The Weyl fermion – an elusive massless particle theorized 85 years ago – has been confirmed by direct observations for the first time.
A detector image signals the existence of Weyl fermion. The plus and minus signs note whether the particle’s spin is in the same direction as its motion – which is known as being right-handed – or in the opposite direction in which it moves, or left-handed. This dual ability allows Weyl fermions to have high mobility. Image credit: Su-Yang Xu / M. Zahid Hasan / Princeton University.
In 1928 Paul Dirac discovered a crucial equation in particle physics and quantum mechanics, now known as Dirac equation.
Very fast electrons were solutions to the Dirac equation. Moreover, the equation predicted the existence of anti-electrons, or positrons: particles with the same mass as electrons but having opposite charge. True to Dirac’s prediction, positrons were discovered in 1932 by the American physicist Carl Anderson.
In 1929 the German-born mathematician Hermann Weyl found another solution to the Dirac equation, this time massless.
A year later, Wolfgang Pauli postulated the existence of the neutrino, which was then thought to be massless, and it was assumed to be the sought-after solution to the Dirac equation found by Weyl.
Neutrinos had not been detected yet in nature, but the case seemed to be closed. It would be decades before American physicists Frederick Reines and Clyde Cowan finally discovered neutrinos in 1957, and numerous experiments shortly thereafter indicated that neutrinos could have mass.
In 1998, the Super-Kamiokande Collaboration neutrino observatory in Japan announced what had now been speculated for years: neutrinos have non-zero mass. This discovery opened a new question: what then was the zero-mass solution found by Weyl?
Two international teams of physicists independently found the answer. In two separate papers, published online in the journal Science, they report first observations of a massless particle called Weyl fermion.
Unlike electrons, Weyl fermions are massless and possess a high degree of mobility; the particle’s spin is both in the same direction as its motion – which is known as being right-handed – and in the opposite direction in which it moves, or left-handed.
“The physics of the Weyl fermion are so strange, there could be many things that arise from this particle that we’re just not capable of imagining now,” said Prof Zahid Hasan of Princeton University, senior author of one of the two papers.
“The find differs from the other particle discoveries in that the Weyl fermion can be reproduced and potentially applied.”
“Typically, particles such as the famous Higgs boson are detected in the fleeting aftermath of particle collisions. The Weyl fermion, however, was discovered inside a synthetic metallic crystal called tantalum arsenide,” Prof Hasan said.
The Weyl fermion possesses two characteristics that could make its discovery a boon for future electronics, including the development of the highly prized field of efficient quantum computing.
“For a physicist, the Weyl fermions are most notable for behaving like a composite of monopole- and antimonopole-like particles when inside a crystal. This means that Weyl particles that have opposite magnetic-like charges can nonetheless move independently of one another with a high degree of mobility,” Prof Hasan said.
The physicists also found that Weyl fermions can be used to create massless electrons that move very quickly with no backscattering, wherein electrons are lost when they collide with an obstruction. In electronics, backscattering hinders efficiency and generates heat.
“It’s like they have their own GPS and steer themselves without scattering. They will move and move only in one direction since they are either right-handed or left-handed and never come to an end because they just tunnel through. These are very fast electrons that behave like unidirectional light beams and can be used for new types of quantum computing,” Prof Hasan said.
In pursuing the Weyl fermion, the scientists had to pull from a number of disciplines, as well as just have faith in their quest and scientific instincts.
“Solving this problem involved physics theory, chemistry, material science and, most importantly, intuition. This work really shows why research is so fascinating, because it involved both rational, logical thinking, and also sparks and inspiration,” said Dr Su-Yang Xu of Princeton University, a member of Prof Hasan’s team.
“After more than 80 years, we found that this fermion was already there, waiting. It is the most basic building block of all electrons,” Prof Hasan said.
“It is exciting that we could finally make it come out following Weyl’s 1929 theoretical recipe.”
“Every single paper written about Weyl fermions (points) was theoretical, until now,” said Prof Marin Soljači of MIT, who is the senior author of the other paper published in the journal Science.
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