We discuss leptogenesis in a model with heavy right-handed Majorana neutrinospropagating in a constant but otherwise generic CPT-violating axial time-likebackground (which could be motivated by string theory considerations). Attemperatures much higher than the temperature of the electroweak phasetransition we solve analytically but approximately (using Pade approximants)the corresponding Boltzmann equations, which describe lepton asymmetrygeneration due to the tree-level decays of the heavy neutrinos into standardmodel leptons. These leptons are effectively massless at such temperatures. Thecurrent work completes in a rigorous way a preliminary treatment of the samesystem, by some of the present authors. In this earlier work, lepton asymmetrywas crudely estimated considering the decay of a right-handed neutrino at rest.Our present analysis includes thermal momentum modes for the heavy neutrino andthis leads to a total lepton asymmetry which is bigger by a factor of two ascompared to the previous estimate. Nevertheless, our current and preliminaryresults for the freezeout are found to be in agreement (within a 12.5%uncertainty). Our analysis depends on a novel use of Pade approximants to solvethe Boltzmann equations and may be more widely useful in cosmology.
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