Leptogenesis from loop effects in curved spacetime

Abstract

We describe a new mechanism -radiatively-induced gravitational leptogenesis -for generating the matter-antimatter asymmetry of the Universe. We show how quantum loop effects in C and CP violating theories cause matter and antimatter to propagate differently in the presence of gravity, and prove this is forbidden in flat space by CPT and translation symmetry. This generates a curvature-dependent chemical potential for leptons, allowing a matter-antimatter asymmetry to be generated in thermal equilibrium in the early Universe. The time-dependent dynamics necessary for leptogenesis is provided by the interaction of the virtual self-energy cloud of the leptons with the expanding curved spacetime background, which violates the strong equivalence principle and allows a distinction between matter and antimatter. We show here how this mechanism is realised in a particular BSM theory, the see-saw model, where the quantum loops involve the heavy sterile neutrinos responsible for light neutrino masses. We demonstrate by explicit computation of the relevant two-loop Feynman diagrams how the size of the radiative corrections relevant for leptogenesis becomes enhanced by increasing the mass hierarchy of the sterile neutrinos, and show how the induced lepton asymmetry may be sufficiently large to play an important rôle in determining the baryon-to-photon ratio of the Universe.

Dynamical evolution of gravitational leptogenesis

“…The BSM Lagrangian is introduced in section 2, where explicit calculations of the one-loop diagrams determining the coefficientsã, c,d are presented. The two-loop coefficient b was previously calculated in [2]. These Feynman diagrams for the neutrino self-energies are matched to the effective action in section 3.…”

“…At finite temperature, this gives rise to an asymmetry n eq L = 1 3 bṘ T 2 . The two-loop BSM calculation of this operator, its interpretation in a modified Boltzmann equation, and its implications for cosmology have been described in detail in [1][2][3].…”

“…The origin of matter-antimatter asymmetry in the universe is one of the most important outstanding issues in cosmology. Radiatively-induced gravitational leptogenesis (RIGL) [1][2][3] is a particularly elegant and economical mechanism to generate a lepton number asymmetry in the early universe with the potential to explain the presently observed baryon-tophoton ratio η = (n B − nB)/n γ 6 × 10 −10 .…”

Baryon asymmetry from primordial black holes

“…where y N is the neutrino Yukawa coupling, and M N is the mass of the right handed neutrinos. Recently, Mcdonald and Shore [45,46] explicitly calculated loop diagrams containing the above interactions coupled with the gravity in the external lines. They took the conformally flat metric, g µν = (1 + h)η µν where the field h is treated as a background field, and then calculated the two-loop Feynman diagrams in Fig.…”

A new twist on the geometry of gravitational plane waves

“…In previous work, quantum loop effects have been shown to lead to a variety of novel effects related to the induced violation of the strong equivalence principle, including ultra-high energy scattering [8,9] and the generation of matter-antimatter asymmetry [10][11][12], challenging conventional understanding of how fundamental concepts such as causality, analyticity and unitarity are realised in quantum field theory in curved spacetime [4][5][6][7].…”