribution measurements from light nuclei to heavy nuclei. And EIC detector systems will capture almost everything about the nucleus breakup, so we can study in even more detail how nucleons interact with each other.”
Additional key contributors who collaborated to perform the complicated data analyses for this study include Brookhaven Lab physicists Jaroslav Adam, Zilong Chang, and Thomas Ullrich.
The strongest of nature’s four basic forces is a strong force (strong, weak, electromagnetic, and gravitational force). Additionally, the interaction strength grows stronger with distance, unlike any other forces. At distances, more than 10-15 meters (or greater than a millionth of a billionth of a meter), the binding force between two quarks is greater than 10 tons!
