Thermal LeptogenesisEdit
Thermal leptogenesis is a leading framework for explaining the observed imbalance between matter and antimatter in the universe. It rests on extending the Standard Model with heavy right-handed neutrinos that interact with the known light neutrinos through the seesaw mechanism. In the hot, early cosmos, these heavy neutrinos can be produced in thermal processes after reheating. As they decay, they do so in a way that violates CP symmetry and proceeds out of equilibrium, generating a net lepton number. Electroweak sphaleron processes then convert part of this lepton asymmetry into a baryon asymmetry, yielding the baryon-to-photon ratio that cosmological observations constrain to be about ηB ≈ 6 × 10^−10. The mechanism ties neatly to the origin of tiny neutrino masses and their mixing, as encoded in the seesaw framework seesaw mechanism and neutrino physics.
The essential idea is simple in outline but rich in detail. Heavy, early-universe neutrinos (often denoted N) decay into Standard Model leptons and Higgs bosons with a small, CP-violating asymmetry. If these decays occur out of thermal equilibrium, a net lepton number builds up. Because the electroweak theory allows certain processes that violate baryon and lepton number in a correlated way (the sphalerons), part of the lepton asymmetry is reprocessed into a baryon asymmetry. This chain—heavy neutrino decays producing a lepton asymmetry, and sphalerons translating that into a baryon asymmetry—embodies the core logic of thermal leptogenesis CP violation sphaleron baryogenesis.
Mechanism
The seesaw extension
The canonical setup adds heavy right-handed neutrinos with Majorana masses. Their Yukawa couplings to the lepton doublets generate light Majorana masses for the observed neutrinos through the seesaw mechanism, providing a natural explanation for why neutrino masses are so small compared to other fermions seesaw mechanism neutrino.
CP violation and out-of-equilibrium decays
A key ingredient is CP violation in the decays N → ℓH and N → ℓ̄H†. The interference between tree-level and loop diagrams yields a net lepton asymmetry proportional to CP-violating phases in the Yukawa sector. The asymmetry must be generated while the decays are out of equilibrium, a condition controlled by the decay rate relative to the Hubble expansion at the relevant temperature. The resulting lepton asymmetry is often characterized by a parameter ε1, whose magnitude depends on the heavy-neutrino masses and their couplings to light neutrinos CP violation Boltzmann equations.
Sphalerons and baryon asymmetry
Electroweak sphalerons convert part of the lepton asymmetry into a baryon asymmetry before the electroweak phase transition. This conversion depends on Standard Model processes and the overall lepton number generated, producing the observed BAU without requiring new electric charge or lepton-number-violating interactions at temperatures below the sphaleron freeze-out sphaleron.
Flavor effects and Boltzmann dynamics
A refined treatment tracks the flavor composition of the lepton sector. Flavor effects can alter efficiency and sign of the generated asymmetry and can shift the favored mass scales of the heavy neutrinos. The evolution is described by a set of coupled Boltzmann equations that account for decays, inverse decays, and washout processes that can dilute the asymmetry Boltzmann equations flavored leptogenesis.
Parameter space and variants
Davidson–Ibarra bound and high-scale viability
In the simplest thermal leptogenesis scenarios, there is a lower bound on the lightest heavy neutrino mass (M1) and on the reheat temperature (TR) after inflation. A widely cited result, the Davidson–Ibarra bound, ties the CP asymmetry and washout to M1 and the light neutrino masses, implying that successful thermal leptogenesis typically requires M1 around 10^9 GeV or higher and a reheat temperature not far below that scale. This places the mechanism squarely in a high-energy regime and links cosmology to grand-scale physics Davidson–Ibarra bound.
Resonant and low-scale leptogenesis
Because the high-scale requirement can clash with other aspects of cosmology, several refinements explore lower-scale realizations. In resonant leptogenesis, nearly degenerate heavy neutrinos lead to an enhanced CP asymmetry even at TeV-scale masses, easing the need for extremely high TR. Other low-scale variants, sometimes termed flavored or N2-dominated leptogenesis, exploit the dynamics of multiple heavy neutrinos and flavor effects to generate the observed BAU at accessible mass scales. These ideas aim to preserve predictive power while improving testability and compatibility with other aspects of beyond‑Standard‑Model physics resonant leptogenesis N2-dominated leptogenesis.
Non-thermal production channels
Beyond purely thermal production, inflaton decays or other post-inflationary processes can populate the heavy-neutrino sector non-thermally. In such scenarios, the resulting leptogenesis can proceed under different temperature histories, potentially relaxing some of the traditional bounds and providing alternative links to the inflationary sector reheating (cosmology).
Cosmological implications and tensions
The gravitino problem and high-temperature constraints
In theories that extend the Standard Model with supersymmetry, high reheat temperatures can lead to an overproduction of gravitinos, which challenges Big Bang nucleosynthesis or overcloses the universe depending on their mass and decay channels. This tension motivates exploring lower-temperature mechanisms or non-thermal avenues and motivates a pragmatic approach to model-building where leptogenesis is compatible with other cosmological constraints supersymmetry. Critics and proponents debate how robust these constraints are across the full landscape of theories, but the interplay between leptogenesis and the thermal history remains a central concern for those seeking a minimal, coherent cosmology.
Neutrino properties and experimental tests
Thermal leptogenesis makes predictions tied to neutrino masses and mixing. While direct tests of the heavy neutrino sector are out of reach with current technology, the framework motivates precision measurements of the light-neutrino sector, neutrinoless double-beta decay, and related phenomena. Progress in these areas constrains the allowed parameter space and helps differentiate among viable variants of leptogenesis neutrinoless double beta decay.
Controversies and debates
Scope and testability: A common debate centers on how falsifiable thermal leptogenesis is. High-scale versions rest on physics at energies far beyond current colliders, relying on cosmological and neutrino data for indirect tests. Proponents emphasize the economy of the extension—linking neutrino masses, CP violation, and the BAU in a single, coherent story—while skeptics push for low-scale, more directly testable alternatives or for non-thermal channels that tie more closely to experimental probes baryogenesis.
Naturalness and model-building: Critics sometimes argue that very high-scale leptogenesis requires accepting new physics at scales that are difficult to probe directly, which can clash with a preference for models with sooner-testable consequences. Advocates respond that the approach minimizes new field content and remains tightly constrained by observed neutrino properties, offering a parsimonious bridge between particle physics and cosmology seesaw mechanism.
Competition with other BAU explanations: Some research emphasizes alternative baryogenesis mechanisms (for example, electroweak baryogenesis in specific extensions of the Standard Model or other leptogenesis variants) as equally viable routes to the BAU. Defenders of thermal leptogenesis point to its natural fit with the observed neutrino mass scale and the Standard Model’s accidental symmetries, arguing that it remains among the most robust explanations consistent with current data baryogenesis.
Woke criticism and scientific debate: In the ecosystem of scientific discourse, some criticisms focus on broader cultural or epistemic questions about what counts as evidence or what kind of explanations deserve emphasis. In this field, the productive stance is to weigh the physics—CP violation, out-of-equilibrium dynamics, and the connection to neutrino masses—against alternative models, while keeping discussions grounded in empirical constraints and theoretical consistency.