Using adaptive mesh refinement, supercomputer simulation narrows axion mass range — ScienceDaily

Physicists looking out — unsuccessfully — for at the moment’s most favored candidate for darkish matter, the axion, have been wanting within the mistaken place, in keeping with a brand new supercomputer simulation of how axions had been produced shortly after the Huge Bang 13.6 billion years in the past.

Utilizing new calculational methods and one of many world’s largest computer systems, Benjamin Safdi, assistant professor of physics on the College of California, Berkeley; Malte Buschmann, a postdoctoral analysis affiliate at Princeton College; and colleagues at MIT and Lawrence Berkeley Nationwide Laboratory simulated the period when axions would have been produced, roughly a billionth of a billionth of a billionth of a second after the universe got here into existence and after the epoch of cosmic inflation.

The simulation at Berkeley Lab’s Nationwide Analysis Scientific Computing Heart (NERSC) discovered the axion’s mass to be greater than twice as massive 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 mentioned, that the mass is near 65 ?eV. Since physicists started in search of the axion 40 years in the past, estimates of the mass have ranged extensively, from a couple of ?eV to 500 ?eV.

“We offer over a thousandfold enchancment within the dynamic vary of our axion simulations relative to prior work and clear up a 40-year previous query concerning the axion mass and axion cosmology,” Safdi mentioned.

The extra definitive mass implies that the commonest sort of experiment to detect these elusive particles — a microwave resonance chamber containing a powerful magnetic area, wherein scientists hope to snag the conversion of an axion right into a faint electromagnetic wave — will not be capable to detect them, irrespective of how a lot the experiment is tweaked. The chamber must be smaller than a couple of centimeters on a aspect to detect the higher-frequency wave from a higher-mass axion, Safdi mentioned, and that quantity could be too small to seize sufficient axions for the sign to rise above the noise.

“Our work gives probably the most exact estimate to this point of the axion mass and factors to a particular vary of plenty that’s not at the moment being explored within the laboratory,” he mentioned. “I actually do suppose it is sensible to focus experimental efforts on 40 to 180 ?eV axion plenty, however there’s quite a lot of work gearing as much as go after that mass vary.”

One newer sort of experiment, a plasma haloscope, which seems to be for axion excitations in a metamaterial — a solid-state plasma — ought to be delicate to an axion particle of this mass, and will probably detect one.

“The fundamental research of those three-dimensional arrays of advantageous wires have labored out amazingly properly, significantly better than we ever anticipated,” mentioned Karl van Bibber, a UC Berkeley professor of nuclear engineering who’s constructing a prototype of the plasma haloscope whereas additionally collaborating in a microwave cavity axion search known as the HAYSTAC experiment. “Ben’s newest outcome may be very thrilling. If the post-inflation situation is true, after 4 a long time, discovery of the axion might be significantly accelerated.”

If axions actually exist.

The work will likely be revealed Feb. 25 within the journal Nature Communications.

Axion high candidate for darkish matter

Darkish matter is a mysterious substance that astronomers know exists — it impacts the actions of each star and galaxy — however which interacts so weakly with the stuff of stars and galaxies that it has eluded detection. That does not imply darkish matter cannot be studied and even weighed. Astronomers know fairly exactly how a lot darkish matter exists within the Milky Manner Galaxy and even in the complete universe: 85% of all matter within the cosmos.

So far, darkish matter searches have centered on large compact objects within the halo of our galaxy (known as large compact halo objects, or MACHOs), weakly interacting large particles (WIMPs) and even unseen black holes. None turned up a possible candidate.

“Darkish matter is many of the matter within the universe, and we do not know what it’s. Some of the excellent questions in all of science is, ‘What’s darkish matter?'” Safdi mentioned. “We suspect it’s a new particle we do not learn about, and the axion might be that particle. It might be created in abundance within the Huge Bang and be floating on the market explaining observations which were made in astrophysics.”

Although not strictly a WIMP, the axion additionally interacts weakly with regular matter. It passes simply by the earth with out disruption. It was proposed in 1978 as a brand new elementary particle that would clarify why the neutron’s spin doesn’t precess or wobble in an electrical area. The axion, in keeping with concept, suppresses this precession within the neutron.

“Nonetheless to at the present time, the axion is the very best thought now we have about the right way to clarify these bizarre observations concerning the neutron,” Safdi mentioned.

Within the Eighties, the axion started to be seen additionally as a candidate for darkish matter, and the primary makes an attempt to detect axions had been launched. Utilizing the equations of the well-vetted concept of elementary particle interactions, the so-called Normal Mannequin, along with the idea of the Huge Bang, the Normal Cosmological Mannequin, it’s doable to calculate the axion’s exact mass, however the equations are so troublesome that to this point now we have solely estimates, which have different immensely. For the reason that mass is thought so imprecisely, searches using microwave cavities — primarily elaborate radio receivers — should tune by hundreds of thousands of frequency channels to attempt to discover the one similar to the axion mass.

