A Long-Ignored Heavy Neutrino May Explain Why Matter Beat Antimatter

A particle that physicists have largely brushed aside for decades may help explain why the universe contains matter at all, according to a new theory reported by ScienceDaily. Researchers from the University of Southampton and the Indian Institute of Technology Kanpur say the particle could also leave a detectable trace in ancient gravitational waves.

The work, which is theoretical, was published in Physical Review Letters, a journal of the American Physical Society.

The puzzle of the missing antimatter

Scientists believe matter and antimatter were produced in nearly equal amounts in the early universe. When the two meet, they annihilate and release energy. If they had existed in perfectly equal quantities, virtually all matter could have vanished, leaving nothing to form stars, planets or life.

Instead, the early universe ended up with slightly more matter than antimatter. That small difference, known as matter and antimatter asymmetry, allowed some matter to survive after most particles and their antiparticles destroyed one another. One of the study's authors, Rishav Roshan of the University of Southampton, said that the fact that we are here at all proves such an asymmetry occurred.

A heavy neutrino that was dismissed

The team focused on what is called a decoupled heavy neutrino. It is a hypothetical, heavier relative of the neutrino, an extremely lightweight particle that rarely interacts with ordinary matter. This heavier version interacts so weakly with other particles that scientists generally assumed it was too disconnected to matter, and earlier attempts to explain the imbalance left it out.

The new calculations suggest otherwise. The particle decays extremely slowly, Roshan said, so it may have lived long enough to take over the universe's energy density for a time as the cosmos cooled after the Big Bang. That would have created a matter-dominated period and changed how the early universe evolved.

According to the researchers, the particle shifted the temperature at which fundamental particles such as electrons and other leptons interacted with their antiparticles with the help of the Standard Model Higgs boson. Roshan likened that temperature to a switch that shapes how the matter-antimatter imbalance was built up. By changing when those interactions took place, the heavy neutrino could have influenced how the imbalance developed.

A possible signal in gravitational waves

The idea also makes a prediction about the expansion of the early universe. Co-author Sudipta Show, of the Indian Institute of Technology Kanpur, said the dramatic shift in early expansion would leave distinct but detectable imprints, for example on primordial gravitational waves. Gravitational waves are ripples in space and time, and scientists believe processes in the extremely early universe may have produced primordial ones that carry information about that era.

If the theory is right, the temporary dominance of the heavy neutrino could have left recognizable signatures in those ancient signals. Detecting such patterns could eventually offer a way to test whether the proposed particle really influenced the universe, the report said.

Why it matters

This research does not change anything in daily life, and it does not prove the particle exists. The heavy neutrino remains hypothetical, and the result is a proposed explanation that other physicists will need to examine. What the study offers is a new candidate for a role that earlier explanations overlooked, along with a possible way to check it.

That is useful because the origin of the matter-antimatter imbalance is among the most fundamental open questions in physics. Show said the work offers a new explanation for how matter won, and the gravitational wave signatures it points to open new avenues for studying the early universe after the Big Bang.

Around Prague, based on reporting by ScienceDaily