Leptogenesis as the origin of matter

TL;DR

The review argues that the observed baryon asymmetry $oldsymbol{ i_B}$ can naturally originate from a primordial lepton asymmetry generated via Leptogenesis, tightly connected to the neutrino mass spectrum through the seesaw mechanism. It develops the thermal Leptogenesis framework with heavy Majorana neutrino decays, CP violation, and sphaleron processes that convert $B-L$ into the observed $B$; Boltzmann equation analyses yield bounds on neutrino masses, heavy-neutrino scales, and reheating temperatures, predominantly placing light neutrino masses in the $10^{-3}-0.1$ eV range. The review also examines compatibility with dark matter scenarios, notably axions and SUSY DM, while addressing the gravitino problem and possible resolutions, including resonant or nonthermal Leptogenesis. Together these results imply a high-scale origin of matter with testable implications for neutrino physics, cosmology, and collider phenomenology, and they motivate exploring nonthermal options and broader model-building avenues. The work emphasizes that Leptogenesis offers a falsifiable, theoretically attractive bridge between early-Universe dynamics and present-day neutrino properties.

Abstract

We explore in some detail the hypothesis that the generation of a primordial lepton-antilepton asymmetry (Leptogenesis) early on in the history of the Universe is the root cause for the origin of matter. After explaining the theoretical conditions for producing a matter-antimatter asymmetry in the Universe we detail how, through sphaleron processes, it is possible to transmute a lepton asymmetry -- or, more precisely, a (B-L)-asymmetry -- into a baryon asymmetry. Because Leptogenesis depends in detail on properties of the neutrino spectrum, we review briefly existing experimental information on neutrinos as well as the seesaw mechanism, which offers a theoretical understanding of why neutrinos are so light. The bulk of the review is devoted to a discussion of thermal Leptogenesis and we show that for the neutrino spectrum suggested by oscillation experiments one obtains the observed value for the baryon to photon density ratio in the Universe, independently of any initial boundary conditions. In the latter part of the review we consider how well Leptogenesis fits with particle physics models of dark matter. Although axionic dark matter and Leptogenesis can be very naturally linked, there is a potential clash between Leptogenesis and models of supersymmetric dark matter because the high temperature needed for Leptogenesis leads to an overproduction of gravitinos, which alter the standard predictions of Big Bang Nucleosynthesis. This problem can be resolved, but it constrains the supersymmetric spectrum at low energies and the nature of the lightest supersymmetric particle (LSP). Finally, as an illustration of possible other options for the origin of matter, we discuss the possibility that Leptogenesis may occur as a result of non-thermal processes.

Figures (5)

• Figure 1: Tree level and one-loop diagrams contributing to heavy neutrino decays whose interference leads to Leptogenesis.
• Figure 2: The evolution of the $N_1$ abundance and the $B-L$ asymmetry for a typical choice of parameters, $M_1=10^{10}\,$GeV, $\varepsilon_1=10^{-6}$, $\widetilde{m}_1=10^{-3}\,$eV and $\overline{m}=0.05\,$eV. From bdp02.
• Figure 3: Final efficiency factor when the washout term $\Delta W$ is neglected. From bdp04.
• Figure 4: Analytical lower bounds on $M_1$ (circles) and $T_{\rm i}$ (dotted line) for $m_1 = 0$, $\eta_B^{CMB} = 6\times 10^{-10}$ and $m_{\rm atm} = 0.05\,{\rm eV}$. The analytical results for $M_1$ are compared with the numerical ones (solid lines). Upper and lower curves correspond to zero and thermal initial $N_1$ abundance, respectively. The vertical dashed lines indicate the range ($m_{\rm sol}$,$m_{\rm atm}$). The gray triangle at large $M_1$ and large $\widetilde{m}_1$ is excluded by theoretical consistency. From bdp04.
• Figure 5: Upper bounds on the reheating temperature as function of the gravitino mass for the case where the gravitino dominantly decays into a gluon-gluino pair. From KKM.