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Non Thermal LeptogenesisEdit

Non-thermal leptogenesis is a mechanism in cosmology and particle physics that explains the matter–antimatter asymmetry of the universe by producing heavy right-handed neutrinos not from a hot primordial plasma, but through processes tied to the dynamics after inflation. In these scenarios, the heavy neutrinos decay in a way that violates lepton number and CP symmetry, generating a lepton asymmetry that is partially converted into a baryon asymmetry by electroweak sphaleron processes. This route sits alongside thermal leptogenesis, where the same ingredients arise from a sufficiently hot early universe.

Non-thermal variants are particularly attractive in models where the reheat temperature after inflation is constrained or where the physics of the inflaton and its couplings naturally yield heavy neutrinos without requiring an extremely hot early cosmos. The framework is consistent with the idea that the Standard Model can be extended by a see-saw sector involving heavy right-handed neutrino states, linking the origin of the baryon asymmetry to the origin of tiny neutrino masses via the see-saw mechanism. It sits comfortably within broader beyond-Standard Model research, including ideas from inflation (cosmology) and various ultraviolet completions such as Grand Unified Theories.

Overview

  • Non-thermal leptogenesis generates a net lepton number through out-of-equilibrium decays of heavy neutrinos that are produced by non-thermal processes, such as inflaton decay or non-perturbative post-inflation dynamics.
  • The created lepton asymmetry is transformed into the observed baryon asymmetry of the universe by electroweak sphalerons, which operationally couple lepton and baryon number under the thermal history of the early universe.
  • The mechanism ties into measurable aspects of neutrino physics, including CP-violating phases in the lepton sector and the mass pattern generated by the see-saw mechanism.

Key concepts to understand include the Sakharov conditions for baryogenesis (baryon number violation, C and CP violation, and departure from thermal equilibrium), the role of heavy Majorana neutrinos in leptogenesis, and the conversion of lepton into baryon number via sphaleron processes.

Mechanisms

  • Inflaton decay into heavy neutrinos: After inflation, the inflaton field can decay directly into heavy right-handed neutrinos. If these neutrinos are sufficiently long-lived and decay out of equilibrium, their CP-violating decays generate a lepton asymmetry without requiring a hot thermal bath. This pathway keeps the reheat temperature relatively modest while still producing the needed asymmetry. See inflation (cosmology) and reheating (cosmology) for related dynamics.
  • Preheating and non-perturbative production: In some models, rapid, non-thermal production of heavy neutrinos occurs during preheating, a phase just after inflation characterized by non-perturbative particle production. Such scenarios can avoid a fully thermalized plasma while still sourcing CP-violating decays that yield a lepton asymmetry.
  • Decays of other heavy states: In certain ultraviolet completions, other heavy fields associated with grand unification or additional scalar sectors can radiatively or non-perturbatively feed heavy right-handed neutrinos, initiating leptogenesis through their decays.

The essential physics is that the heavy neutrino decays proceed out of thermal equilibrium and violate CP to generate a net lepton number. The created lepton asymmetry then gets partly converted into a baryon asymmetry by the electroweak sector, via the sphaleron transitions that are active at temperatures above the electroweak scale.

Model space and connections

  • Mass scales and naturalness: Non-thermal leptogenesis often operates with heavy neutrino masses around the traditional high-scale see-saw values, which can be well above the reach of terrestrial experiments. But because production is non-thermal, the exact height of the reheat temperature and the coupling structure can be adjusted to maintain naturalness and compatibility with other cosmological constraints, such as the absence of unwanted relics.
  • See-saw link to neutrino physics: The heavy neutrino sector gives rise to small observed neutrino masses through the see-saw mechanism and introduces CP-violating phases that can influence low-energy observables. This creates a potential bridge between early-universe processes and measurable properties like neutrino oscillation parameters and CP phases in the lepton sector.
  • Model-building options: Non-thermal scenarios are explored within various frameworks, including minimal extensions of the Standard Model and more ambitious schemes that place the heavy neutrinos in ideas such as Grand Unified Theories or supersymmetric models. The choice of inflationary sector, reheating dynamics, and neutrino Yukawa couplings all shape the viability and predictions of a given model.

Observational implications and tests

  • Baryon asymmetry: The primary observational constraint is the measured baryon asymmetry of the universe, typically quantified as a small excess of matter over antimatter in the cosmic baryon density. Successful non-thermal leptogenesis must reproduce this value in a consistent cosmological history.
  • Neutrino sector: Because the mechanism relies on heavy neutrinos and CP violation in the lepton sector, any compatibility with low-energy neutrino data—such as mass splittings, mixing angles, and the Dirac CP phase—matters. This makes upcoming and ongoing experiments in neutrino physics relevant for constraining or guiding models.
  • Lepton-number and CP-violation signals: While the heavy states are out of reach directly, indirect consequences such as predictions for neutrinoless double beta decay rates or correlations with low-energy CP-violating observables can provide circumstantial tests of the framework.
  • Gravitational waves and early-universe dynamics: In some non-thermal scenarios, the post-inflationary dynamics that produce heavy neutrinos can leave imprints in the stochastic gravitational-wave background or in other cosmological relics, depending on the detailed inflationary and reheating history.

Controversies and debates

  • Testability and model dependence: Critics argue that non-thermal leptogenesis often relies on specific inflationary models, particular couplings, or fine-tuned parameters, making it hard to test directly. Proponents counter that cosmology routinely deals with indirect evidence and that linking leptogenesis to measurable neutrino properties provides a plausible, falsifiable path forward.
  • Naturalness and high scales: A longstanding tension in leptogenesis frameworks is the requirement for very high mass scales for the heavy neutrinos. Non-thermal mechanisms sometimes aim to alleviate this by modifying production, but some observers question whether these constructions remain natural or predictive without additional assumptions.
  • Comparison with thermal leptogenesis: Some physicists prefer the thermal approach because it exemplifies a simple, robust picture: a hot early universe naturally supplies the heavy neutrinos, CP violation from the neutrino sector, and sphaleron conversion. Non-thermal variants are valued when a model wants to avoid high reheat temperatures or to fit a particular inflationary story, but they inherently involve more moving parts.
  • Woke criticisms and debates about cosmology discourse: In public discussions, some critics argue that cosmology theorizing descends into speculative territory with uncertain empirical footing. Advocates for non-thermal leptogenesis respond that the framework builds on established physics (neutrino masses, CP violation, and sphalerons) and engages with testable aspects of the neutrino sector and cosmology. They would mark as unhelpful the claim that such work is inherently non-scientific or politically motivated, emphasizing instead the coherence and potential for indirect falsifiability through precision neutrino measurements and cosmological data. If one encounters objections framed as political or social accusations, the productive response is to focus on the physics claims, the range of plausible models, and the specific observables that could confirm or refute the scenarios.

See also