Deep inside every atom is an unstable world of quarks and gluons – the tiny building blocks that hold everything together, from rocks to stars. For decades, physicists have been trying to understand how these particles behave, especially under extreme conditions.
“We need to know how these gluons behave in these extreme conditions because gluons maintain the universe. And now, the photonuclear interaction is the best way we have to study the behavior of gluons,” said Gian Michele Innocenti, an experimental physicist and assistant professor at MIT.
Now, using the Large Hadron Collider (LHC), Innocenti and his team have found a new way to observe this hidden world—not in violent collisions, but in intimate ways. Instead of breaking up the particles exactly, they focused on times when the particles don’t collide as hard.
These temporary encounters, once thought of as noise, have now revealed a new behavior in nuclear energy—a vital force that binds things together. This discovery could change the way scientists study nuclear matter and open a new way to understand the universe at its most fundamental level.
Rare and overlooked events
Accelerators like the LHC usually work by shooting beams of particles at nearly the speed of light and smashing them together. These collisions produce showers of tiny particles, which scientists analyze to reconstruct what’s inside the atoms.
However, there is a problem. Alongside these head-on collisions, the episodes also feature a series of near-misses. When fast-moving particles come close together, they are surrounded by flat electric fields—like invisible energy cakes. These fields emit high-energy photons (particles of light).
Sometimes, one of these photons hits the nucleus of a nearby atom. This interaction is called photonuclear interaction. For many years, scientists ignored these events because they were rare and hidden in the mass of collision data.
Innocenti says: “These nuclear events were seen as a background that people wanted to erase. The MIT team decided to do something different – they treated these close-ups as signals rather than noise.
Transforming past conflicts into a powerful microscope
To do this work, the researchers first simulated what these photonuclear events should look like. They focus on a very specific outcome: D’s output0 meson, a particle containing an unusual quark.
Charm quarks do not normally exist in everyday matter and only appear at high energies, making them excellent tools for probing the nuclear interior. Next, the team developed a special algorithm that could analyze billions of particle collisions in real time and pick out a few rare situations where a photon hits the nucleus and produces D.0 mason/
They implemented this method in the Compact Muon Solenoid (CMS) detector, one of the largest instruments in the collider. However, even with such a highly developed speaker, picking out the rare events that went wrong was a very difficult task.
“We had to collect billions of collisions in order to find several hundred of these rare events where a photon hits the nucleus and produces one of the rare D.0 meson particles,” said Innocenti.
By studying the strength, direction and number of D0 produced mesons, the researchers could work backwards to estimate how gluons, particles that stick together, are distributed in the nucleus.
What they found was surprising. When nuclear matter is tightly packed and moving at extreme speeds, gluons begin to behave in unusual ways. This confirms the long-term predictions about the high-energy nuclear issue, but more importantly, it proves that this new method can measure such effects.
Simply put, the team turned the neglected noise into a kind of highly accurate microscope—one that uses light itself to probe the heart of things.
Small parts, big success
This study has many implications for physics. For example, it is important to understand how gluons behave because they control their energy. A clearer picture of these forces could improve theories that explain everything from nuclear reactions to the early universe just after the Big Bang.
“An explanation of the energetic forces underlying everything we see in nature.” Now we have a way to fully confirm or show deviations from that definition,” Innocenti added.
This method also provides a cleaner and more accurate way to study nuclear structure compared to conventional collision methods. By using nuclear interactions, scientists can investigate matter without the chaos of full particle collisions.
However, there are still limitations. These events are rare, requiring large datasets and highly developed detection methods. Current measurements are still not precise enough to fully map the behavior of gluons under all conditions.
This is exactly what the researchers plan to do next. By improving their algorithms and collecting more data, the team hopes to improve their measurements and perhaps discover differences from existing theories. If such differences are found, they could point to new physics beyond what scientists currently understand.
The study was published in the journal Physical Examination Letters.
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