mathematical signature of an axion found in Weyl semimetal

"mathematical signature of an axion found in Weyl semimetal"

https://www.livescience.com/axion-found-in-weyl-semimetal.html

axion-found-in-weyl-semimetal

"mathematical signature of an axion"

excerpts:

Condensed-matter experiments like the one the researchers conducted have been used to "find" elusive predicted particles in several well-known cases, including that of the majorana fermion. The particles are not detected in the usual sense, but are instead found as collective vibrations in materials that behave and respond exactly as the particle would.

"The problem with looking at outer space is that you cannot control your experimental environment very well," said study co-author Johannes Gooth, a physicist at the Max Planck Institute for Chemical Physics of Solids in Germany. "You wait for an event to happen and try to detect it. I think one of the beautiful things of getting these concepts of high-energy physics into condensed matter is that you can actually do much more."

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The research team worked with a Weyl semimetal, a special and strange material in which electrons behave as if they have no mass, don't interact with each other and are split into two types: right-handed and left-handed. The property of being either right- or left-handed is called chirality; chirality in Weyl semimetals is conserved, meaning there are equal numbers of right- and left-handed electrons. Cooling the semimetal to 12 degrees Fahrenheit (minus 11 degrees Celsius) allowed the electrons to interact and to condense themselves into a crystal of their own. 

Waves of vibrations traveling through crystals are called phonons. Since the strange laws of quantum mechanics dictate that particles can also behave as waves, there are certain phonons that have the same properties as common quantum particles, such as electrons and photons. Gooth and his colleagues observed phonons in the electron crystal that responded to electric and magnetic fields exactly like axions are predicted to. These quasiparticles also did not have equal numbers of right- and left-handed particles. (Physicists also predicted that axions would break conservation of chirality.)

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"It's encouraging that these equations [describing the axion] are so natural and compelling that they are realized in nature in at least one circumstance," said MIT theoretical physicist and Nobel laureate Frank Wilczek, who originally named the axion in 1977. "If we know that there are some materials that host axions, well, maybe the material we call space also houses axions." Wilczek, who was not involved in the current study, also suggested that a material like Weyl semimetal could one day be used as a kind of "antenna" for detecting fundamental axions, or axions that exist in their own right as particles in the universe, rather than as collective vibrations.

While the search for the axion as an independent, lone particle will continue, experiments like this help more traditional detection experiments by providing limits on and estimates of the particle's properties, such as mass. This gives other experimentalists a better idea of where to look for these particles. It also robustly demonstrates that the particle's existence is possible.

"A theory first is a mathematical concept," said Gooth. "And the beauty of these condensed-matter physics experiments is that we can show that this kind of mathematics exists in nature at all."

The research was published online Oct. 7 in the journal Nature.

Originally published on Live Science.

Editor's Note: This second paragraph of this story was updated at 10:05 a.m. E.D.T. to clarify that what was found in this study was a mathematical signature of an axion and not a dark matter axion found in space.

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Weyl fermions with no mass created in lab

Weyl fermions with no mass created in lab

excerpts:

Mathematician Hermann Weyl first proposed the mysterious massless particle in 1929. The particles would have a spin, but would also have "chirality," meaning they would spin as they traveled through space in either a left- or right-handed orientation, Xu said. When a left- and right-handed Weyl fermion come into contact, they would annihilate each other.

According to the Standard Model, the reigning model that describes subatomic particles, two major types of particles exist: Bosons and fermions. Bosons carry force and fermions are the teensy constituents of matter. However, scientists have long thought that fermions came in three types: Dirac, Majorana and Weyl. So far, scientists have found evidence in particle accelerators of the first two, but no hint of the latter.

However, in a 2011 study in the journal Physical Review B, researchers proposed that a crystal lattice with certain properties could produce Weyl fermions under the right conditions. In order to produce the ghostly particles, the material would need a certain kind of asymmetry, and would also have to be a semi-metal, a material with properties between an insulator and a conductor. The catch was that nobody knew exactly which materials to try.

So Xu and his colleagues pored over a database containing nearly 1 million descriptions of crystal lattices. They decided that a lattice made up of tantalum and arsenic would be a promising place to look. So they bombarded a tantalum-arsenide lattice with a beam of photons (particles of light), which energize electrons in the material. The extra bump of energy provided by the photons kicked the electrons out of their normal positions in the lattice and sent them moving. By detecting these displaced electrons, the team could understand how they were moving through the lattice.

By analyzing those properties, the team found that the electrons were acting very strangely. "The electron quasi-particle behaves exactly like a Weyl fermion," Xu said.

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https://phys.org/news/2017-07-evidence-majorana-fermion-particle-antiparticle.html

In 1928, physicist Paul Dirac made the stunning prediction that every fundamental particle in the universe has an antiparticle – its identical twin but with opposite charge. When particle and antiparticle met they would be annihilated, releasing a poof of energy. Sure enough, a few years later the first antimatter particle – the electron's opposite, the positron – was discovered, and antimatter quickly became part of popular culture.

But in 1937, another brilliant physicist, Ettore Majorana, introduced a new twist: He predicted that in the class of particles known as fermions, which includes the proton, neutron, electron, neutrino and quark, there should be particles that are their own antiparticles.

Now a team including Stanford scientists says it has found the first firm evidence of such a Majorana fermion. It was discovered in a series of lab experiments on exotic materials at the University of California in collaboration with Stanford University. The experimental team was led by UCLA Professor Kang Wang, and precise theoretical predictions were made by Stanford Professor Shoucheng Zhang's group, in collaboration with experimental groups led by Associate Professor Jing Xia at UC-Irvine and Professor Kai Liu at UC-Davis. The team reported the results July 20 in Science.

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What they've been looking for are "quasiparticles" – particle-like excitations that arise out of the collective behavior of electrons in superconducting materials, which conduct electricity with 100 percent efficiency. The process that gives rise to these quasiparticles is akin to the way energy turns into short-lived "virtual" particles and back into energy again in the vacuum of space, according to Einstein's famous equation E = mc2. While quasiparticles are not like the particles found in nature, they would nonetheless be considered real Majorana fermions.

Over the past five years, scientists have had some success with this approach, reporting that they had seen promising Majorana fermion signatures in experiments involving superconducting nanowires.

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The results of these experiments are not likely to have any effect on efforts to determine if the neutrino is its own antiparticle, said Stanford physics Professor Giorgio Gratta, who played a major role in designing and planning EXO-200.

"The quasiparticles they observed are essentially excitations in a material that behave like Majorana particles," Gratta said. "But they are not elementary particles and they are made in a very artificial way in a very specially prepared material. It's very unlikely that they occur out in the universe, although who are we to say? On the other hand, neutrinos are everywhere, and if they are found to be Majorana particles we would show that nature not only has made this kind of particles possible but, in fact, has literally filled the universe with them."

He added, "Where it gets more interesting is that analogies in physics have proved very powerful. And even if they are very different beasts, different processes, maybe we can use one to understand the other. Maybe we will discover something that is interesting for us, too."