Forgotten Particle May Explain Matter-Antimatter Asymmetry, Physicists Propose

Scientists may have found a key to one of the universe's most profound mysteries: why matter exists in such abundance while antimatter is scarce. Researchers propose that a poorly understood particle, the decoupled heavy neutrino, played a crucial role in the early universe, leading to the matter-dominated cosmos we observe today.

The Big Bang, according to current theories, should have produced equal amounts of matter and antimatter. However, when matter and antimatter particles meet, they annihilate each other, leaving behind only radiation. The visible universe, composed overwhelmingly of matter, presents a paradox. This imbalance, known as the matter-antimatter asymmetry, is critical for the existence of stars, planets, and life.

In a paper published in Physical Review Letters, physicists from the University of Southampton and the Indian Institute of Technology Kanpur suggest that the decoupled heavy neutrino, a particle often overlooked due to its weak interactions and high mass, could be the missing piece of the puzzle. "The decoupled right-handed neutrino has often been dismissed due to its weak interactions and high mass, making it appear irrelevant in many frameworks of particle physics," said Dr. Rishav Roshan, a co-author from the University of Southampton. "However, its potential implications for cosmology and particle interactions had not been fully appreciated until recent studies."

Neutrinos are fundamental particles that interact very rarely with other matter. Some theories posit the existence of three heavy 'right-handed' neutrinos to explain the small mass of the standard neutrino. The third of these, the 'decoupled' heavy neutrino, has been largely ignored because of its minimal interactions. The new research suggests this particle, despite its elusive nature, may have significantly influenced the universe's earliest moments.

These particles, due to their weak interactions and slow decay, might have briefly possessed more energy than their surrounding radiation. This could have driven a period of rapid expansion in the nascent universe, altering the conditions under which matter and antimatter formed. "A neutrino, particularly a heavy right-handed neutrino, can influence the creation of matter and antimatter," Dr. Roshan explained. "By affecting the interactions and decays of particles during the early universe, these neutrinos can help favour the production of matter over antimatter."

Using theoretical models, the researchers calculated that the presence of decoupled heavy neutrinos could create a minute imbalance between matter and antimatter. Even a slight excess, perhaps one extra matter particle per billion, would be sufficient for the surviving matter to form the vast structures observed today after annihilation processes concluded.

While this remains a theoretical proposal, and the existence of these heavy neutrinos has not yet been experimentally confirmed, the researchers are optimistic about future verification. "This theory could be tested experimentally through upcoming gravitational wave detectors and particle physics experiments," Dr. Roshan stated. "These tools may help us detect signatures related to heavy neutrinos and their role in the early universe, providing insights into the matter-antimatter asymmetry."

Scientists believe that the universe contains three heavy neutrino particles that make up about 0.67 per cent of the mass in the universe.

Neutrinos are a type of electrically neutral particle, which interacts with matter so rarely that scientists need to use specialised detectors sunk to the bottom of the ocean to find them.

Scientists believe that a forgotten particle known as a decoupled heavy neutrino could be the key to understanding why the universe as we know it exists today.