Publications (34)137.59 Total impact
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ABSTRACT: We use a Green's function approach in order to develop a method for calculating the tunneling rate between radiativelygenerated nondegenerate vacua. We apply this to a model that exhibits spontaneous symmetry breaking via the ColemanWeinberg mechanism, where we determine the selfconsistent tunneling configuration and illustrate the impact of gradient effects that arise from accounting for the underlying spacetime inhomogeneity.  [Show abstract] [Hide abstract]
ABSTRACT: We propose a novel method of evaluating the effective action, wherein the physical one and twopoint functions are obtained in the limit of nonvanishing external sources. We illustrate the selfconsistency of this method by recovering the usual 2PI effective action due to Cornwall, Jackiw and Tomboulis, differing only by the fact that the saddlepoint evaluation of the path integral is performed along the extremal quantum, rather than classical, path. As such, this approach is of particular relevance to situations where the dominant quantum and classical paths are nonperturbatively far away from oneanother. A pertinent example is the decay of false vacua in radiativelygenerated potentials, as may occur for the electroweak vacuum of the Standard Model. In addition, we describe how the external sources may instead be chosen so as to yield the twoparticlepointirreducible (2PPI) effective action of Coppens and Verschelde. Finally, in the spirit of the symmetryimproved effective action of Pilaftsis and Teresi, we give an example of how the external sources can be used to preserve global symmetries in truncations of the 2PI effective action. Specifically, in the context of an O(2) model with spontaneous symmetry breaking, we show that this approach allows the HartreeFock approximation to be reorganized, such that the Goldstone boson remains massless algebraically in the symmetrybroken phase and we obtain the correct secondorder thermal phase transition.  [Show abstract] [Hide abstract]
ABSTRACT: We introduce a Green's function method for handling radiative effects on false vacuum decay. In addition to the usual thinwall approximation, we achieve further simplification by treating the bubble wall in the planar limit. As an application, we take the $\lambda\phi^4$ theory, extended with $N$ additional heavier scalars, wherein we calculate analytically both the functional determinant of the quadratic fluctuations about the classical soliton configuration as well as the first correction to the soliton configuration itself.  [Show abstract] [Hide abstract]
ABSTRACT: We consider a massive scalar field with quartic selfinteraction $\lambda/4!\,\phi^4$ in de~Sitter spacetime and present a diagrammatic expansion that describes the field as driven by stochastic noise. This is compared with the Feynman diagrams in the Keldysh basis of the Amphichronous (ClosedTimePath) Field Theoretical formalism. For all orders in the expansion, we find that the diagrams agree when evaluated in the leading infrared approximation, i.e. to leading order in $m^2/H^2$, where $m$ is the mass of the scalar field and $H$ is the Hubble rate. As a consequence, the correlation functions computed in both approaches also agree to leading infrared order. This perturbative correspondence shows that the stochastic Theory is exactly equivalent to the Field Theory in the infrared. The former can then offer a nonperturbative resummation of the Field Theoretical Feynman diagram expansion, including fields with $0\leq m^2\ll\sqrt \lambda H^2$ for which the perturbation expansion fails at late times.  [Show abstract] [Hide abstract]
ABSTRACT: Mixing lepton doublets of the Standard Model can lead to lepton flavour asymmetries in the Early Universe. We present a diagrammatic representation of this recently identified source of $CP$ violation and elaborate in detail on the correlations between the lepton flavours at different temperatures. For a model where two sterile righthanded neutrinos generate the light neutrino masses through the seesaw mechanism, the lower bound on reheat temperatures in accordance with the observed baryon asymmetry turns out to be $\gsim 1.2\times 10^9\,{\rm GeV}$. With three righthanded neutrinos, substantially smaller values are viable. This requires however a tuning of the Yukawa couplings, such that there are cancellations between the individual contributions to the masses of the light neutrinos.  [Show abstract] [Hide abstract]
ABSTRACT: We show that the effective decay asymmetry for resonant Leptogenesis in the strong washout regime with two sterile neutrinos and a single active flavour can in wide regions of parameter space be approximated by its latetime limit $\varepsilon=X\sin(2\varphi)/(X^2+\sin^2\varphi)$, where $X=8\pi\Delta/(Y_1^2+Y_2^2)$, $\Delta=4(M_1M_2)/(M_1+M_2)$, $\varphi=\arg(Y_2/Y_1)$, and $M_{1,2}$, $Y_{1,2}$ are the masses and Yukawa couplings of the sterile neutrinos. This approximation in particular extends to parametric regions where $Y_{1,2}^2\gg \Delta$, {\it i.e.} where the width dominates the mass splitting. We generalise the formula for the effective decay asymmetry to the case of several flavours of active leptons and demonstrate how this quantity can be used to calculate the lepton asymmetry for phenomenological scenarios that are in agreement with the observed neutrino oscillations. We establish analytic criteria for the validity of the latetime approximation for the decay asymmetry and compare these with numerical results that are obtained by solving for the mixing and the oscillations of the sterile neutrinos. For phenomenologically viable models with two sterile neutrinos, we find that the flavoured effective latetime decay asymmetry can be applied throughout parameter space.  [Show abstract] [Hide abstract]
