Long-sought subatomic particle ‘seen’ at last

A subatomic particle that was first predicted to exist before the discovery of Pluto, 85 years ago, has finally been spotted on Earth. Known as Weyl fermions, they are like electrons. But unlike electrons, they have no mass. Physicists found them inside a material made of the elements tantalum and arsenic. These fermions dart around and through it in strange and exciting ways.

“It’s definitely a big deal,” says Leon Balents. He’s a condensed matter theorist at the University of California, Santa Barbara.

Researchers reported their new discovery July 16 in Science.

The material is called tantalum arsenide. The newfound particles’ behavior gives this material metal-like features. Called a “semimetal,” it shares features with materials such as graphene, which is a sheet of carbon that’s just one atom thick. Its novel structure gives graphene unusual superstrong characteristics that have excited researchers over the last decade or so. “There are a lot of reasons to be interested in these materials,” notes Balents, who was not involved with the new fermion discovery.

Some scientists think that like graphene, tantalum arsenide could change the future of electronics. It could let devices use a fast-moving electrical current that easily evades any bumps or valleys in its path. Physicists can also use tantalum arsenide to learn more about Weyl fermions. These particles are stuck inside the material. But some physicists suspect free-floating Weyl fermions might also exist.

Fermions make up a family of subatomic particles that includes electrons, neutrinos and more. So far, all known fermions obey rules laid out by English physicist Paul Dirac in 1928. But shortly after that time, scientists proposed two more types of fermions. One of them was named for German mathematician and physicist Hermann Weyl. What makes these special: Weyl fermions would have no mass.

Physicists had suggested that electrons interacting in certain materials would create ripples of energy. These ripples should act just like Weyl fermions would in space.

Earlier this year, with that in mind, Su-Yang Xu, Zahid Hasan and their colleagues proposed that tantalum arsenide could hold Weyl fermions. Xu and Hasan are condensed matter physicists working at Princeton University in New Jersey. These researchers fired a beam of high-energy X-rays to peer deep inside crystals of tantalum arsenide. Then they studied the energies and motion of its electrons. And they saw the signature of massless particles. They appeared to be Weyl fermions.

“I’m kind of amazed that someone was able to really see these things experimentally so quickly,” Balents says.

Tantalum arsenide is the first “Weyl semimetal” scientists have found. In some ways, such a semimetal is similar to “topological insulators.” Those are relatively newfound materials that don’t conduct electricity well on the inside, but let electrons run laps around their surface. Tantalum arsenide does not have the same kind of interior. But it does have high-speed electron superhighways on its surface.

The new twist with the Weyl semimetal, Xu says, is that its surface electrons don’t race around a closed track. Instead, they move from one side to the other. Then they disappear into the material and pop back out on the opposite surface.

Weyl semimetals also are similar to graphene, Balents says. Both materials let electrons zip around at extreme speeds and act like they have no mass. All these features make Weyl semimetals an exciting possibility for future electronics, Hasan says.

Now that physicists have found two of the three fermion types, Xu says, discovery of the still elusive third kind — the so-called Majorana fermions — may be just around the corner. In recent years physicists have seen hints of them. But they still lack proof. Says Xu, Weyl semimetals might be a good place to find not only Weyl fermions, but Majorana particles as well.

Power Words

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accelerator   (in physics) Also known as a particle accelerator, this massive machine revs up the motion of subatomic particles to great speed, and then beams them at targets. Sometimes the beams are used to deliver radiation at a tissue for cancer treatment. Other times, scientists crash the particles into solid targets in hopes of breaking the particles into their building blocks.

arsenic   A highly poisonous metallic element. It occurs in three chemically different forms, which also vary by color (yellow, black and gray). The brittle, crystalline (gray) form is the most common. Some manufacturers tap its toxicity by adding it to insecticides.

condensed matter physics   A field of research that investigates how matter a large number of interacting atoms and electrons give rise to new matter, and what properties that matter will have as a result of these interactions.

crystal  A solid consisting of a symmetrical, ordered, three-dimensional arrangement of atoms or molecules. It’s the organized structure taken by most minerals. Apatite, for example, forms six-sided crystals. The mineral crystals that make up rock are usually too small to be seen with the unaided eye.

current   (in electricity) The flow of electricity or the amount of electricity moving through some point over a particular period of time.

electron   A negatively charged particle, usually found orbiting the outer regions of an atom; also, the carrier of electricity within solids.

electronics  Devices that are powered by electricity but whose properties are controlled by the semiconductors or other circuitry that channel or gate the movement of electric charges.

fermions   A family of subatomic particles. The Weyl fermion, for instance, is a massless, subatomic particle named after the German mathematician and physicist Hermann Weyl.

graphene   A superthin, superstrong material made from a single layer of carbon atoms connected together.

insulator   A substance or device that does not readily conduct electricity.

mass   A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from. For objects on Earth, we know the mass as “weight.”

neutrino   A subatomic particle with a mass close to zero. Neutrinos rarely react with normal matter. Three kinds of neutrinos are known.

physicist  A scientist who studies the nature and properties of matter and energy.

radiation  One of the three major ways that energy is transferred. (The other two are conduction and convection.) In radiation, electromagnetic waves carry energy from one place to another. Unlike conduction and convection, which need material to help transfer the energy, radiation can transfer energy across empty space.

semimetal   A material that is similar to metal but does not share all of its properties.

subatomic  Anything smaller than an atom, which is the smallest bit of matter that has all the properties of whatever chemical element it is (like hydrogen, iron or calcium).

tantalum   A metallic element used in electronics, such as computers and DVD players.

theory  (in science)  A description of some aspect of the natural world based on extensive observations, tests and reason. A theory also can be a way of organizing a broad body of knowledge that applies in a broad range of circumstances to explain what will happen. Unlike the common definition of theory, a theory in science is not just a hunch. Ideas or conclusions that are based on a theory — and not yet on firm data or observations — are referred to as theoretical. Scientists who use mathematics and/or existing data to project what might happen in new situations are known as theorists.

Weyl fermion   A massless subatomic particle named after German mathematician and physicist Hermann Weyl.

X-ray        A type of radiation analogous to gamma rays, but of somewhat lower energy.