Why Discovery of Axions Is Important for Particle Physics

Axion has remained an elusive particle for the particle physicists ever since its existence was predicted 40 years back. Physicists have struggled to see the axions, but failed. Now, a recent study published in the journal Nature, scientists in the Max Planck Institute for Chemical Physics of Solids in Germany have reported to have seen traces of axions in their experiment.

It has been suggested in theoretical predictions that dark matter, the invisible matter permeating the universe may be made of axions. The characteristic of axions that it can’t interact with regular matter, a property similar to that of the dark matters have rendered axions the candidature for dark matters. This also makes axions, even if they exist, extremely difficult to detect. That’s the reason dark matter axion has not been seen deep in outer space; rather, scientists have discovered its traces as mathematical signature in an exotic material developed in lab.

The newly discovered axion is not exactly a matter as we perceive a matter to be. It manifests as a wave of electrons in a supercooled material known as a semimetal.

This discovery is important from another perspective as well. Axion discovery could be the first step in addressing one of the major unsolved problems in particle physics.

The unsolved problem is the long standing conundrum in physics known as the “strong CP problem”. It has been found that for some strange reasons, the laws of physics seem to act similarly on the particles and their anti-particle counterparts, even when their spatial coordinates are inverted. This is famously known as the Charge parity (CP) symmetry. The existing physics theory says that there should not be any reason why this happens. The unexpected symmetry could be explained with the existence of a special field and detection of axion might be helpful in proving that this field exists and thus in solving the mystery.

The axions interact with normal matters very rarely and it is extremely hard to find them with the help of present day telescopes. So, the researchers tried their hands on a strange material known as condensed matter. In fact, condensed matter experiments like the one the researchers have conducted have been used earlier also to find elusive particles, for example the majorana fermions.

“The problem with looking at outer space is that you cannot control your experimental environment very well. 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,” said study co-author Johannes Gooth, a physicist at the Max Planck Institute for Chemical Physics of Solids in Germany.

The research team worked with the condensed matter called the Weyl Semimetal. This is a really strange material designed in the lab a few years back. In this material, electrons behave as if they have no mass, they don’t interact with each other and are split into two parts—right-handed and left-handed. This is called the chirality—existence in either right or left handed. Cooling Weyl Semimetal to -11 degree Celsius allows the electrons to interact and they also condense to form a crystal of their own.

Gooth and his colleagues observed waves of vibrations traveling through the electron crystal of Weyl Semimetal. Also they found that these waves respond to electric and magnetic fields in a fashion that the axions are predicted to.

MIT theoretical physicist and Nobel laureate Frank Wilczek, who originally named the axion in 1977 said: “It's encouraging that these equations [describing the axion] are so natural and compelling that they are realised in nature in at least one circumstance. If we know that there are some materials that host axions well, maybe the material we call space also houses axions.”