10 research outputs found
Medium corrections to the CP-violating parameter in leptogenesis
In two recent papers, arXiv:0909.1559 and arXiv:0911.4122, it has been
demonstrated that one can obtain quantum corrected Boltzmann kinetic equations
for leptogenesis using a top-down approach based on the
Schwinger-Keldysh/Kadanoff-Baym formalism. These "Boltzmann-like" equations are
similar to the ones obtained in the conventional bottom-up approach but differ
in important details. In particular there is a discrepancy between the
CP-violating parameter obtained in the first-principle derivation and in the
framework of thermal field theory. Here we demonstrate that the two approaches
can be reconciled if causal n-point functions are used in the thermal field
theory approach. The new result for the medium correction to the CP-violating
parameter is qualitatively different from the conventional one. The analogy to
a toy model considered earlier enables us to write down consistent quantum
corrected Boltzmann equations for thermal leptogenesis in the Standard Model
(supplemented by three right-handed neutrinos) which include quantum
statistical terms and medium corrected expressions for the CP-violating
parameter.Comment: 13 pages, 9 figure
Systematic approach to leptogenesis in nonequilibrium QFT: self-energy contribution to the CP-violating parameter
In the baryogenesis via leptogenesis scenario the self-energy contribution to
the CP-violating parameter plays a very important role. Here, we calculate it
in a simple toy model of leptogenesis using the Schwinger-Keldysh/Kadanoff-Baym
formalism as starting point. We show that the formalism is free of the
double-counting problem typical for the canonical Boltzmann approach. Within
the toy model, medium effects increase the CP-violating parameter. In contrast
to results obtained earlier in the framework of thermal field theory, the
medium corrections are linear in the particle number densities. In the resonant
regime quantum corrections lead to modified expressions for the CP-violating
parameter and for the decay width. Most notably, in the maximal resonant regime
the Boltzmann picture breaks down and an analysis in the full Kadanoff-Baym
formalism is required.Comment: 28 pages, 14 figure
Systematic approach to leptogenesis in nonequilibrium QFT: vertex contribution to the CP-violating parameter
The generation of a baryon asymmetry via leptogenesis is usually studied by
means of classical kinetic equations whose applicability to processes in the
hot and expanding early universe is questionable. The approximations implied by
the state-of-the-art description can be tested in a first-principle approach
based on nonequilibrium field theory techniques. Here, we apply the
Schwinger-Keldysh/Kadanoff-Baym formalism to a simple toy model of
leptogenesis. We find that, within the toy model, medium effects increase the
vertex contribution to the CP-violating parameter. At high temperatures it is a
few times larger than in vacuum and asymptotically reaches the vacuum value as
the temperature decreases. Contrary to the results obtained earlier in the
framework of thermal field theory, the corrections are only linear in the
particle number densities. An important feature of the Kadanoff-Baym formalism
is that it is free of the double-counting problem, i.e. no need for real
intermediate state subtraction arises. In particular, this means that the
structure of the equations automatically ensures that the asymmetry vanishes in
equilibrium. These results give a first glimpse into a number of new and
interesting effects that can be studied in the framework of nonequilibrium
field theory.Comment: 27 pages, 21 figure
Leptogenesis from first principles in the resonant regime
The lepton asymmetry generated by the out-of-equilibrium decays of heavy
Majorana neutrinos with a quasi-degenerate mass spectrum is resonantly
enhanced. In this work, we study this scenario within a first-principle
approach. The quantum field theoretical treatment is applicable for mass
splittings of the order of the width of the Majorana neutrinos, for which the
enhancement is maximally large. The non-equilibrium evolution of the mixing
Majorana neutrino fields is described by a formal analytical solution of the
Kadanoff-Baym equations, that is obtained by neglecting the back-reaction.
Based on this solution, we derive approximate analytical expressions for the
generated asymmetry and compare them to the Boltzmann result. We find that the
resonant enhancement obtained from the Kadanoff-Baym approach is smaller
compared to the Boltzmann approach, due to additional contributions that
describe coherent transitions between the Majorana neutrino species. We also
discuss corrections to the masses and widths of the degenerate pair of Majorana
neutrinos that are relevant for very small mass splitting, and compare the
approximate analytical result for the lepton asymmetry with numerical results.Comment: 47 pages, 5 figure
Systematic approach to thermal leptogenesis
In this work we study thermal leptogenesis using nonequilibrium quantum field theory. Starting from fundamental equations for correlators of the quantum fields we describe the steps necessary to obtain quantum-kinetic equations for quasiparticles. These can easily be compared to conventional results and overcome conceptional problems inherent in the canonical approach. Beyond CP-violating decays we include also those scattering processes which are tightly related to the decays in a consistent approximation of fourth order in the Yukawa couplings. It is demonstrated explicitly how the S-matrix elements for the scattering processes in the conventional approach are related to two- and three-loop contributions to the effective action. We derive effective decay and scattering amplitudes taking medium corrections and thermal masses into account. In this context we also investigate CP-violating Higgs decay within the same formalism. From the kinetic equations we derive rate equations for the lepton asymmetry improved in that they include quantum-statistical effects and medium corrections to the quasiparticle properties