Leptogenesis

TL;DR

Leptogenesis connects neutrino mass generation to the cosmological baryon asymmetry by invoking the out-of-equilibrium decays of heavy sterile neutrinos that generate a lepton asymmetry, subsequently converted into a baryon asymmetry by sphalerons. The review clarifies the seesaw parameterizations, CP-violating sources, and the dynamical evolution via Boltzmann equations, while detailing finite-temperature effects, spectator processes, and crucial flavour dynamics that can dramatically alter outcomes. It shows that flavour, thermal corrections, and resonant or nonstandard scenarios can ease lower bounds on $M_1$ and open lower reheating temperatures in some models, though universal bounds persist in others. The work systematically catalogs the methods and limitations, highlighting how leptogenesis remains a compelling, testable framework within extensions of the Standard Model, and outlining the phenomenological implications for neutrino physics and cosmology.

Abstract

Leptogenesis is a class of scenarios where the baryon asymmetry of the Universe is produced from a lepton asymmetry generated in the decays of a heavy sterile neutrino. We explain the motivation for leptogenesis. We review the basic mechanism, and describe subclasses of models. We then focus on recent developments in the understanding of leptogenesis: finite temperature effects, spectator processes, and in particular the significance of flavor physics.

Figures (8)

• Figure 3.1: Evolution with time of the energy eigenstates of chiral fermions in a gauge field background with $\tilde{F} F \neq 0$.
• Figure 5.1: The diagrams contributing to the CP asymmetry $\epsilon_{\alpha \alpha}$. The flavour of the internal lepton $\ell_\beta$ is summed. The internal $\ell_\beta$ and Higgs $\phi$ are on-shell. The X represents a Majorana mass insertion. Line direction is "left-handedness", assigning to scalars the handedness of their SUSY partners. The loop diagrams on the first line are lepton flavour and lepton number violating. The last diagram is lepton flavour changing but "lepton number conserving", in the sense that it makes no contribution to the total CP asymmetry $\epsilon$. It is suppressed by an additional factor $M_1/M_{2,3}$ [see eqn (\ref{['flavour-CPasym']})].
• Figure 6.1: Diagrams for various $2\leftrightarrow 2$ scattering processes: $(a)$ scatterings with the top-quarks, $(b),\,(c)$ scatterings with the gauge bosons ($A=B,W_i$ with $i=1,2,3$), $(d)$$\Delta L=2$ scatterings mediated by $N_1$.
• Figure 7.1: Comparison between scattering rates in the Standard Model with (solid lines) and without (dashed lines) thermal corrections. We use $\tilde{m}=0.06\,$eV and $M_1=10^{10}\,$GeV. $(a)$$\gamma_{N_t}$: the rate density of the $t$-channel $N_1$-exchange scattering $\ell\,\ell\leftrightarrow\bar{\phi}\,\bar{\phi}$, normalized to $n_\ell H$. ($b$) $\gamma^{\rm top}_{\phi_s}$ and $\gamma^{\rm top}_{\phi_{t+u}}$: the rate densities of the Higgs exchange scaterrings in, respectively, the $s$-channel ($q_3\,\bar{t}\leftrightarrow\ell\,N$) and the $t$- and $u$-channels ($q_3\, N\leftrightarrow\ell\, t$ and $\bar{t}\, N \leftrightarrow \ell\, \bar{q}_3$), normalized to $n_{N_1} H$. (Figures adapted from ref. munoz:2007.)
• Figure 9.1: The total baryon asymmetry in the two flavour calculation (upper curves) and within the one-flavour approximation (lower curves) as a function of $z$, for two different sets of washout parameters. Left picture: $\tilde{m}_{\tau\tau}/m_*=10$, $\tilde{m}_{\mu\mu}/m_*=30$, $\tilde{m}_{ee}/m_*=30$. Right picture: $\tilde{m}_{\tau\tau}/m_*=10$, $\tilde{m}_{\mu\mu}/m_*=30$, $\tilde{m}_{ee}/m_*=10^{-2}$. In both cases $\epsilon_{\tau \tau}=2.5\times 10^{-6}$, $\epsilon_{\mu\mu}=-2\times 10^{-6}$, $\epsilon_{ee}=10^{-7}$ and $M_1=10^{10}$ GeV. These plots are from Abada:2006ea.