New simulations refine axion mass, refocusing dark matter search
Using adaptive mesh refinement, supercomputer simulation narrows axion
mass range

Date:
February 25, 2022
Source:
University of California - Berkeley
Summary:
Axions are today's most popular candidate for dark matter, and
numerous experiments are trying to detect them in microwave cavities
where the axion should rarely convert into an electromagnetic
wave. But a new simulation of the production of axions in the
early universe provides a more refined mass estimate, and higher
frequency for the EM wave, that is outside the range of these
experiments. The new mass comes from adaptive mesh refinement in
supercomputer simulations.



FULL STORY ========================================================================== Physicists searching -- unsuccessfully -- for today's most favored
candidate for dark matter, the axion, have been looking in the wrong
place, according to a new supercomputer simulation of how axions were
produced shortly after the Big Bang 13.6 billion years ago.


========================================================================== Using new calculational techniques and one of the world's largest
computers, Benjamin Safdi, assistant professor of physics at the
University of California, Berkeley; Malte Buschmann, a postdoctoral
research associate at Princeton University; and colleagues at MIT and
Lawrence Berkeley National Laboratory simulated the era when axions
would have been produced, approximately a billionth of a billionth of a billionth of a second after the universe came into existence and after
the epoch of cosmic inflation.

The simulation at Berkeley Lab's National Research Scientific Computing
Center (NERSC) found the axion's mass to be more than twice as big as
theorists and experimenters have thought: between 40 and 180 microelectron volts (micro-eV, or?eV), or about one 10-billionth the mass of the
electron. There are indications, Safdi said, that the mass is close
to 65 ?eV. Since physicists began looking for the axion 40 years ago,
estimates of the mass have ranged widely, from a few ?eV to 500 ?eV.

"We provide over a thousandfold improvement in the dynamic range of
our axion simulations relative to prior work and clear up a 40-year old question regarding the axion mass and axion cosmology," Safdi said.

The more definitive mass means that the most common type of experiment
to detect these elusive particles -- a microwave resonance chamber
containing a strong magnetic field, in which scientists hope to snag
the conversion of an axion into a faint electromagnetic wave -- won't be
able to detect them, no matter how much the experiment is tweaked. The
chamber would have to be smaller than a few centimeters on a side to
detect the higher-frequency wave from a higher-mass axion, Safdi said,
and that volume would be too small to capture enough axions for the
signal to rise above the noise.

"Our work provides the most precise estimate to date of the axion mass
and points to a specific range of masses that is not currently being
explored in the laboratory," he said. "I really do think it makes sense
to focus experimental efforts on 40 to 180 ?eV axion masses, but there's
a lot of work gearing up to go after that mass range." One newer type
of experiment, a plasma haloscope, which looks for axion excitations
in a metamaterial -- a solid-state plasma -- should be sensitive to an
axion particle of this mass, and could potentially detect one.



==========================================================================
"The basic studies of these three-dimensional arrays of fine wires have
worked out amazingly well, much better than we ever expected," said
Karl van Bibber, a UC Berkeley professor of nuclear engineering who is
building a prototype of the plasma haloscope while also participating
in a microwave cavity axion search called the HAYSTAC experiment. "Ben's
latest result is very exciting. If the post-inflation scenario is right,
after four decades, discovery of the axion could be greatly accelerated."
If axions really exist.

The work will be published Feb. 25 in the journal Nature Communications.

Axion top candidate for dark matter Dark matter is a mysterious substance
that astronomers know exists -- it affects the movements of every star
and galaxy -- but which interacts so weakly with the stuff of stars and galaxies that it has eluded detection. That doesn't mean dark matter
can't be studied and even weighed. Astronomers know quite precisely how
much dark matter exists in the Milky Way Galaxy and even in the entire universe: 85% of all matter in the cosmos.



==========================================================================
To date, dark matter searches have focused on massive compact objects in
the halo of our galaxy (called massive compact halo objects, or MACHOs),
weakly interacting massive particles (WIMPs) and even unseen black
holes. None turned up a likely candidate.

"Dark matter is most of the matter in the universe, and we have no idea
what it is. One of the most outstanding questions in all of science is,
'What is dark matter?'" Safdi said. "We suspect it is a new particle
we don't know about, and the axion could be that particle. It could be
created in abundance in the Big Bang and be floating out there explaining observations that have been made in astrophysics." Though not strictly
a WIMP, the axion also interacts weakly with normal matter.

It passes easily through the earth without disruption. It was proposed
in 1978 as a new elementary particle that could explain why the neutron's
spin does not precess or wobble in an electric field. The axion, according
to theory, suppresses this precession in the neutron.

"Still to this day, the axion is the best idea we have about how to
explain these weird observations about the neutron," Safdi said.

In the 1980s, the axion began to be seen also as a candidate for dark
matter, and the first attempts to detect axions were launched. Using the equations of the well-vetted theory of fundamental particle interactions,
the so-called Standard Model, in addition to the theory of the Big Bang,
the Standard Cosmological Model, it is possible to calculate the axion's precise mass, but the equations are so difficult that to date we have
only estimates, which have varied immensely. Since the mass is known
so imprecisely, searches employing microwave cavities -- essentially
elaborate radio receivers -- must tune through millions of frequency
channels to try to find the one corresponding to the axion mass.