“With these axion experiments, they do not know what station they’re speculated to be tuning to, so that they need to scan over many alternative potentialities,” Safdi mentioned.

Safdi and his staff produced the latest, although incorrect, axion mass estimate that experimentalists are at the moment focusing on. However as they labored on improved simulations, they approached a staff from Berkeley Lab that had developed a specialised code for a greater simulation method known as adaptive mesh refinement. Throughout simulations, a small a part of the increasing universe is represented by a three-dimensional grid over which the equations are solved. In adaptive mesh refinement, the grid is made extra detailed round areas of curiosity and fewer detailed round areas of area the place nothing a lot occurs. This concentrates computing energy on an important elements of the simulation.

The method allowed Safdi’s simulation to see 1000’s of instances extra element across the areas the place axions are generated, permitting a extra exact willpower of the entire variety of axions produced and, given the entire mass of darkish matter within the universe, the axion mass. The simulation employed 69,632 bodily pc processing unit (CPU) cores of the Cori supercomputer with practically 100 terabytes of random entry reminiscence (RAM), making the simulation one of many largest darkish matter simulations of any type to this point.

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

“You may consider these strings as composed of axions hugging the vortices whereas these strings whip round forming loops, connecting, present process quite a lot of violent dynamical processes in the course of the enlargement of our universe, and the axions hugging the perimeters of those strings try to carry on for the experience,” Safdi mentioned. “However when one thing too violent occurs, they only get thrown off and whip away from these strings. And people axions which get thrown off of the strings find yourself changing into the darkish matter a lot in a while.”

By retaining observe of the axions which are whipped off, researchers are in a position to predict the quantity of darkish matter that was created.

Adaptive mesh refinement allowed the researchers to simulate the universe for much longer than earlier simulations and over a a lot larger patch of the universe than earlier simulations.

“We resolve for the axion mass each in a extra intelligent means and in addition by throwing simply as a lot computing energy as we may probably discover onto this downside,” Safdi mentioned. “We may by no means simulate our whole universe as a result of it is too massive. However we needn’t stimulate our whole universe. We simply must simulate a large enough patch of the universe for a protracted sufficient time period, such that we seize the entire dynamics that we all know are contained inside that field.”

The staff is working with a brand new supercomputing cluster now being constructed at Berkeley Lab that can allow simulations that can present an much more exact mass. Referred to as Perlmutter, after Saul Perlmutter, a UC Berkeley and Berkeley Lab physicist who gained the 2011 Nobel Prize in Physics for locating the accelerating enlargement of the universe pushed by so-called darkish power, the next-generation supercomputer will quadruple the computing energy of NERSC.

“We wish to make even larger simulations at even larger decision, which is able to permit us to shrink these error bars, hopefully all the way down to the ten% degree, so we will let you know a really exact quantity, like 65 plus or minus 2 micro-eV. That then actually adjustments the sport experimentally, as a result of then it might turn out to be a better experiment to confirm or exclude the axion in such a slender mass vary,” Safdi mentioned.

For van Bibber, who was not a member of Safdi’s simulation staff, the brand new mass estimate exams the boundaries of microwave cavities, which work much less properly at excessive frequencies. So, whereas the decrease restrict of the mass vary remains to be throughout the potential of the HAYSTAC experiment to detect, he’s enthused concerning the plasma haloscope.

“Over time, new theoretical understanding has loosened the constraints on the axion mass; it may be wherever inside 15 orders of magnitude, when you take into account the likelihood that axions fashioned earlier than inflation. It is turn out to be an insane activity for experimentalists,” mentioned van Bibber, who holds UC Berkeley’s Shankar Sastry Chair of Management and Innovation. “However a latest paper by Frank Wilczek’s Stockholm concept group could have resolved the conundrum in making a resonator which might be concurrently each very massive in quantity and really excessive in frequency. An precise resonator for an actual experiment remains to be some methods away, however this might be the way in which to go to get to Safdi’s predicted mass.”

As soon as simulations give an much more exact mass, the axion could, in actual fact, be simple to seek out.

“It was actually essential that we teamed up with this pc science staff at Berkeley Lab,” Safdi mentioned. “We actually expanded past the physics area and really made this a computing science downside.”

Safdi’s colleagues embody Malte Buschmann of Princeton; MIT postdoctoral fellow Joshua Foster; Anson Hook of the College of Maryland; and Adam Peterson, Don Willcox and Weiqun Zhang of Berkeley Lab’s Heart for Computational Sciences and Engineering. The analysis was largely funded by the U.S. Division of Vitality by the Exascale Computing Venture (17-SC-20-SC) and thru the Early Profession program (DESC0019225).

Video: https://youtu.be/hrCN6tF087c

Video on measuring an axion: https://youtu.be/hikmvEbO-vA

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