New simulations refine axion mass, refocusing dark matter search

Physicists searching for the most favored candidate for dark matter, the axion, have been unsuccessful, according to a new simulation.

Using new calculational techniques and one of the world's largest computers, a group of people, including an assistant professor of physics at the University of California, Berkeley, and a research associate at Princeton University, recreated an era when computers were not as powerful as they are today.

The axion mass was found to be between 40 and 180 microelectron volts, more than twice as big as theorists and experimenters have thought. There are indications that the mass is close to 65 BC. Estimates of the mass have ranged from a few BC to 500 BC.

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.

A microwave resonance chamber containing a strong magnetic field is the most common type of experiment to detect these elusive particles. The volume of the chamber would be too small to capture enough axions for the signal to rise above the noise, so it would have to be smaller than a few centimeters.

The most precise estimate to date of the axion mass is provided by our work, and it points to a specific range of mass that is not currently being explored in the laboratory.

A new type of experiment, a plasma haloscope, which looks for axion excitations in a metamaterial, should be sensitive to an axion particle of this mass, and could potentially detect one.

The basic studies of these three-dimensional array 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 the project. If the post-inflation scenario is correct, the axion could be discovered quickly.

If axions exist.

The work will be published in February.

Axion is a top candidate for dark matter.

Astronomers know that dark matter affects the movements of every star and galaxy, but they don't know how it interacts with the stuff of stars and galaxies. Dark matter can be studied and weighed. 85% of all matter in the universe is dark matter, which is found in the Milky Way.

Massive compact objects in the halo of our galaxy, weakly interacting massive particles, and even unseen black holes have been the focus of dark matter searches to date. None of them turned out to be a likely candidate.

We have no idea what dark matter is. One of the most outstanding questions in all of science is, "What is dark matter?" It could be created in abundance in the Big Bang and be floating out there explaining observations that have been made in astrophysics.

The axion interacts weakly with normal matter. It is easy to pass through the earth. It was proposed in 1978 as a new elementary particle that could explain why the neutron does not spin in an electric field. The axion suppresses the precession in the neutron.

Still to this day, the axion is the best idea we have for explaining the weird observations about the neutron.

The first attempts to detect axions were launched in the 1980s, when the axion was seen as a candidate for dark matter. It is possible to calculate the axion's precise mass using the equations of the Standard Model, but they are so difficult. Since the mass is so vague, searches using microwave cavities must be done through millions of channels.

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

The most recent, though incorrect, axion mass estimate was produced by the team. As they worked on improved simulations, they approached a team from Berkeley Lab that had developed a specialized code for a better simulation technique. A small part of the expanding universe is represented by a three-dimensional grid during simulations. The grid is more detailed around areas of interest and less detailed around areas of space where nothing happens. The most important parts of the simulation are the ones with the most computing power.

The technique allowed the 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 the axion mass. One of the largest dark matter simulations of any kind to date utilized 69,632 physical computer processing unit (CPU) cores of the Cori supercomputer with over 100 terabytes of random access memory.

The simulation showed that after the inflationary epoch, small tornadoes 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. The axions which are thrown off of the strings become the dark matter later on.

Researchers can predict the amount of dark matter created by keeping track of the axions that are whipped off.

The researchers were able to simulation the universe much longer than previous simulations and over a larger patch of the universe.

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. We don't need to make our universe bigger. We need to recreate a patch of the universe for a long time so that we can capture all of the dynamics that we know are contained within that box.

A new supercomputing cluster being built at Berkeley Lab will enable simulations that will provide an even more precise mass. Saul Perlmutter, a UC Berkeley and Berkeley Lab physicist who won the 2011 Nobel Prize in Physics for discovering the accelerated expansion of the universe driven by so-called dark energy, was called Perlmutter.

We want to make even bigger simulations at even higher resolution, which will allow us to shrink these error bars, so we can tell you a very precise number, like 65 plus or minus 2 micro-eV. It would become an easier experiment to verify or exclude the axion in such a narrow mass range after that.

The new mass estimate tests the limits of microwave cavities, which work less well at high frequencies, for van Bibber, who was not a member of the simulation team. The lower limit of the mass range is still within the ability of the HAYSTAC experiment to detect, but he is excited about the plasma haloscope.

If you consider the possibility that axions formed before inflation, the axion mass can be anywhere within 15 orders of magnitude. It has become an insane task for experimentalists, according to the chair of leadership and innovation at UC Berkeley. An actual resonator for a real experiment is still some way away, but this could be the way to get to the predicted mass.

The axion may be easy to find once simulations give an even more precise mass.

It was crucial that we collaborated with the computer science team at Berkeley Lab.

More information: Dark matter from axion strings with adaptive mesh refinement, Nature Communications (2022). DOI: 10.1038/s41467-022-28669-y Journal information: Nature Communications Citation: New simulations refine axion mass, refocusing dark matter search (2022, February 25) retrieved 25 February 2022 from https://phys.org/news/2022-02-simulations-refine-axion-mass-refocusing.html This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.