"With these axion experiments, they don't know what station they're
supposed to be tuning to, so they have to scan over many different possibilities," Safdi said.

Safdi and his team produced the most recent, though incorrect, axion
mass estimate that experimentalists are currently targeting. But as they
worked on improved simulations, they approached a team from Berkeley Lab
that had developed a specialized code for a better simulation technique
called adaptive mesh refinement. During simulations, a small part of
the expanding universe is represented by a three-dimensional grid over
which the equations are solved. In adaptive mesh refinement, the grid
is made more detailed around areas of interest and less detailed around
areas of space where nothing much happens.

This concentrates computing power on the most important parts of the simulation.

The technique allowed Safdi's simulation to see thousands of times
more detail around the areas where axions are generated, allowing a
more precise determination of the total number of axions produced and,
given the total mass of dark matter in the universe, the axion mass. The simulation employed 69,632 physical computer processing unit (CPU) cores
of the Cori supercomputer with nearly 100 terabytes of random access
memory (RAM), making the simulation one of the largest dark matter
simulations of any kind to date.

The simulation showed that after the inflationary epoch, little tornadoes,
or vortices, form like ropey strings in the early universe and throw
off axions like riders bucked from a bronco.

"You can think of these strings as composed of axions hugging the vortices while these strings whip around forming loops, connecting, undergoing a
lot of violent dynamical processes during the expansion of our universe,
and the axions hugging the sides of these strings are trying to hold
on for the ride," Safdi said. "But when something too violent happens,
they just get thrown off and whip away from these strings. And those
axions which get thrown off of the strings end up becoming the dark
matter much later on." By keeping track of the axions that are whipped
off, researchers are able to predict the amount of dark matter that
was created.

Adaptive mesh refinement allowed the researchers to simulate the universe
much longer than previous simulations and over a much bigger patch of
the universe than previous simulations.

"We solve for the axion mass both in a more clever way and also by
throwing just as much computing power as we could possibly find onto
this problem," Safdi said. "We could never simulate our entire universe
because it's too big.

But we don't need to stimulate our entire universe. We just need to
simulate a big enough patch of the universe for a long enough period
of time, such that we capture all of the dynamics that we know are
contained within that box." The team is working with a new supercomputing cluster now being built at Berkeley Lab that will enable simulations
that will provide an even more precise mass. Called Perlmutter, after
Saul Perlmutter, a UC Berkeley and Berkeley Lab physicist who won the
2011 Nobel Prize in Physics for discovering the accelerating expansion
of the universe driven by so-called dark energy, the next-generation supercomputer will quadruple the computing power of NERSC.

"We want to make even bigger simulations at even higher resolution,
which will allow us to shrink these error bars, hopefully down to the
10% level, so we can tell you a very precise number, like 65 plus or
minus 2 micro-eV. That then really changes the game experimentally,
because then it would become an easier experiment to verify or exclude
the axion in such a narrow mass range," Safdi said.

For van Bibber, who was not a member of Safdi's simulation team, the
new mass estimate tests the limits of microwave cavities, which work
less well at high frequencies. So, while the lower limit of the mass
range is still within the ability of the HAYSTAC experiment to detect,
he is enthused about the plasma haloscope.

"Over the years, new theoretical understanding has loosened the
constraints on the axion mass; it can be anywhere within 15 orders of magnitude, if you consider the possibility that axions formed before
inflation. It's become an insane task for experimentalists," said van
Bibber, who holds UC Berkeley's Shankar Sastry Chair of Leadership and Innovation. "But a recent paper by Frank Wilczek's Stockholm theory
group may have resolved the conundrum in making a resonator which
could be simultaneously both very large in volume and very high in
frequency. An actual resonator for a real experiment is still some ways
away, but this could be the way to go to get to Safdi's predicted mass."
Once simulations give an even more precise mass, the axion may, in fact,
be easy to find.

"It was really crucial that we teamed up with this computer science
team at Berkeley Lab," Safdi said. "We really expanded beyond the
physics field and actually made this a computing science problem."
Safdi's colleagues include Malte Buschmann of Princeton; MIT postdoctoral fellow Joshua Foster; Anson Hook of the University of Maryland; and
Adam Peterson, Don Willcox and Weiqun Zhang of Berkeley Lab's Center for Computational Sciences and Engineering. The research was largely funded
by the U.S. Department of Energy through the Exascale Computing Project (17-SC-20-SC) and through the Early Career program (DESC0019225).

Video: https://youtu.be/hrCN6tF087c Video
on measuring an axion: https://youtu.be/hikmvEbO-vA ========================================================================== Story Source: Materials provided by
University_of_California_-_Berkeley. Original written by Robert
Sanders. Note: Content may be edited for style and length.


========================================================================== Related Multimedia:
* Snapshots_from_simulation_of_the_early_universe ========================================================================== Journal Reference:
1. Malte Buschmann, Joshua W. Foster, Anson Hook, Adam Peterson, Don E.

Willcox, Weiqun Zhang, Benjamin R. Safdi. Dark matter from axion
strings with adaptive mesh refinement. Nature Communications,
2022; 13 (1) DOI: 10.1038/s41467-022-28669-y ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220225085845.htm

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