Thinking small: New ideas in the search for dark matter

Date:
June 16, 2020
Source:
California Institute of Technology
Summary:
Magnetic 'quasiparticles' called magnons may help scientists detect
dark matter.



FULL STORY ========================================================================== Since the 1980s, researchers have been running experiments in search of particles that make up dark matter, an invisible substance that permeates
our galaxy and universe. Coined dark matter because it gives off no light,
this substance, which constitutes more than 80 percent of matter in our universe, has been shown repeatedly to influence ordinary matter through
its gravity.

Scientists know it is out there but do not know what it is.


==========================================================================
So researchers at Caltech, led by Kathryn Zurek, a professor of
theoretical physics, have gone back to the drawing board to think of new
ideas. They have been looking into the possibility that dark matter is
made up of "hidden sector" particles, which are lighter than particles
proposed previously, and could, in theory, be found using small,
underground table-top devices. In contrast, scientists are searching
for heavier dark matter candidates called WIMPs (weakly interacting
massive particles) using large-scale experiments such as XENON, which
is installed underground in a 70,000-gallon tank of water in Italy.

"Dark matter is always flowing through us, even in this room" says Zurek,
who first proposed hidden sector particles over a decade ago. "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." In a new paper accepted for publication in the journal Physical Review Letters, the physicists
outline how the lighter-weight dark matter particles could be detected
via a type of quasiparticle known as a magnon. A quasiparticle is an
emergent phenomenon that occurs when a solid behaves as if it contains
weakly interacting particles. Magnons are a type of quasiparticle in
which electron spins -- which act like little magnets -- are collectivity excited. In the researchers' idea for a table-top experiment, a magnetic crystalized material would be used to look for signs of excited magnons generated by dark matter.

"If the dark matter particles are lighter than the proton, it becomes
very difficult to detect their signal by conventional means," says study
author Zhengkang (Kevin) Zhang, a postdoctoral scholar at Caltech. "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. 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."
Such an experiment is only theoretical at this point but may eventually
take place using small devices housed underground, likely in a mine,
where outside influences from other particles, such as those in cosmic
rays, can be minimized.

One telltale sign of a dark matter detection in the table-top experiments
would be changes to the signal that depend on the time of day. This is
due to the fact that the magnetic crystals that would be used to detect
the dark matter can be anisotropic, meaning that the atoms are naturally arranged in such a way that they tend to interact with the dark matter
more strongly when the dark matter comes in from certain 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,"
says Zhang.

"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 researchers have other ideas about how
dark matter may reveal itself, in addition to through magnons. They
have proposed that the lighter dark matter particles could be detected
via photons as well as with another type of quasiparticle called a
phonon, which is caused by vibrations in a crystal lattice. Preliminary experiments based on photons and phonons are underway at UC Berkeley,
where the team was based prior to Zurek joining the Caltech faculty in
2019. The researchers say that the use of these multiple strategies to
look for dark matter is crucial because they complement each other and
would help confirm 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," says Zhang.


========================================================================== Story Source: Materials provided by
California_Institute_of_Technology. Original written by Whitney
Clavin. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Tanner Trickle, Zhengkang Zhang, Kathryn M. Zurek. Detecting
Light Dark
Matter with Magnons. Physical Review Letters, 2020; 124 (20) DOI:
10.1103/PhysRevLett.124.201801 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200616155946.htm

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