Shining Light on the Dark Sector: Search for Axion-like Particles and Other New Physics in Photonic Final States with FASER

Deep within the subatomic world, physicists have long suspected that the universe is far more crowded than the standard model of particle physics suggests. This established framework successfully describes the known building blocks of matter and the forces that govern them, yet it leaves out a crucial piece of the puzzle: dark matter. While we know dark matter exists because of its gravitational pull on galaxies, we have never directly detected the particles that make it up. Many theories propose that these hidden particles are light, weakly interacting, and perhaps long-lived, meaning they can travel vast distances before vanishing. If such particles exist, they might be produced in the violent collisions of protons at the Large Hadron Collider, only to slip through the massive detectors designed to catch ordinary matter and travel hundreds of meters into the surrounding rock. Finding them would not only reveal the nature of dark matter but also open a window into a "dark sector" of physics that operates alongside the one we know.

At the Large Hadron Collider, a machine that smashes protons together at nearly the speed of light, a specialized experiment called FASER is positioned 480 meters away from the main collision point, tucked inside a narrow tunnel. This location is strategic; it sits directly in the path of particles that shoot forward from the collisions, a region where light, elusive particles are most likely to appear. While the main detectors at the collision site are surrounded by thick layers of concrete and rock that stop almost everything, FASER is designed to catch the few particles that can punch through this barrier. In its latest work, the FASER collaboration turned its attention to a specific type of hypothetical particle called an axion-like particle. These are theoretical cousins of the axion, a particle originally proposed to solve a deep mystery in physics regarding why the universe behaves the way it does under the strong nuclear force. Unlike the heavy particles usually sought in high-energy physics, these axion-like particles are light and could decay into pairs of high-energy photons, or particles of light, as they travel through the detector.

The researchers analyzed data collected during 2022 and 2023, corresponding to a massive amount of proton-proton collisions. They were looking for a very specific signature: a sudden, intense burst of energy in their electromagnetic calorimeter, a device that measures the energy of particles, with absolutely no signal in the surrounding veto systems that would indicate a charged particle had entered. Since the hypothetical particles are neutral, they would pass through the initial detectors without leaving a trace, only to decay into two photons deep inside the instrument, creating a clean, high-energy flash. The team had to be incredibly careful to distinguish this signal from the background noise, which is dominated by neutrinos. Neutrinos are ghostly particles that also pass through the rock and can interact with the detector material, sometimes mimicking the energy signature of the sought-after particles. To manage this, the scientists used sophisticated computer simulations to predict exactly how many neutrino interactions should occur in their specific energy range.

The result of this meticulous search was a single event. The detector recorded one instance where a high-energy deposit appeared in the calorimeter with no upstream activity, matching the expected pattern of a decaying axion-like particle. However, the team's calculations showed that they expected to see about 0.44 neutrino interactions that could look exactly the same, with a margin of uncertainty that makes the observation of one event perfectly consistent with random chance. In other words, the single event they found was likely just a neutrino doing what neutrinos do, rather than a discovery of new physics. Because the observation did not significantly exceed the background expectation, the researchers could not claim to have found a new particle. Instead, they used this result to set new, world-leading limits on where these particles could exist.

By showing that no excess of events appeared beyond what neutrinos could explain, the experiment effectively ruled out a wide range of possibilities for these axion-like particles. The team determined that if these particles exist and interact with the weak nuclear force in the way they modeled, they cannot have certain combinations of mass and strength of interaction. Specifically, they excluded the possibility of these particles having masses between 100 and 250 million electronvolts with interaction strengths between 3 times 10 to the minus 5 and 5 times 10 to the minus 4 inverse GeV. They also extended these limits to heavier masses, ruling out particles up to 300 million electronvolts for weaker interactions. This is a significant step forward because it probes a region of parameter space that had never been explored before, complementing other experiments that look for lighter or heavier versions of these particles.

The study did not stop at just one type of particle. The same data and analysis methods were applied to several other theoretical models that would produce a similar flash of light in the detector. These included models involving particles that interact with gluons, a type of force carrier in the nucleus, as well as new types of force-carrying particles related to the number of protons and neutrons in matter, and light scalar particles that prefer to interact with up-quarks. For each of these scenarios, the lack of a significant excess of events allowed the team to draw new boundaries on where these particles could hide. They found that their experiment is sensitive to regions of the universe's hidden physics that were previously unconstrained, effectively narrowing the search space for future discoveries.

The success of this search relied heavily on the unique geometry of the FASER detector and its ability to filter out the overwhelming background of ordinary particles. The detector is equipped with layers of scintillating material that act as a gatekeeper, stopping any charged particles like muons from entering the sensitive volume. Only neutral particles can pass through this gate. Once inside, the detector looks for the specific pattern of energy deposition that would result from a particle decaying into two photons. The analysis required precise timing to ensure the event happened in sync with the proton collisions and careful calibration to ensure the energy measurements were accurate. The team also had to account for the fact that the beam of particles is not perfectly straight but has a slight angle, which changes the path of the particles slightly over the 480-meter journey.

This work represents the first time FASER has searched for new particles that decay into photons, marking a new chapter in its exploration of the dark sector. While the single event observed was not a discovery, the ability to rule out so many theoretical possibilities is a powerful scientific outcome. It tells us that if these light, long-lived particles exist, they are either much weaker in their interactions or have different properties than the models tested here. The experiment demonstrates that looking far downstream from the collision point is a viable and powerful strategy for finding physics that the main detectors miss. As the Large Hadron Collider continues to operate and collect more data, FASER will be able to push these limits even further, potentially one day catching the faint signature of a particle that has traveled through the earth to reveal the secrets of the dark sector.