‘Strange’ particle found to have mass when moving in one direction, not another Premium

Physicists explore semi-Dirac fermions in condensed matter physics to understand the universe.

Since the start of the 20th century, physicists have discovered a veritable zoo of subatomic particles. Matter can be both wave and particle. If you take the particle route, these subatomic particles are what you could say the universe and everything in it is made of. There are many ways to further categorise them.

A common one is as fermions and bosons: fermions make up matter and bosons mediate the forces between matter. For example, electrons and protons are fermions whereas photons are bosons.

Fermions can be further classified as Dirac or Majorana fermions. Dirac fermions are fermions that may or may not have mass but are always different from their anti-particles. Majorana fermions are fermions that are also their own antiparticles (neutrinos are suspected to be Majorana fermions).

Even if these distinctions seem too fine, they’re of considerable interest to physicists. They know something’s up in the subatomic zoo. Some animals that should obviously be there are missing, like the particle for the force of gravity. Some animals are much heavier than they should be (Higgs bosons and neutrinos). One enclosure, dark matter, remains empty even though physicists have been looking for it under every rock and leaf. Their knowledge of quite a few animals is just incomplete or at odds with what they studied in school. There’s a lot of work left if the zoo is to be a fully understood place without any surprises.

To do this physicists have one advantage: a common theory that collects all these animals under a single, unified description, called the Standard Model (SM) of particle physics. Physicists can explore ‘new physics’ in terms of whether it agrees or disagrees with the SM. Right now it’s like a big jigsaw puzzle with a few important pieces missing. If physicists find a new piece in their calculations or their particle collider experiments, they can check if it fits into the puzzle. If it doesn’t, maybe the puzzle itself needs to be changed.

In a sense, grouping fermions into fine categories is an exercise in meticulously cataloguing the exact shapes of the puzzle’s pieces. This way, if physicists find a piece whose shape is new even in a very small way, they stand to make a big update.

A particle as it exists in the wilderness of space is slightly different from a particle that exists inside solids and liquids. “In condensed-matter physics, every material can behave like a new universe,” IIT Kanpur assistant professor Adhip Agarwala said. “Here strange particles can arise and be experimentally detectable, which are otherwise not usually seen in three dimensions.”