“Near-misses” in particle accelerators can illuminate new physics, study finds

Particle accelerators reveal the heart of nuclear matter by smashing together atoms at close to the speed of light. The high-energy collisions produce a shower of subatomic fragments that scientists can then study to reconstruct the core building blocks of matter.

An MIT-led team has now used the world's most powerful particle accelerator to discover new properties of matter, through particles' "near-misses." The approach has turned the particle accelerator into a new kind of microscope - and led to the discovery of new behavior in the forces that hold matter together.

In a study appearing this week in the journal Physical Review Letters, the team reports results from the Large Hadron Collider (LHC) - a massive underground, ring-shaped accelerator in Geneva, Switzerland. Rather than focus on the accelerator's particle collisions, the MIT team searched for instances when particles barely glanced by each other.

When particles travel at close to the speed of light, they are surrounded by an electromagnetic halo that flattens when particles pass close but don't collide. The pancaked energy fields produce extremely high-energy photons. Occasionally, a photon from one particle can ping off another particle, like an intense, quantum-sized pinprick of light.

The MIT team was able to pick out such near-miss pinpricks, or what scientists call "photonuclear interactions," from the LHC's particle-collision data. They found that when some photons pinged off a particle, they kicked out a type of subatomic particle, known as a D⁰ meson, that the scientists could measure for the first time.

D⁰ mesons are subatomic particles that contain a charm quark, a rare type of quark not normally found in ordinary nuclear matter. Quarks are the fundamental building blocks of all matter, and are bound by gluons, which are massless particles that are the invisible glue, or "strong force" that holds matter together. The rare charm quarks can only be created in high-energy interactions. As such, they provide an especially clean, unambiguous probe of quarks and gluons inside a nucleus.

Through their measurements of D⁰ mesons , the researchers could estimate how tightly gluons are packed, and, essentially, how strong the strong force is within a particle's nucleus.

"Our result gives an indication that when nuclear matter is squeezed together, then gluons start behaving in a funny way," says lead author Gian Michele Innocenti, an assistant professor of physics at MIT. "We need to know how these gluons behave in these extreme conditions because gluons keep the universe together. And at this point, photonuclear interactions are the best way we have to study gluon behavior."

The study's co-authors include members of the CMS Collaboration - a global consortium of physicists who operate and maintain the Compact Muon Solenoid (CMS) experiment, which is one of the largest detectors within the LHC that was used to collect the study's data.

Bringing a "background" into focus

With each run, the Large Hadron Collider fires off needle-thin beams of particles in opposite directions around a 27-kilometer-long underground ring. When the beams cross paths, particles can collide. If the collisions happen to take place in a region of the ring where the CMS detector is set up, the detector can record the collisions, and scientists can then analyze the aftermath to reconstruct the fragments that make up the original particles.

Since the LHC began operations in 2008, the focus has been overwhelmingly on the detection and analysis of "head-on" collisions. Physicists have known that by accelerating particle beams, they would also produce photonuclear interactions - near-miss events where a particle might collide not with another particle, but with its cloud of photons. But such light-nucleus interactions were thought to be simply noise.

"These photonuclear events were considered a background that people wanted to cancel," Innocenti says. "But now people want to use it as a signal because a collision between a photon and a nucleus can essentially be like a super-high-accuracy microscope for nuclear matter."

When a photon pings off a particle, the abundance, direction, and energy of the produced D⁰ meson relates directly to the energy and density of the gluons in the nucleus. If scientists can detect and measure this photon interaction, it would be like using an extremely small and powerful flashlight to illuminate the nuclear structures. But until now, it was assumed that photonuclear interactions would be impossible to pick out amid the various physics processes that can occur in such collisions.

"People didn't think it was possible to remove the huge mess of all these other collisions, to zoom in on single photons hitting single nuclei producing a D⁰ meson," Innocenti says. "We had to devise a system to recognize those very rare photonuclear interactions while data was being taken of particle collisions."

Illuminating charm

For their new study, Innocenti and his colleagues first simulated what a photonuclear interaction would look like amid a shower of other particle collisions. In particular, they simulated a scenario in which a photon pings off a nucleus and produces a D⁰ meson. Although these events are rare, D⁰ mesons are among the most abundant particles that contain a charm quark. The team reasoned that if they could detect signs of a charm quark in D⁰ mesons that are produced in a photonuclear interaction, it could give valuable information about the gluons that hold the nucleus together.

With their simulations, the researchers then developed an algorithm to detect photonuclear interactions. They implemented the algorithm at the CMS detector to search for signals in real-time during the LHC's particle-colliding runs.

"We had to collect tens of billions of collisions in order to extract a few hundred of these rare instances where a photon hits a nucleus and produces one of these exotic D⁰ meson particles," Innocenti explains.

From this enormous dataset, the team identified a clean sample of these rare events by exploiting CMS's advanced detector capabilities to select near-miss events and reconstruct the properties of the D⁰ mesons.

Through this process, the team detected instances of D⁰ meson production and then worked back to calculate properties of the particles' charm quarks and the gluons that would have held them together in the original nucleus.

"We are constraining what happens to gluons when they are squeezed in ions that are very large that are traveling very fast," Innocenti says. "So far, our data confirms what people expect in terms of high-density nuclear matter. In reality, this is the first time we've shown this kind of measurement is feasible. "

The team is working to improve the measurement's accuracy in order to provide a clearer picture of how quarks and gluons are arranged inside a nucleus.

"Gluons are a very strong force that keeps the universe together," Innocenti says. "The description of the strong force is at the basis of everything we see in nature. Now we have a way to either fully confirm, or show deviations from, that description."

This work was supported, in part, by the U.S. Department of Energy, including support from a DOE Early Career Research Program award, and it builds on the contributions of a large MIT team of graduate students, undergraduate researchers, scientists, and postdocs.