Jun 17 2020
Ever since the 1980s, scientists have been conducting experiments to look for particles that constitute dark matter, an imperceptible substance that spreads across the universe and the Milky Way galaxy.
Caltech physicists are designing new ways to detect dark matter using magnons, which are quasiparticles that arise when electron spins—which act like little magnets—are collectivity excited. In this scheme, a magnetic crystal material would be used to look for signs of excited magnons generated by dark matter. Image Credit: California Institute of Technology/Zhengkang “Kevin” Zhang.
Dubbed dark matter because it does not emit any light, it has been repeatedly demonstrated that this invisible substance, which makes up over 80% of matter in the universe, impacts normal matter via its gravity. Although researchers know it is out there, they do not exactly know what it is.
Therefore, a team of researchers from the California Institute of Technology, headed by Kathryn Zurek, a professor of theoretical physics, started planning something again to dredge up new concepts.
The researchers have been searching for the possibility that dark matter is composed of “hidden sector” particles, which are lighter than particles as indicated before, and could, theoretically, be detected using tiny, underground table-top devices.
On the contrary, some researchers are looking for heavier dark matter candidates known as weakly interacting massive particles (WIMPs) through large-scale experiments, like XENON, which has been set up underground in a 70,000-gallon tank of water in Italy.
Dark matter is always flowing through us, even in this room. As we move around the center of the galaxy, this steady wind of dark matter mostly goes unnoticed. But we can still take advantage of that source of dark matter, and design new ways to look for rare interactions between the dark matter wind and the detector.
Kathryn Zurek, Professor, Walter Burke Institute for Theoretical Physics, California Institute of Technology
Zurek had first suggested the hidden sector particles more than 10 years ago.
In a recent article accepted for publication in the Physical Review Letters journal, the physicists showed how the lighter dark matter particles can possibly be identified through a type of quasiparticle called a magnon. A quasiparticle is essentially an emergent phenomenon that takes place when a solid acts as if it comprises weakly interacting particles.
Magnons can be described as a form of quasiparticle, where electron spins—acting similar to tiny magnets—are collectivity activated. In the table-top experiment idea proposed by the researchers, a magnetic crystallized material would be employed to look for signs of activated magnons produced by dark matter.
If the dark matter particles are lighter than the proton, it becomes very difficult to detect their signal by conventional means. But, according to many well-motivated models, especially those involving hidden sectors, the dark matter particles can couple to the spins of the electrons, such that once they strike the material, they will induce spin excitations, or magnons.
Zhengkang (Kevin) Zhang, Study Author and Postdoctoral Scholar, California Institute of Technology
Zhang continued, “If we reduce the background noise by cooling the equipment and moving it underground, we could hope to detect magnons generated solely by dark matter and not ordinary matter.”
An experiment like that is only theoretical at this juncture but may ultimately occur using miniature devices buried underground, perhaps in a mine, where outside effects from other kinds of particles, like those in cosmic rays, can be decreased.
One evident sign of the detection of dark matter in the table-top experiments would be the signal variations that rely on the time of day. This is because the magnetic crystals that would be used for detecting the dark matter may turn out to be anisotropic. This means the atoms are naturally organized such that they are likely to communicate with the dark matter more robustly when the dark matter comes in from specific directions.
As Earth moves through the galactic dark matter halo, it feels the dark matter wind blowing from the direction into which the planet is moving. A detector fixed at a certain location on Earth rotates with the planet, so the dark matter wind hits it from different directions at different times of the day, say, sometimes from above, sometimes from the side.
Zhengkang (Kevin) Zhang, Study Author and Postdoctoral Scholar, California Institute of Technology
Zhang continued, “During the day, for example, you may have a higher detection rate when the dark matter comes from above than from the side. If you saw that, it would be pretty spectacular and a very strong indication that you were seeing dark matter.”
The scientists have other concepts regarding how dark matter may expose itself, besides magnons. They have now suggested that the lighter dark matter particles can potentially be identified through photons and also with another type of quasiparticle known as a phonon, which is induced by vibrations in a crystal lattice.
Initial experiments based on phonons and photons are ongoing at UC Berkeley, where the researchers were based before Zurek joined the California Institute of Technology faculty in 2019. According to scientists, the use of various strategies to search for dark matter is very important because they complement one another and would help validate each other’s results.
“We’re looking into new ways to look for dark matter because, given how little we know about dark matter, it's worth considering all the possibilities,” concluded Zhang.
The study titled “Detecting Light Dark Matter with Magnons” was financially supported by the National Science Foundation (NSF) and the Department of Energy (DOE). Tanner Trickle is another co-author of the study and a graduate student at UC Berkeley.
Journal Reference:
Trickle, T., et al. (2020) Detecting Light Dark Matter with Magnons. Physical Review Letters. doi.org/10.1103/PhysRevLett.124.201801.