Southampton and IIT Kanpur authors propose heavy neutrino explains matter-antimatter imbalance

Rishav Roshan of the University of Southampton and Sudipta Show of the Indian Institute of Technology Kanpur published their study in Physical Review Letters in September 2026, proposing that the heaviest right-handed neutrino helped produce the present-day matter-dominated universe.

The early universe held matter and antimatter in roughly equal measure, and exact balance would have brought mutual annihilation, leaving almost no ordinary matter behind. Enough matter survived to build galaxies, stars, planets and eventually life. Yet physicists still do not fully understand why matter dominates the observable universe. Ordinary particle interactions later eliminated leftover antimatter, so matter became the material seen in the cosmos.

Rishav Roshan and Sudipta Show wrote Delayed Charged Lepton Yukawa Equilibration in Minimal Seesaw, which appeared in Physical Review Letters, volume 137, issue 11, article 111802, in September 2026. Prior work in the minimal seesaw framework — an approach used to explain why ordinary neutrinos are so light — focused on the two lighter heavy neutrinos and set aside the heaviest right-handed neutrino because its weak interactions seemed unable to change the outcome. The new study centers on anti-neutrinos with the greatest mass, which the authors call the decoupled heavy neutrino.

At the start, neutrons, protons, electrons and neutrinos shared one temperature as they interacted with each other, and this balance ended when expansion pushed the universe past a critical point. About one second after the Big Bang, neutrons separated from other material particles and began behaving like present-day neutrinos. Those separated neutrinos form the cosmic neutrino background, an echo of the Big Bang resembling the cosmic microwave background, though they now weakly shape cosmic evolution. Neutrinos occur in three flavors linked to the electron, muon and tauon, each with its own mass, while antineutrinos carry corresponding mass values.

Neutrinos and antineutrinos differ by handedness rather than by electric charge. Left-handed neutrinos take part in ordinary interactions with matter in the modern universe, while right-handed states remain sterile apart from a gravitational effect. Because the heaviest right-handed neutrino interacts so weakly, it can stay out of balance with the rest of the early universe for a long time. Its slow decay let it briefly command the universe’s energy density and open a matter-dominated era while the cosmos cooled after the Big Bang.

The changed equilibration temperature works like a timing switch, altering the order and efficiency of the processes that built matter’s advantage over antimatter. The idea connects to leptogenesis, where particles and antiparticles behave differently at high temperatures and can create the surplus of matter seen in the cosmos. Interactions between charged leptons bring electrons and other leptons into contact with their antiparticles through the Standard Model Higgs boson. The long half-life of heavy neutrinos delayed their decay until electrons, protons, positrons and anti-protons already existed.

Dr. Roshan argued that our existence demonstrates a matter-antimatter asymmetry happened. Dr. Roshan said, “This decoupled heavy neutrino has long been dismissed as an irrelevant background detail in particle physics.” Roshan holds a master’s from Jamia Millia in New Delhi and a doctorate from IIT Guwahati, and he joined Southampton in 2023. Show received her doctorate from the Physical Research Laboratory in Ahmedabad.

The authors present a model-based scenario rather than a confirmed explanation, and their study does not establish that the particle existed in the early universe. Heavy right-handed neutrinos have never been detected in any experiment. Proposed tests include measurements of how neutrinos change type, dedicated neutrino experiments and searches for primordial gravitational waves. According to Dr. Show, the results change how we see early cosmic expansion and explain how matter came to dominate.

These primordial gravitational waves are faint ripples in the fabric of spacetime from the universe’s earliest moments, and current equipment cannot confirm either test definitively. If the model is correct, the era of neutrino dominance would leave a measurable mark on primordial gravitational waves and on the cosmic neutrino background.

Any claimed detection would need signal characteristics that match the model’s predictions to back the proposed mechanism. Roshan and Show also point to possible effects on dark-matter production and on the primordial gravitational-wave spectrum. If the idea survives more theoretical scrutiny, researchers could add the heaviest decoupled neutrino to future early-universe calculations.