A strange particle can gain or lose mass depending on the direction it is traveling

Scientists accidentally discover a particle that has mass when it travels in one direction, but not mass while traveling in the other direction. Known as semi-Dirac fermions, particles with strange behavior were first predicted 16 years ago.

The discovery was made of a semi-metallic material called ZrSiS, which is composed of zirconium, silicon and sulfur, while studying the properties of quasiparticles. It arises from the collective behavior of many particles within a solid material.

“This was completely unexpected,” said Yinming Shao, lead author of the study. “We weren’t looking for a semi-Dirac fermion when we started working on this material, but we found signatures we didn’t understand – and this became our first observation of these wild quasiparticles that sometimes behave like they have mass and sometimes act like they don’t have it.”

It seems like an impossible feat – how can something be acquired and lost so quickly? But it actually goes back to the classic formula that everyone has heard but many don’t understand – E = mc². It describes the relationship between the energy of a particle (E) and mass (m), with the speed of light (c) squared.

According to Einstein’s theory of special relativity, nothing with any mass can reach the speed of light, because it requires an infinite amount of energy to accelerate it to that speed. But a funny thing happens when you flip that on its head – when a small particle slows down from the speed of light, it actually gains mass.

And that’s what happened here. When quasiparticles travel in one dimension within ZrSiS crystals, they do so at the speed of light and are therefore massless. But when they try to travel in a different direction, they hit resistance, slow down and gain mass.

“Think of the particle as a small train set on a network of tracks, which is the underlying electronic structure of the material,” Shao said. “Now, at some point the tracks intersect, so our particle train is moving along its fast track, at high speed, but then it hits an intersection and has to move to a perpendicular track. It experiences resistance, it has mass. The particles can be all energy or have mass depending on the direction of their movement along the ‘tracks’ of the material.

Researchers initially began studying quantum interactions in material by looking at how electrons respond to light. They found that the energy levels of the electrons followed an unexpected pattern as the strength of the magnetic field increased, which turned out to be an important prediction of semi-Dirac fermions.

Although this is a bit of a high concept in physics, the team says the discovery could open up a range of applications for ZrSiS similar to graphene.

“This is a layered material, which means once we know how a layer is cut in this compound, we can use the power of semi-Dirac fermions, control its properties with the same with the same precision as graphene,” Shao said. “But the most exciting part of this experiment is that the data has yet to be fully explained. There are many unsolved mysteries in what we observed, so that’s what we’re trying to understand.”

The research was published in the journal Physical Review X.

Source: Penn State