Physicists link overlooked neutrino to matter’s existence

Physicists at the University of Southampton and the Indian Institute of Technology Kanpur say a heavy neutrino that most theorists have ignored may help explain why the universe is made of matter and not empty of everything. In a study published in Physical Review Letters, they propose that this "decoupled" particle decayed slowly enough to briefly dominate the early universe's energy density, according to the University of Southampton.phys.org

The work goes after a long-standing puzzle. The Big Bang should have produced equal amounts of matter and antimatter, and the two should have destroyed each other, leaving a universe of pure energy. The authors say the overlooked particle could have shifted the conditions under which the imbalance formed, and that it may have left traces in primordial gravitational waves that future detectors could measure.phys.org

A slow decay that reshaped the cosmos

In the standard "type-I seesaw" theory of neutrino mass, three heavy right-handed neutrinos are added to the Standard Model. Most studies keep only two of them and drop the heaviest, assuming it has almost no effect on neutrino masses or on how matter was created. The paper by Rishav Roshan and Sudipta Show argues that this third particle's very weak couplings make it long-lived. That would let it take over the universe's energy budget and set off an early period of matter domination.arxiv.org

"This decoupled heavy neutrino has long been dismissed as an irrelevant background detail in particle physics," Roshan said. "However, it decays incredibly slowly". He said the particle changed the temperature at which leptons, such as electrons, interacted with their antiparticles through the Higgs boson. "This temperature acts like a switch that shapes how the matter-antimatter imbalance was built up."phys.org

Testable predictions

Show, of IIT Kanpur, said the change in early expansion "leaves distinct but detectable imprints," including on primordial gravitational waves. The letter also predicts that the lightest ordinary neutrino has a mass of about 10⁻¹² to 10⁻¹⁴ electronvolts. The authors say this makes the idea falsifiable if experiments such as KATRIN or Project 8 can probe that range.arxiv.org+1

A follow-up preprint posted October 13 by Roshan, Show and Southampton physicist Stephen King develops the idea further. It finds that the early matter era would also put a break in the gravitational-wave background from cosmic strings, and that future observatories including LISA, SKA, DECIGO and the Einstein Telescope could detect it.arxiv.org

The proposal is still theoretical. As the Daily Mail noted, scientists have not shown that these heavy neutrinos exist. Roshan said the theory "could be tested experimentally through upcoming gravitational wave detectors and particle physics experiments."dailymail.com