There are particles that we discover in a detector. And there are others that are born much earlier, on paper, because something in the equations does not fit. Axions belong to this second category. They are hypothetical particles since we have not detected them yet, but they have a very specific profile: They would be extremely light, without an electrical charge and with almost no ability to interact with matter. In other words, they would traverse the world without leaving a trace. And, precisely for that reason, they could be everywhere. Their importance does not come from what they do, but from what they could explain.
For starters, axions are one of the best candidates for solving the biggest mystery of modern cosmology: dark matter. We know that about 85% of the matter in the universe is invisible, but its gravity holds galaxies together. Axions fit that role surprisingly well: They would be abundant, stable, and nearly impossible to detect directly.
But his story doesn’t begin there. In reality, axions are born from a smaller, more uncomfortable problem within particle physics. The theory that describes the strong nuclear force, Quantum Chromodynamics, describes behavior that experiments do not observe. Simply put: The equations say that neutrons should show a slight “preference” over electric fields… but they don’t. That mismatch is known as the strong CP problem.
Axions appear as an elegant solution: they introduce a mechanism that “adjusts” that behavior automatically. It is not about forcing the theory, but about adding a piece that makes the universe behave as we really observe. A solution that could also explain dark matter. The problem is that all this only works if axions exist. And detecting them is extraordinarily difficult.
This is where a new study published on Arxiv comes in. The experiment, known as SPACE (Student Project for an Axonal Cavity Experiment), takes a strategy that, at first glance, seems almost simple. It uses a resonant cavity (a kind of metal box designed with millimeter precision) placed within an extremely intense magnetic field. In this case, of up to 14 Tesla, a magnetic field about 300,000 times more intense than that of the Earth.
The idea is to take advantage of a very particular property: if axions exist, they could transform into photons (the particles of light, so to speak) when passing through that magnetic field. But not just any light, but an extremely weak signal, at a very specific frequency.
Thus, the experiment works like an ultra-precise radio, tuned to a very specific range of the “dial” of the universe: in this case, axions with a mass around 16.6 microelectronvolts, a particle billions of times lighter than an electron.The result was silence. No signal was detected. But, paradoxically, that silence is full of information. The experiment has made it possible to rule out that axions, if they exist, have certain properties in that specific range. And it has done so with remarkable precision: improving previous limits by more than two orders of magnitude.
And here comes something especially interesting: the spirit of the project itself. According to the authors, led by Agit Akgümüs, the objective was not to compete with large international experiments, but to demonstrate that even smaller devices can provide relevant results. The key is precision and exploring regions of parameter space that other experiments have not yet covered.
“The advantage of working with dark matter, or axions, is that we expect it to be present throughout our galaxy – concludes Akgümüs -. So, in essence, regardless of where the experiment is carried out, there is dark matter available to experiment with. Our experiment covers only a small region, with limited sensitivity, but it still helps narrow down the possibilities. To find the particle, we need much larger experiments or many different experiments, each exploring a specific region. We were told that setups like ours could become some day in standard laboratory experiments for students. In a way, we may have anticipated that future, demonstrating that it is already possible to build and operate such an experiment on a small scale.”