Abstract
The gravitational Chern–Simons term coupled to an evolving axion is known to generate lepton number through the gravitational anomaly. We examine this leptogenesis scenario in the presence of the Gauss–Bonnet term over and above the gravitational Chern–Simons term. We find that the lepton production can be exponentially enhanced. The Gauss–Bonnet term creates CP-violating instability of gravitational waves that may appear transiently after inflation, and during the period of instability elliptically polarized gravitational waves are exponentially amplified at sub-horizon scales. This instability does not affect the spectrum of the cosmic microwave background as it occurs at much shorter length scales. In a typical scenario based on natural inflation, the observed baryon asymmetry of the Universe corresponds to the UV cutoff scale at 1014–16 GeV.
Highlights
The gravitational Chern-Simons term coupled to an evolving axion is known to generate lepton number through the gravitational anomaly
The Universe is observed to be baryon-asymmetric and the origin of this asymmetry remains as an unsettled problem of particle physics
Popular scenarios of baryogenesis, such as the GUT baryogenesis [3] and the Fukugita-Yanagida scenario of leptogenesis [4], typically assume new physics at energy scales of 1014−16 GeV. These exquisite scenarios are difficult to test experimentally since the new physics is at such high energy
Summary
We take a conservative view that the higher curvature terms are corrections to the Einstein gravity, and discuss a scenario in which the effects of such terms are minor during the slow roll This view is natural in our model, as the GB and gCS terms are topological terms that become trivial if the axion field is held constant. Assuming the BunchDavies vacuum we choose the positive frequency mode vk± These solutions are valid near the end of inflation when the slow roll conditions are marginally satisfied. The Planck constraints on the tensor/scalar ratio r 0.1 set an upper bound on H at the time of the horizon exit of the CMB scale, giving
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