Scientists have found a new way to “see” within the simplest atomic nuclei to better understand the “glue” that holds the building blocks of matter together. The results, which have just been published in physical examination letters, they come from collisions of photons (light particles) with deuteros, the simplest atomic nuclei (consisting of a single proton attached to a neutron).
The collisions occurred at the Relativistic Heavy Ion Collider (RHIC), a user facility of the U.S. Department of Energy’s Office of Scientific Users (DOE) for nuclear physics research in the United States. DOE Brookhaven National Laboratory. Scientists around the world are analyzing RHIC subatomic collision data to better understand the particles and forces that make up the visible matter in our world.
In these particular collisions, the photons acted as an X-ray beam to provide the first glimpse of how particles called gluons are arranged in deuteron.
“The gluon is very mysterious,” said Zhoudunming Tu, a Brookhaven lab physicist who led the project for the RHIC STAR collaboration. Gluons, as “carriers” of strong force *, are the glue that binds quarks, the internal building blocks of protons and neutrons. They also hold protons and neutrons together to form atomic nuclei. “We want to study the distribution of gluons because it is one of the keys that unites quarks. This measurement of the distribution of gluons in a deuteron has never been done before.”
Also, because photon-deuteron collisions sometimes separate deuterons, collisions can help scientists understand this process.
“Measuring deuterium rupture tells us a lot about the basic mechanisms that hold these particles together in nuclei in general,” Tu said.
Understanding gluons and their role in nuclear matter will be the focus of research at the Electron-Ion Collider (EIC), a future nuclear physics research facility in the planning stages of Brookhaven Laboratory. In EIC, physicists will use electrons generated by electrons to probe the distribution of gluons within protons and nuclei, as well as the force that holds nuclei together. But Tu, who made research plans at the EIC, realized he could get clues by looking at existing data from the 2016 RHIC deuteron experiments.
“The motivation to study deuteron is because it’s simple, but it still has everything a complex core has,” Tu explained. “We want to study the simplest case of a nucleus to understand these dynamics, including how they change as we move from a single proton to more complex nuclei that we will study in the EIC.”
So he began examining data collected by STAR on hundreds of millions of accidents in 2016.
“The data was there. No one had looked at the distribution of deuteron gluon until I started when I was a Goldhaber Fellow in 2018. I had just joined Brookhaven and found that link to the EIC.”
shine the light
RHIC can accelerate a wide range of ions: atomic nuclei stripped of their electrons. It can even send beams of two different types of particles moving in opposite directions through the twin rings of its 2.4-mile circular circuit at a speed close to light. But it cannot directly accelerate photons.
But thanks to physics, recently discussed here, fast-moving particles with a lot of positive charge emit their own light. Thus, in 2016, when RHIC collided deuterons with highly charged gold ions, these fast gold ions were surrounded by photon clouds. By identifying the “outermost collisions”, where the deuteron only looks at the photon cloud of a gold ion, you realized that you could study the photons that interact with the deuteros to see inside.
The telltale sign of these interactions is the production of a particle called J / psi, triggered by the interaction of the photon with the gluons inside the deuteron.
“I found 350 J / psi. There are only 350 events out of the hundreds of millions of collisions recorded by the STAR experiment. It’s actually a very rare fact,” Tu said.
Although J / psi decays rapidly, the STAR detector can track decay products to measure the amount of impulse transferred from the interaction. Measuring the distribution of impulse transfer between all collisions allows scientists to infer the distribution of gluons.
“There is a one-to-one relationship between impulse transfer (the ‘kick’ given to J / psi) and the location of the gluon in the deuteron,” Tu explained. “On average, gluons within the deuteron nucleus themselves give a very large boost. Gluons in the periphery give a smaller stitch. Therefore, the examination of the global distribution of the moment can be used to map the distribution of gluons in deuteró. ‘
“The results of our study filled a gap in our understanding of gluon dynamics between a free proton and a heavy nucleus,” said Shuai Yang, a STAR collaborator at South China Normal University. Yang was a leading physicist in the use of light-emitting light from fast-moving ions to study the properties of nuclear matter in nuclei-core collisions outside RHIC and the Large Hadron Collider (LHC). in Europe. “This work combines particle physics and nuclear physics,” he said.
Another key contributor, William Schmidke of Brookhaven Lab, said: “We’ve actually been studying this process for many years. But it’s the first result that tells us the dynamics of gluons for the two individual nucleons Proton and neutron) and the nucleus in the same system. “
Studying Deuteron’s breakup
In addition to generating a J / psi particle, each photon-gluon interaction also gives an impulse that deflects the deutron or breaks this simple nucleus into a proton and a neutron. The study of the rupture process provides an insight into the force generated by the gluons that holds the nuclei together.
When it breaks, the positively charged proton bends in the magnetic field of the RHIC accelerator. But the neutral neutron is still straight. To capture these “viewing neutrons”, STAR has a detector located 18 meters from its center, along the line of light at one end.
“This process is very simple,” Tu said. “Only one J / psi is produced in the STAR center. The only other particles that can be created come from this deuteron rupture. So every time you get a neutron, you know it’s because of the deutron rupture. The STAR detector can unambiguously measure this high-energy process. ”
Measuring how the rupture process is associated with a J / psi particle produced by the interaction of gluons can help scientists understand the role of gluons in the interaction between protons and neutrons. This knowledge may be different from what scientists understand about these low-energy interactions.
“At high energy, the photon” sees “almost nothing but gluons inside the deuteron,” Tu said. “After gluons ‘pat’ the J / psi particle, the way this ‘kick’ leads to a rupture is most likely related to the dynamics of the gluon between the proton and the neutron. The advantage of this measure is that we can experimentally identify the gluon-dominated chain and the nuclear rupture at the same time. ”
In addition, Tu points out that measuring neutrons produced by nuclear rupture, commonly known as “spectator tagging”, is a broad and useful technique that will no doubt be used in the future EIC.
But at the EIC, “the instrumentation will be much better and will have more coverage,” he explained. “We will be able to further improve the accuracy of gluon spatial distribution measurements, from light nuclei to heavy nuclei. And EIC detection systems will pick up almost anything related to nuclear rupture, so we can study it in even more detail. how the nucleons interact with each other. ”
Other key contributors to the complex data analysis of this study include Brookhaven Lab physicists Jaroslav Adam, Zilong Chang, and Thomas Ullrich.
* The strong force is the strongest of the four fundamental forces of nature (strong, weak, electromagnetic and gravitational). And unlike all other forces, the force of interaction increases with increasing distance. The binding force between two quarks at a distance greater than 10-15 meters (more than one millionth of a meter) is over 10 tons!