Dark matter: Our new experiment aims to convert ghostly matter into actual light

A ghost is haunting our universe. This has been known in astronomy and cosmology for decades. Observations show that approximately 85% of the matter in the universe is mysterious and invisible. These two qualities are reflected in its name: dark matter.

Multiple experiments aimed to reveal its composition, but despite decades of exploration, scientists have found nothing. Now, a new experiment we are building at Yale University in the United States offers a new strategy.

Dark matter has been around the universe since ancient times, pulling stars and galaxies together. It is invisible and subtle and does not appear to interact with light or any other matter. In fact, it has to be something completely new.

The Standard Model of particle physics is incomplete, and that’s a problem. We have to find new elementary particles. Surprisingly, the same flaws in the standard model provide valuable clues as to where they might be hiding.

The trouble with neutrons

Let’s take neutrons as an example. Together with protons, it makes up the nucleus. Although generally neutral, the theory states that it is composed of three types of charged particles called quarks. So we expect some parts of the neutron to be positively charged and other parts to be negatively charged, meaning it has what physicists call an electric dipole moment.

However, many attempts to measure it have yielded the same result: it is too small to be detected. Another ghost. We are not talking about a shortcoming of the instrument, but about a parameter that must be less than one part per 10 billion. It’s so small that one wonders if it could be exactly zero.

In physics, however, mathematical zero is always a strong statement. In the late 1970s, particle physicists Roberto Peccei and Helen Quinn (and later Frank Wilczek and Steven Weinberg) tried to combine theory and evidence.

Maybe the parameter is not zero, they thought. Instead, it is a dynamic quantity that slowly lost charge after the Big Bang, evolving to zero. Theoretical calculations show that if such an event occurred, it would leave behind a large number of light, sneaky particles.

These are called axions, named after the brand of detergent because they solve the problem of neutrons. even more. If axions were created in the early universe, then they have been around ever since. Best of all, their properties match all expectations for dark matter. For these reasons, axions have become one of the favorite candidates for dark matter.

Axions interact only weakly with other particles. However, this means they will still have some interactions. Invisible axions can even transform into ordinary particles, including ironically photons, which are the essence of light. This may occur under certain circumstances, such as in the presence of a magnetic field. This is a godsend for experimental physicists.

experimental design

Many experiments have attempted to evoke the axion ghost in the controlled environment of the laboratory. For example, some are designed to convert light into axions, which are then converted back into light on the other side of the wall.

Currently, the most sensitive method is to use a device called a halograph to target the halo of dark matter that permeates the Milky Way (and thus the Earth). It’s a conducting cavity immersed in a strong magnetic field; the former captures the dark matter around us (assuming it’s an axion), while the latter converts it into light. The result is an electromagnetic signal inside the cavity, which oscillates at a characteristic frequency that depends on the axion mass.

The system works like a receiving radio. It needs to be tuned appropriately to intercept the frequencies we are interested in. If the axion and cavity frequencies don’t match, it’s like tuning a radio to the wrong channel.

Unfortunately, the pipeline we are looking for cannot be predicted in advance. We have no choice but to scan all potential frequencies. It’s like picking a radio station in a sea of ​​white noise, like finding a needle in a haystack, the old radios needed to get louder or quieter every time we turned the frequency knob.

However, these are not the only challenges. Cosmology points to tens of gigahertz as the latest, promising frontier in the search for axions. Since higher frequencies require smaller cavities, the cavities required to explore this region are too small to capture a meaningful amount of signal.

New experiments are trying to find alternative pathways. Our Axion Longitudinal Plasma Halo (Alpha) experiment uses a new concept of metamaterial-based cavities.

A metamaterial is a composite material that has overall properties that are distinct from its components and are more than the sum of its parts. The characteristic frequency of a cavity filled with conductive rods appears to be reduced by a factor of a million, while its volume barely changes. This is exactly what we need. Additionally, these rods offer a built-in, easy-to-adjust tuning system.

We are currently building the setup and will be ready to acquire data in a few years. The technology holds great promise. Its development was the result of collaboration between solid-state physicists, electrical engineers, particle physicists and even mathematicians.

As elusive as the axion is, it’s driving progress that no ghost can take away.