Particle physics is not everyone’s cup of tea. A team of physicists have theorised the existence of a strange type of particle that behaves differently depending on its direction of travel—massless in one direction but possessing mass when moving the other way! This strange, elusive particle, known as a semi-Dirac fermion or “quasiparticle,” has actually been observed in action. To detect it, researchers cooled a semi-metal crystal to near absolute zero, exposed it to a powerful magnetic field and infrared light, and successfully captured the signal of these unusual quasiparticles.
Particle physics is the branch of physics that studies the fundamental make up of matter and the forces that govern their interactions. It focuses on the smallest building blocks of the universe—particles such as quarks, leptons, and bosons—which make up atoms and everything around us. These subatomic particles interact through fundamental forces like electromagnetism, gravity, the strong nuclear force, and the weak nuclear force. The study of particle physics often involves high-energy experiments, where particles are accelerated to near the speed of light and collided, allowing for observations of their behaviour and properties.
Discoveries in particle physics are not all that common but a team of researchers from the Penn State University have announced their discovery of a new type of particle known as a quasiparticle. Quasiparticles are a quantum of energy in a crystal structure or other lattice structure that has momentum and position and can in some cases be considered a particle. They have named their new quasiparticle the semi-Dirac fermion, until the announcement it had been 16 years since this strange particle had been theorised.
Often in particle physics, things can go against every thing your common sense tells you. It’s most definitely the case with the semi-Dirac fermion which was discovered in a ZrSiS crystal (Zirconium silicon sulfide.) When it is moving in one direction it seems to have mass but in the other direction it appears massless! This is possible when a particle derives its energy from its motion and in this case its almost pure energy travelling at the speed of light.
The discovery is in accordance with Einstein’s theory of Special Relativity that says anything travelling at the speed of light cannot have mass. According to lead researcher Yinming Shao ‘In solid materials, the collective behaviour of many particles, also known as quasiparticles, can have different behaviour than the individual particles, which in this case gave rise to particles having mass in only one direction.’
The team used the hybrid magnet at the National High Magnetic Field Laboratory in Florida to generate a magnetic field 900,000 times stronger than the Earth’s! They cooled a piece of ZrSiS crystal down to just a few degrees above absolute zero and exposed it to the magnetic field while directing infrared light at it to explore its quantum properties. This enabled them to study how electrons inside the material responded to the light revealing many features that were expected, plus a few more that puzzled the team.
The magnetic field was a crucial element to their experiment which caused the electrons inside the crystal to become quantised into discrete states called Landau Levels that have fixed values. The difference between the levels depends on the mass of the electrons and the strength of the magnetic field. If the magnetic field increases, the energy level of the electrons should increase based on their mass, but they didn’t!
Shao went on to explain their findings ‘Imagine the particle is a tiny train confined to a network of tracks, which are the material’s underlying electronic structure. Now, at certain points the tracks intersect, so our particle train is moving along its fast track, at light speed, but then it hits an intersection and needs to switch to a perpendicular track. Suddenly, it experiences resistance, it has mass. The particles are either all energy or have mass depending on the direction of their movement along the material’s tracks.’
Source : Particle that only has mass when moving in one direction observed for first time