Search for events with one displaced vertex from long-lived neutral particles decaying into hadronic jets in the ATLAS muon spectrometer in $pp$ collisions at $\sqrt{s}=13$ TeV

The universe is built on a set of rules that physicists have spent a century decoding, a framework known as the Standard Model. This model successfully explains how the smallest known particles interact, but it leaves some of the biggest questions in the cosmos unanswered. It cannot explain what dark matter is, why there is more matter than antimatter, or how particles acquire their mass in a way that feels natural. To fill these gaps, scientists have proposed the existence of new, hidden particles. Many of these theoretical particles are predicted to be "long-lived," meaning they do not vanish the instant they are created. Instead, they travel a measurable distance away from the point of their creation before decaying into ordinary matter. Finding these elusive travelers would be a monumental discovery, offering a direct window into the physics that lies beyond our current understanding.

At the Large Hadron Collider, a massive particle accelerator buried beneath the border of France and Switzerland, scientists smash protons together at nearly the speed of light to recreate the conditions of the early universe. The ATLAS experiment, one of the giant detectors surrounding the collision point, acts as a high-speed camera capturing the debris of these collisions. In a recent study, the ATLAS collaboration turned its attention to a very specific and unusual signature: a single point of decay occurring far away from the collision site, deep inside the outer layers of the detector. While most particles decay immediately or leave a clear trail, these hypothetical long-lived particles would travel meters, sometimes even tens of meters, before suddenly bursting into a spray of ordinary particles. The researchers were looking for a "displaced vertex," a term for a collision point that appears in a place where no collision actually happened, signaling that a hidden particle had traveled there first.

The team analyzed a vast amount of data collected between 2015 and 2018, corresponding to 140 units of collision intensity. They focused their search on the muon spectrometer, the outermost shell of the ATLAS detector, which sits between three and fourteen meters away from the center of the collision. This region is typically reserved for catching muons, a type of particle that can penetrate deep into matter. However, if a long-lived particle decays there, it would create a cluster of tracks that look like a tiny explosion occurring in empty space. To find these rare events, the scientists developed specialized computer algorithms capable of reconstructing these distant, isolated bursts of activity, filtering out the overwhelming noise of ordinary particle collisions that happen much closer to the center.

The search was designed to test several different theories about what these hidden particles might be. Some theories suggest they are related to the Higgs boson, the particle discovered in 2012 that gives other particles mass. Others propose they are linked to the imbalance between matter and antimatter in the universe, or that they are a type of "axion-like particle" that interacts very weakly with light. The researchers also looked for scenarios where these particles are produced alongside a known particle called a Z boson. By casting a wide net across these different possibilities, the team could determine if any of these theoretical models matched the reality of their data.

After sifting through the millions of recorded events, the researchers found no evidence of these long-lived particles. The number of events they observed in the outer detector matched exactly what they expected to see from known background processes, such as ordinary particles punching through the detector's inner layers and mimicking a distant decay. Because no new signal appeared, the team was able to set strict limits on the existence of these particles. They determined that if these particles do exist, they cannot be produced at the rates predicted by many popular theories. Specifically, for models involving the Higgs boson, they ruled out the possibility that the Higgs decays into these long-lived particles more than one percent of the time, for particles that travel anywhere from five centimeters to forty meters before decaying.

For the models involving axion-like particles, this search produced the strongest constraints to date for particles that travel more than a few tens of centimeters. The study also combined its results with a previous search that looked for two such decays instead of one, extending the sensitivity to even longer lifetimes. While the search did not find the new particles it was hunting for, the result is a significant step forward in the quest to understand the universe. By eliminating these specific possibilities, the scientists have narrowed the path for future discoveries, telling theorists exactly where they do not need to look and guiding the next generation of experiments toward the remaining, unexplored corners of physics.