Timing-Based Search for Magnetic Monopoles with the NOvA Detector on the Surface

The Invisible Ghost Hunters of Ash River

Imagine the universe is a bustling, chaotic city, constantly bombarded by invisible rain made of tiny, high-speed particles. Most of this "rain" is made of familiar stuff like protons and electrons, but physicists have long suspected there might be a very rare, very heavy, and very strange type of particle raining down on us too: the magnetic monopole. Think of a regular magnet as a pair of twins who are inseparable; if you cut a magnet in half, you don't get a North-only piece and a South-only piece. You just get two smaller magnets, each with both a North and a South. A magnetic monopole, however, would be the ultimate rebel: a single particle that is only a North pole or only a South pole.

For decades, scientists have been hunting for these magnetic loners. They are predicted by some of our biggest theories about how the universe works, but despite looking everywhere from deep underground mines to the tops of mountains, no one has ever found a convincing one. The big question is: do they exist? If they do, how heavy are they, and how fast are they moving? Finding one would be like discovering a new color or a new law of physics; it would rewrite our understanding of the cosmos. But if they are too heavy or too slow, they might get stopped by the Earth's atmosphere before they ever reach our detectors, making them incredibly hard to catch.

The Great Monopole Sweep

In this paper, a massive team of scientists known as the NOvA Collaboration decided to try a different angle. Instead of hiding deep underground where they are safe from the noisy "rain" of cosmic rays, they looked at a giant detector sitting right on the surface of the Earth in Ash River, Minnesota. This detector, called the NOvA Far Detector, is a 14,000-ton box filled with thousands of plastic tubes packed with a special liquid that glows when particles zip through it. It was originally built to study neutrinos (ghostly particles that pass through everything), but the team realized it could also be a perfect trap for slow-moving magnetic monopoles.

The scientists treated the detector like a giant, high-speed camera waiting to snap a picture of a slow-moving ghost. They knew that if a heavy magnetic monopole drifted through the detector, it wouldn't move like a speeding bullet. Instead, it would move very slowly—so slowly that it would take a long time to cross the room. Because it's so heavy and charged, it would leave a very specific, straight, glowing trail as it passed through the liquid, unlike the messy, chaotic trails left by the billions of regular cosmic rays that constantly bombard the surface.

To find these slow ghosts, the team developed a clever "trigger" system. Imagine trying to spot a slow turtle walking across a busy highway while ignoring the thousands of speeding cars. The NOvA computer system looked for pairs of glowing signals that happened very close together in time and space, forming a straight line. If the signals were too fast, it was just a regular cosmic ray. But if the signals were spaced out just right to suggest a slow, steady march, the computer saved the data for a closer look. They ran this search for a massive amount of time—2,743 days, or about 7.5 years—scanning through billions of events.

The Verdict: No Ghosts Found, But a New Map Drawn

After sifting through all that data, the result was clear: no magnetic monopoles were found. Not a single event matched the description of a slow-moving monopole. While this might sound like a disappointment, in science, a "null result" is actually a huge victory because it tells us where not to look.

The team used this lack of sightings to set a very strict rule: if monopoles exist, they must be rarer than a specific number. They calculated that the flow of these particles through the sky is less than 8 × 10⁻¹⁶ cm⁻²s⁻¹sr⁻¹ for monopoles moving at speeds between 6 × 10⁻⁴ and 5 × 10⁻³ times the speed of light, provided they are heavier than 10⁹ GeV. To put that in perspective, this is an incredibly tiny number, meaning that if these particles exist, they are so rare that you might have to wait a very long time to see just one.

What makes this discovery special is the "low-mass" territory they explored. Previous underground experiments were great at finding super-heavy, super-slow monopoles, but they were too deep to catch lighter ones that might get stopped by the Earth's atmosphere. Because the NOvA detector is on the surface, it could spot these lighter, slightly faster (but still very slow) candidates that other detectors missed. They effectively drew a new boundary on the map of the universe, saying, "We are 90% confident that monopoles in this specific speed and weight range do not exist in the quantities we were hoping for."

The paper also explains that they simulated millions of fake monopole events to make sure their detector would have seen them if they were there. They found that their system was about 81% efficient at catching these slow tracks. Since they saw zero real candidates, they can confidently say that the universe is much emptier of these specific types of magnetic loners than some theories had hoped. The search continues, and the NOvA detector keeps watching the sky, ready to catch the next ghost if it decides to show up.