Oct 11 2019
As part of a study that could completely transform the quest for the elusive dark matter, physicists from Stockholm University and the Max Planck Institute for Physics have resorted to the use of plasmas.
The researchers propose a new instrument for searching dark matter axions using tunable plasmas. (Image credit: Alexander Millar/Stockholm University)
Dark matter, a strange substance, forms 85% of the matter in the universe. The so-called axion, which was actually introduced to describe why the Strong Force (that holds neutrons and protons together) is the same forward and backward in time, would offer a natural explanation for dark matter. Instead of discrete particles, axion dark matter would create a ubiquitous wave that flows through the entire space.
Although the axion is one of the ideal explanations for dark matter, only recently has it gained the attention of extensive experimental effort. This revolution has resulted in a race to bring about new concepts on ways to search for the axion in all the areas in which it could be hidden.
Finding the axion is a bit like tuning a radio: you have to tune your antenna until you pick up the right frequency. Rather than music, experimentalists would be rewarded with ‘hearing’ the dark matter that the Earth is travelling through. Despite being well motivated, axions have been experimentally neglected during the three decades since they were named by coauthor Frank Wilczek.
Dr Alexander Millar, Study Author, Postdoctor, Department of Physics, Stockholm University
A Plasma Gives a Larger Signal
The main understanding offered by the researchers’ new study is that within a magnetic field, axions would produce a small electric field that could be employed to trigger oscillations in the plasma. A plasma is a material in which charged particles like electrons can freely flow like a fluid. The oscillations in plasma amplify the signal, resulting in an improved “axion radio.”
In contrast to conventional experiments that are based on resonant cavities, there is almost no restriction on the size of these plasmas, thereby providing a larger signal. The difference is almost similar to the difference between a radio broadcast tower and a walkie talkie.
Without the cold plasma, axions cannot efficiently convert into light. The plasma plays a dual role, both creating an environment which allows for efficient conversion, and providing a resonant plasmon to collect the energy of the converted dark matter.
Dr Matthew Lawson, Study Author, Postdoctor, Department of Physics, Stockholm University
“This is totally a new way to look for dark matter, and will help us search for one of the strongest dark matter candidates in areas that are just completely unexplored. Building a tuneable plasma would allow us to make much larger experiments than traditional techniques, giving much stronger signals at high frequencies” stated Dr Alexander Millar.
According to the researchers, this “axion radio” can be tuned using a so-called “wire metamaterial,” which is a framework of wires thinner than a strand of human hair that can be moved to alter the plasma’s characteristic frequency. Within a huge, powerful magnet, analogous to those used in Magnetic Resonance Imaging machines in hospitals, a wire metamaterial changes into a highly sensitive axion radio.
Using plasmas to look for dark matter will not just be an interesting concept. Working closely with the researchers, an experimental team from Berkeley has been performing research and development using the idea, with the aim of developing such an experiment very soon.
Plasma haloscopes are one of the few ideas that could search for axions in this parameter space. The fact that the experimental community has latched onto this idea so quickly is very exciting and promising for building a full scale experiment.
Dr Alexander Millar, Study Author, Postdoctor, Department of Physics, Stockholm University
Source: https://www.su.se/cmlink/stockholm-university