Leptogenesis and Baryogenesis

Core Idea

The observed universe contains far more matter than antimatter (baryon-to-photon ratio eta ~ 6 x 10^{-10}), an asymmetry that cannot be explained by the Standard Model alone. Baryogenesis mechanisms must satisfy the three Sakharov conditions: baryon number violation, C and CP violation, and departure from thermal equilibrium. Leptogenesis -- generating a lepton asymmetry from the decay of heavy right-handed neutrinos, which is then partially converted to a baryon asymmetry by electroweak sphalerons -- is one of the most compelling scenarios.

Explainer

The matter-antimatter asymmetry of the universe is one of the most profound puzzles in physics. The observed baryon-to-photon ratio eta ~ 6 x 10^{-10}, measured from Big Bang nucleosynthesis and the CMB, means that for every billion antiprotons in the early universe, there were one billion and one protons. This tiny excess survived after all the matter-antimatter pairs annihilated, leaving the residual baryons that make up all visible matter today. Generating this asymmetry dynamically (baryogenesis) requires physics beyond the Standard Model.

Electroweak baryogenesis attempts to generate the asymmetry at the electroweak phase transition (~100 GeV). If the transition were strongly first-order, expanding bubbles of the broken phase would provide the out-of-equilibrium condition, and CP-violating interactions of particles with the bubble walls would produce a baryon asymmetry through sphaleron processes. However, in the SM with m_H = 125 GeV, the transition is a smooth crossover, not first-order. Extensions of the Higgs sector (additional scalars, as in the two-Higgs-doublet model or NMSSM) can make the transition first-order, but these models are constrained by Higgs coupling measurements and direct searches. Electroweak baryogenesis also requires new sources of CP violation beyond the CKM phase.

Leptogenesis is the leading alternative, elegantly connecting the baryon asymmetry to neutrino physics. In the type-I seesaw mechanism, heavy right-handed Majorana neutrinos N_i with masses M_i ~ 10^{9-15} GeV generate tiny left-handed neutrino masses through m_nu ~ m_D^2/M_N. These same heavy neutrinos, decaying out of equilibrium in the early universe with CP-violating asymmetry, produce a lepton asymmetry that sphalerons partially convert to a baryon asymmetry. The elegance of leptogenesis is that it uses particles (right-handed neutrinos) already motivated by neutrino masses and requires CP violation already hinted at by neutrino oscillation data.

Testing leptogenesis is challenging because the right-handed neutrinos are typically too heavy to produce at colliders. However, the connection to low-energy neutrino parameters provides indirect tests: the CP phase delta_CP measured in oscillation experiments is related (though not identical) to the high-energy CP violation driving leptogenesis. Resonant leptogenesis (where M_1 ~ M_2, enhancing the CP asymmetry) and ARS (Akhmedov-Rubakov-Smirnov) leptogenesis (using GeV-scale sterile neutrinos) offer scenarios testable at the LHC or future experiments like SHiP. The discovery of neutrinoless double beta decay would confirm the Majorana nature of neutrinos, a necessary ingredient for the seesaw mechanism and standard leptogenesis.