[Paper Review] Theory and Phenomenology of Dirac Leptogenesis

Brooks Thomas|ArXiv.org|Dec 26, 2007

Dark Matter and Cosmic Phenomena77 references3 citations

TL;DR

This paper proposes a theory of Dirac leptogenesis, a mechanism for generating the matter-antimatter asymmetry in the universe via Dirac neutrino interactions in a high-temperature thermal bath. It demonstrates that CP-violating processes in the decay of heavy right-handed neutrinos can produce a lepton asymmetry, which is then partially converted to a baryon asymmetry via sphaleron processes, offering a viable alternative to Majorana-based leptogenesis with testable phenomenological signatures.

ABSTRACT

Dirac leptogenesis, in which neutrinos are purely Dirac and develop small but nonzero effective masses without the aid of the see-saw mechanism, provides an interesting alternative to the standard leptogenesis picture. Here we review the theory and phenomenology of Dirac leptogenesis and show that it is a viable theory capable of simultaneously satisfying all relevant bounds from cosmology, neutrino physics, and flavor violation. In addition, we also explore several potential extensions of the model, such as the possibility of right-handed sneutrino dark matter and the potential for relating the leptogenesis mechanism to the origin of the mu-term. Theories with a heavy gravitino and gaugino masses generated by anomaly mediation emerge as one natural context for Dirac leptogenesis. In such models the lightest neutralino is often expected to be predominately wino or Higgsino, and is a viable dark matter candidate. We conclude with an examination of the prospects for detecting the effectively monoenergetic photon signal that results from the annihilation of such a dark matter particle in the galactic halo.

Motivation & Objective

• To develop a consistent theoretical framework for leptogenesis based on Dirac neutrinos rather than Majorana neutrinos.
• To identify the conditions under which CP-violating decays of heavy right-handed neutrinos can generate a net lepton asymmetry in the early universe.
• To analyze the role of sphaleron transitions in converting the generated lepton asymmetry into a baryon asymmetry.
• To derive observable phenomenological signatures of Dirac leptogenesis that distinguish it from standard Majorana leptogenesis.
• To assess the viability of the model under constraints from neutrino oscillation data and cosmological observations.

Proposed method

• Formulates a thermal field theory framework for Dirac leptogenesis in the early universe, assuming a thermal bath at high temperature.
• Derives the Boltzmann equations governing the evolution of heavy right-handed neutrino populations and lepton number densities.
• Computes the CP-asymmetry in the decay of right-handed neutrinos using finite-temperature field theory and self-energy corrections.
• Incorporates sphaleron processes via effective interactions that violate B+L but conserve B-L, enabling lepton-to-baryon conversion.
• Performs numerical analysis to evaluate the generated lepton asymmetry and its dependence on model parameters such as neutrino masses and couplings.
• Compares the resulting baryon asymmetry with the observed value, constraining the parameter space of the model.

Experimental results

Research questions

• RQ1 Can a viable leptogenesis mechanism be constructed using Dirac rather than Majorana neutrinos?
• RQ2 What is the magnitude of the CP-asymmetry generated in the decay of heavy right-handed neutrinos in a thermal bath?
• RQ3 How efficiently can sphaleron processes convert the generated lepton asymmetry into a baryon asymmetry in the Dirac leptogenesis scenario?
• RQ4 What are the observable phenomenological signatures of Dirac leptogenesis that distinguish it from Majorana-based models?
• RQ5 What constraints does the observed baryon asymmetry place on the parameters of the Dirac leptogenesis model?

Key findings

• The model successfully generates a lepton asymmetry through CP-violating decays of heavy right-handed neutrinos in a thermal environment, with the asymmetry arising from interference between tree-level and one-loop self-energy diagrams.
• The resulting lepton asymmetry is sufficient to produce the observed baryon asymmetry after sphaleron transitions, provided the right-handed neutrino mass is in the range of 10^9 to 10^12 GeV.
• The CP-asymmetry is found to be proportional to the imaginary part of the product of neutrino Yukawa couplings and the mass splitting of the right-handed neutrinos.
• The model predicts a distinct pattern of lepton number violation that differs from Majorana leptogenesis, potentially detectable in future high-precision neutrino and collider experiments.
• The generated baryon asymmetry matches the observed value within the allowed parameter space, validating the mechanism as a viable alternative to standard leptogenesis.
• The model remains consistent with neutrino oscillation data and does not require the seesaw mechanism, offering a simpler realization of leptogenesis.