Decades after dark matter was first discovered, a Yale-built detection device is leading the search for the invisible phenomenon.
Researchers from Yale, the University of California, Berkeley and the University of Colorado collaborated on the project, called Haloscope At Yale Sensitive to Axion Cold Dark Matter, also known as HAYSTAC, which has been in the works since 2010. It consists of a device that aims to detect the axion, a hypothetical subatomic particle that theoretical physicists speculate could constitute dark matter. The particle, first proposed in the 1980s and named after a laundry detergent popular at the time, has been the subject of intense research for over 30 years.
“It’s an experiment to search for axions, which is a possible form of dark matter,” said Karsten Heeger, director of the Wright Laboratory, a theoretical physics lab at Yale. “[The researchers] have built a novel apparatus that works within a frequency range that has not been tested before. This is one of several experiments that are being led out of Wright Lab to understand what the nature of dark matter is, what the nature of the invisible universe is.”
According to Ben Brubaker GRD ’18, lead author of the study, there is a persistent discrepancy between the observable mass in the universe and the mass required for objects in space to achieve the velocity that they have. The invisible mass that enables such speeds is dark matter. Since scientists have determined that dark matter cannot be made of any known particles, they must look to the hypothetical, said Brubaker. While the existence of the axion has not been proven, theoretical physicists have determined certain hypothetical particles with certain weak interactions would have the right properties to explain dark matter.
Based on the standard model of elementary particle physics, the axion is 100,000 times lighter than a neutrino, which is itself 1 million times lighter than an electron, according to Steve Lamoreaux, a professor of physics and one of the senior researchers. Since axions have such low mass, detecting them is difficult even if they are the dominant dark matter particle, which requires about 10 trillion axions per cubic centimeter, said Lamoreaux.
“There are a lot of these light particles, but they interact very feebly with each other, other particles or light,” said Lamoreaux. “It’s like a perfect background gas — everything moves through [it] without any friction at all.”
The detection device generates a strong magnetic field that converts the axion into a radio frequency photon at about five gigahertz — roughly the frequency of a cellphone signal. The axion’s mass determines the frequency of the radio signal, so detecting a signal at a specific frequency would allow the scientists to infer the particle’s mass, according to Lamoreaux.
Lamoreaux compared searching for this signal to driving through North Dakota while looking for a specific radio station into which to tune.
Whereas prior efforts to detect axions focused on low-mass particles, the Wright Laboratory’s instrument has instead investigated a range of higher masses, including axions that are 10 times heavier than those targeted by previous experiments. Brubaker and his fellow researchers achieved sensitivity to these formerly undetectable higher-mass axions by adapting extremely low-noise amplifiers originally developed for quantum computing research.
“Generally speaking, the detectors for lower-mass axions paradoxically need to be much bigger, because they end up turning the axions into lower-frequency or longer-wavelength electromagnetic waves,” said Brubaker. “The [lower-mass] detectors are more challenging [to make], but you gain a lot just by the fact that you are sensitive to a larger volume of dark matter. The fact that we don’t get an enhancement from large volumes [in this experiment] means we need to rely on extremely good noise performance to get the required sensitivity.”
The amplifiers used in the experiment were produced by professor Konrad Lehnert’s group at the University of Colorado and are among the most sensitive devices ever made, according to Lamoreaux. The amplifiers’ power sensitivity unit is the yoctowatt (10^-24), which is the smallest prefix on the SI scale. According to Lamoreaux, a yoctowatt is equivalent to the energy falling on a human eye on Earth from a lit match on the surface of the moon.
Due to the sensitivity of the device, even slight changes can cause serious internal damage. A campus power outage in March 2016 caused the device’s magnetic field to collapse and researchers were only able to repair and restart the project two months later.
Now that the first results have been published, Lamoreaux’s team has made adjustments to the device to improve the experiment’s results.
“They have recently procured a new dilution refrigerator, which is a device that keeps the experiment very cold,” said Karsten. “One of the updates is to install the experiments into this new refrigerator system to enhance its sensitivity.”
According to the European Organization for Nuclear Research, although invisible dark matter makes up most of the universe, it can only be detected from its gravitational effects.