The origin of matter in the Universe is a fascinating cosmological puzzle that has triggered a formidable intellectual enterprise, started in 1967 with the prescient paper by Andrej Sakharov (1967 Pisma Zh. Eksp. Teor. Fiz. 5 32; 1967 JETP Lett. 52 4; 1991 Sov. Phys.—Usp. 34 392; 1991 Usp. Fiz. Nauk 161 61) aimed at relating a cosmological observation to the fundamental laws of physics, the goal of baryogenesis. A successful model of baryogenesis should ultimately identify the required source of charge parity violation and the origin of the cosmological matter–antimatter asymmetry. This focus issue is not only a review of the main ideas that have been proposed in baryogenesis but should also bear witness to the great vitality of the field and to show how future experimental results could bring a breakthrough in baryogenesis during the coming years. For this reason we selected, out of the multitude of proposed baryogenesis models, those that will more likely experience a significant experimental test during the coming years.
Focus on the Origin of Matter
Figure. Baryogenesis represents a very early stage in the history of the early Universe that should have occurred just at the end or after inflation and, most likely, before electroweak symmetry breaking. Therefore, it implies an initial temperature much greater than the highest values currently tested with Big Bang Nucleosynthesis (BBN). Figure courtesy of Pasquale Di Bari.
Pasquale Di Bari, University of Southampton, UK
Antonio Masiero, Università degli Studi di Padova, Italy
Rabindra Mohapatra, University of Maryland, College Park, USA
Baryogenesis provides the first example of a theory suggesting an extension of the laws of physics tested in Earth-based laboratories in order to explain a basic cosmological property: the non-observation of primordial anti-matter in the Universe. This is seemingly at odds with the symmetric description of matter and anti-matter contained within the established fundamental laws elegantly encoded by the Dirac equation.
In 1967 Andrej Sakharov, three years after the discovery of CP violation in the decays of neutral kaons by Cronin and Fitch, proposed that the origin of such a fundamental new property could be linked to the matter–antimatter asymmetry of the Universe. This implies that a successful model of baryogenesis should ultimately identify both the source of CP violation and the origin of the cosmological matter–antimatter asymmetry.
It has been 45 years since the Sakharov paper that established baryogenesis as a new field in particle cosmology, which is a good opportunity to review the state-of-the-art of the field—discussing the achieved result and what advances toward a solution to the problem we can expect in the future.
However, this recurrence is not the only reason for this focus issue on baryogenesis. We have just entered a stage of experimental effort in particle physics and cosmology that will probably have a tremendous impact on fundamental physics and specifically on baryogenesis.
In collider physics, the Large Hadron Collider is exploring in detail the physics of the electroweak scale, testing both the sources of CP violation within the quark sector and also, especially if the evidence for the Higgs boson is confirmed, possible sources of departure from thermal equilibrium in the early Universe. At the same time, neutrino physics experiments, after the discovery of a non-vanishing reactor mixing angle, will certainly make further substantial progress in the completion of the measurement of the low-energy neutrino parameters, with potentially some further exciting surprises, such as the discovery of light sterile neutrino states. On the cosmological side, the Planck satellite could provide a definitive 'smoking gun' for an inflationary stage, opening a door to, so far, the most difficult of the proposed models of baryogenesis to be tested. Finally, great efforts are being dedicated to testing the paradigm of missing primordial antimatter in the observable Universe even more accurately.
It is quite probable that at the end of this stage, probably at the end of this decade, we will have to abandon some of the ideas that have driven the investigation of baryogenesis so far. At the same time, there are realistic possibilities that strong signals pinning down the correct model could emerge. Even the possibility that one of the existing models of baryogenesis will be established as the next pillar within the history of the early Universe, together with BBN, the recombination era and, seemingly, inflation, is a legitimate hope. Our choice then should be clear—within the multitude of proposed models of baryogenesis, which will be most likely to experience a significant experimental test during the coming years.
This focus issue is thus meant not only as a review of the results achieved in baryogenesis, but also as a testament to the great vitality of the field and a demonstration of how experimental results could bring a breakthrough in baryogenesis in the next few years.
We wish to thank all the authors for their invaluable contributions that we are sure will determine the success of this focus issue.
Recent evidence for neutrino masses has established the leptogenesis mechanism as a very natural possible explanation for the baryon asymmetry of the Universe. The explicit realization of this mechanism depends on the neutrino mass model considered. If the right-handed type-I seesaw model of neutrino masses is certainly the most straightforward, it is not the only natural one, especially in the framework of explicit GUT realizations of the seesaw. In this paper, we review in detail the various seesaw scenarios that can implement the leptogenesis mechanism successfully, beyond the paradigm of the pure standard type-I seesaw model. This includes scenarios based on the existence of scalar triplets (type-II), of fermion triplets (type-III) as well as mixed seesaw frameworks.
The Affleck–Dine mechanism is an attractive scenario for generating the observed baryon asymmetry of the universe utilizing flat directions in the scalar potential of supersymmetric theories. In this mini review, we describe this mechanism in its original version, its explicit realization within the minimal supersymmetric standard model and its variants. We discuss the formation of a condensate along the flat directions in the inflationary era, its post-inflationary evolution leading to baryogenesis and its fate. In some cases the condensate may fragment into non-topological solitons, known as Q-balls, during its evolution. In models of gravity-mediated supersymmetry breaking, the Q-balls can be long-lived, in which case their decay will be the source of all baryons and dark matter in the form of the lightest supersymmetric particle. In models of gauge-mediated supersymmetry breaking, the Q-balls can be absolutely stable and form dark matter that can be searched for directly.
We review the main features and results of thermal leptogenesis within the type I seesaw mechanism, the minimal extension of the Standard Model explaining neutrino masses and mixing. After presenting the simplest approach, the vanilla scenario, we discuss various important developments of recent years, such as the inclusion of lepton and heavy neutrino flavour effects, a description beyond a hierarchical heavy neutrino mass spectrum and an improved kinetic description within the density matrix and the closed-time-path formalisms. We also discuss how leptogenesis can ultimately represent an important phenomenological tool to test the seesaw mechanism and the underlying model of new physics.
Electroweak baryogenesis (EWBG) remains a theoretically attractive and experimentally testable scenario for explaining the cosmic baryon asymmetry. We review recent progress in computations of the baryon asymmetry within this framework and discuss their phenomenological consequences. We pay particular attention to methods for analyzing the electroweak phase transition and calculating CP-violating asymmetries, the development of Standard Model extensions that may provide the necessary ingredients for EWBG, and searches for corresponding signatures at the high energy, intensity and cosmological frontiers.
We review observational evidence for a matter–antimatter asymmetry in the early universe, which leads to the remnant matter density we observe today. We also discuss bounds on the presence of antimatter in the present-day universe, including the possibility of a large lepton asymmetry in the cosmic neutrino background. We briefly review the theoretical framework within which baryogenesis, the dynamical generation of a matter–antimatter asymmetry, can occur. As an example, we discuss a testable minimal particle physics model that simultaneously explains the baryon asymmetry of the universe, neutrino oscillations and dark matter.
The similar cosmological energy budgets in visible baryons and dark matter motivate one to consider a common origin for the generation of both. We outline the key features of scenarios that can accommodate a unified framework for the genesis of cosmic matter. In doing so, we provide a brief overview of some of the past and recent developments and discuss the main predictions of a number of models.