Monitoring antiproton numbers with a CMOS detector in a dense-track environment

At the heart of the CERN laboratory in Switzerland, scientists are working to understand why the universe is made of matter at all. In the moments after the Big Bang, matter and antimatter should have been created in equal amounts, only to annihilate each other instantly. Yet, here we are, surrounded by matter. To solve this mystery, researchers study antimatter with extreme precision, looking for even the tiniest difference in how it behaves compared to normal matter. One such experiment, called GBAR, aims to create antihydrogen atoms—mirror images of ordinary hydrogen—and drop them to see how gravity affects them. To do this, the team must first gather a specific number of antiprotons, the negatively charged building blocks of antimatter, and slow them down to a gentle crawl. The challenge is that these particles are invisible and vanish the moment they touch ordinary matter, making it incredibly difficult to know exactly how many are present at any given time. Without a precise count, the delicate measurements required to test the laws of physics become impossible.

To solve this counting problem, the GBAR team developed a clever method using a modified digital camera to watch the aftermath of particle collisions. Inside their vacuum chamber, they direct a beam of antiprotons onto a specialized detector surface made of tiny glass tubes. When an antiproton hits this surface, it annihilates, vanishing and releasing a burst of energy that sprays out a cloud of new, fast-moving particles. The team realized that if they could count these spray particles, they could work backward to determine exactly how many antiprotons had arrived. They mounted a commercial digital camera, stripped of its lens and wrapped in light-proof foil, outside the vacuum chamber. This camera is equipped with a highly sensitive sensor made of millions of microscopic pixels, similar to those found in smartphones but far more precise. As the spray of particles from the annihilation hits the sensor, it leaves behind tiny tracks of electrical charge, which the camera records as distinct clusters of light.

The researchers faced a significant hurdle: the environment is crowded with particles, and the surface where the antiprotons hit is complex and uneven. They needed to ensure that the camera could distinguish between a single antiproton event and background noise, and that it could handle millions of events without getting confused. Through careful testing, they found that the camera's sensor is nearly perfect for this job. It is so sensitive that it detects almost every particle that passes through it, yet it remains blind to the background radiation that usually confuses other detectors. By analyzing the size and shape of the clusters of light on the sensor, the team could determine the angle at which the particles arrived and even estimate the thickness of the sensor's active layer. They discovered that for every antiproton that hit the target, a predictable number of new charged particles emerged, creating a reliable signature that the camera could count with high precision.

To turn these counts into an exact number of antiprotons, the team had to account for the geometry of their setup and the behavior of the particles as they traveled from the target to the camera. They used powerful computer simulations to model how the particles moved through the vacuum and interacted with the surrounding equipment. These simulations helped them understand that the number of particles reaching the camera depends on the distance from the target and the specific materials the particles pass through. By comparing the camera's counts at two different locations—one close to the beam source and one further down the line—they could calibrate their system. They found that the ratio of particles detected at the two spots matched their computer models, confirming that their method was sound. This allowed them to create a simple formula: by counting the clusters on the camera and knowing the distance and the average number of particles produced per antiproton, they could calculate the total number of antiprotons in the beam.

The result is a new way to measure antimatter that is both robust and accurate. The team demonstrated that their method can reconstruct millions of antiproton annihilations with a total error margin of roughly ten percent. This level of precision is achieved because the camera counts the particles directly, bypassing many of the uncertainties that plague other measurement techniques. The approach minimizes errors caused by the complex surface of the target or the varying number of particles produced in each collision, as these factors cancel out when the system is calibrated against a known beam intensity. The success of this technique means that the GBAR experiment can now monitor its antiproton supply with great confidence, ensuring that the subsequent steps of creating and studying antihydrogen are based on a solid foundation. It is a quiet but essential victory, turning a chaotic spray of subatomic debris into a clear, countable signal that brings scientists one step closer to understanding the fundamental nature of our universe.