Heavy neutrino model offers new clue to cosmic matter dominance

A theoretical study by researchers at the University of Southampton and the Indian Institute of Technology Kanpur identifies a possible role for a long-overlooked heavy neutrino in the emergence of a matter-dominated early universe. The model, presented in Physical Review Letters , examines how the unusually slow decay of a weakly interacting particle could have changed conditions during the universe’s expansion after the Big Bang. The authors propose that the particle may have influenced the asymmetry between matter and antimatter, but their findings remain a theoretical explanation awaiting observational scrutiny.

The question at the center of the work is one of modern physics’ enduring problems: how matter came to predominate over antimatter. According to current theories, matter and antimatter should have annihilated each other after the Big Bang, leaving an empty universe containing only energy. Most proposals for that process have treated a decoupled heavy neutrino as an irrelevant background detail because it interacts too weakly with other particles to appear consequential.

The Southampton–Kanpur analysis challenges that assumption within a minimal seesaw framework. The decoupled heavy neutrino can decay extremely slowly, allowing it to remain present for a substantial period as the cosmos cooled. According to the proposed scenario, the particle lived long enough to temporarily take over the universe’s energy density and trigger a matter-dominated period.

Its significance therefore lies not in strong interactions with ordinary matter, but in its lifetime and the changing cosmological conditions that prolonged existence could create. The paper, titled “Delayed Charged Lepton Yukawa Equilibration in Minimal Seesaw,” examines what this delayed decay implies for charged leptons, including electrons, and for their interactions with antiparticles through the Standard Model Higgs boson. Rishav Roshan of the University of Southampton said the heavy neutrino shifted the temperature at which these interactions equilibrated, describing the temperature as a switch that shaped how the matter–antimatter imbalance was built up.

In the model, delaying this equilibration alters the sequence of events during the universe’s thermal history and supplies a possible explanation for how matter came to dominate rather than being eliminated by annihilation. The researchers also propose a way to investigate the model through primordial gravitational waves. Paper co-author Sudipta Show said the dramatic shift associated with the early expansion could leave detectable imprints, opening additional avenues for studying the universe’s evolution after the Big Bang.

Such a signal would have to exhibit the characteristics predicted by the model to constitute support for the proposed mechanism.

Subject of Research: Theoretical heavy-neutrino explanation for early-universe matter–antimatter asymmetry

Article Title: Forgotten particle could answer mystery of why we exist, say physicists

Article References: Forgotten particle could answer mystery of why we exist, say physicists. Original publication (Bibliographic details unavailable.)

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Keywords: heavy neutrinos, matter–antimatter asymmetry, early universe, Big Bang cosmology, minimal seesaw, charged leptons, Higgs interactions, primordial gravitational waves, Forgotten, particle, answer, mystery