ABSTRACT: By including spectator fields into the Boltzmann equations for Leptogenesis, we show that partially equilibrated spectator interactions can have a significant impact on the freezeout value of the asymmetry in the strong washout regime. The final asymmetry is typically increased, since partially equilibrated spectators "hide" a part of the asymmetry from washout. We study examples with leptonic and nonleptonic spectator processes, assuming thermal initial conditions, and find up to 50% enhanced asymmetries compared to the limit of fully equilibrated spectators. Together with a comprehensive overview of the equilibration temperatures for various Standard Model processes, the numerical results indicate the ranges when the limiting cases of either fully equilibrated or negligible spectator fields are applicable and when they are not. Our findings also indicate an increased sensitivity to initial conditions and finite density corrections even in the strong washout regime.  [Show abstract] [Hide abstract]
ABSTRACT: Lepton flavour asymmetries generated at the onset of the oscillations of sterile neutrinos with masses above the electroweak scale can be large enough to partly survive washout and to explain the baryon asymmetry of the Universe. This opens up new regions of parameter space, where Leptogenesis is viable within the typeI seesaw framework. In particular, we find it possible that the sterile neutrinos masses are substantially below 10^9 GeV, while not being degenerate. However, the required reheat temperature that is determined by the begin of the oscillations lies some orders of magnitude above the sterile neutrino massscale.  [Show abstract] [Hide abstract]
ABSTRACT: We consider massive \lambda\phi^4 theory in de Sitter background. The mass of the scalar field \phi is chosen small enough, such that the amplification of superhorizon momentum modes leads to a significant enhancement of infrared correlations, but large enough such that perturbation theory remains valid. Using the ClosedTimePath approach, we calculate the infrared corrections to the twopoint function of \phi to 2loop order. To this approximation, we find agreement with the correlation found using stochastic methods. When breaking the results down to individual Feynman diagrams obtained by the two different methods, we observe that these agree as well.  [Show abstract] [Hide abstract]
ABSTRACT: By including all leading quantumstatistical effects at finite temperature, we show that no net asymmetry of leptons and sleptons is generated from soft leptogenesis, save the possible contribution from the resonant mixing of sneutrinos. This result contrasts with different conclusions appearing in the literature that are based on an incomplete inclusion of quantum statistics. We discuss vertex and wavefunction diagrams as well as all different possible kinematic cuts that nominally lead to CPviolating asymmetries. The present example of soft leptogenesis may therefore serve as a paradigm in order to identify more generally applicable caveats relevant to alternative scenarios for baryogenesis and leptogenesis, and it may provide useful guidance in constructing viable models.  [Show abstract] [Hide abstract]
ABSTRACT: Using the ClosedTimePath approach, we perform a systematic leading order calculation of the relaxation rate of flavour correlations of lefthanded Standard Model leptons. This quantity is of pivotal relevance for flavoured Leptogenesis in the Early Universe, and we find it to be 5.19*10^3 T at T=10^7 GeV and 4.83*10^3 T at T=10^13 GeV. These values apply to the Standard Model with a Higgsboson mass of 125 GeV. The dependence of the numerical coefficient on the temperature T is due to the renormalisation group running. The leading linear and logarithmic dependencies of the flavour relaxation rate on the gauge and topquark couplings are extracted, such that the results presented in this work can readily be applied to extensions of the Standard Model. We also derive the production rate of light (compared to the temperature) sterile righthanded neutrinos, a calculation that relies on the same methods. We confirm most details of earlier results, but find a substantially larger contribution from the tchannel exchange of fermions.  [Show abstract] [Hide abstract]
ABSTRACT: The production and decay rate of massive sterile neutrinos at finite temperature receives nexttoleading order corrections from the gauge interactions of lepton and Higgs doublets. Using the ClosedTimePath approach, we demonstrate that the perturbatively obtained inclusive rate is finite. For this purpose, we show that soft, collinear and Bose divergences cancel when adding the treelevel rates from 1<>3 and 2<>2 processes to vertex and wavefunction corrections to 1<>2 processes. These results hold for a general momentum of the sterile neutrino with respect to the plasma frame. Moreover, they do not rely on nonrelativistic approximations, such that the full quantumstatistical effects are accounted for to the given order in perturbation theory. While the neutrino production rate is of relevance for Leptogenesis, the proposed methods may as well be suitable for application to a more general class of relativistic transport phenomena.  [Show abstract] [Hide abstract]
ABSTRACT: It is shown that the mixing of lepton doublets of the Standard Model can yield sizeable contributions to the lepton asymmetry, that is generated through the decays of righthanded neutrinos at finite temperature in the early Universe. When calculating the flavourmixing correlations, we account for the effects of Yukawa as well as of gauge interactions. We compare the freezeout asymmetry from leptondoublet mixing to the standard contributions from the mixing and direct decays of righthanded neutrinos. The asymmetry from lepton mixing is considerably large when the mass ratio between the righthanded neutrinos is of order of a few, while it becomes Maxwellsuppressed for larger hierarchies. For an intermediate range between the case of degenerate righthanded neutrinos (resonant Leptogenesis) and the hierarchical case, lepton mixing can yield the main contribution to the lepton asymmetry.  [Show abstract] [Hide abstract]
ABSTRACT: For Leptogenesis based on the typeI seesaw mechanism, we present a systematic calculation of leptonnumber violating and purely flavoured asymmetries within nonequilibrium Quantum Field Theory. We show that sterile neutrinos with nondegenerate masses in the GeV range can explain the baryon asymmetry of the Universe via flavoured Leptogenesis. This is possible due to the interplay of thermal and flavour effects. Our approach clarifies the relation between Leptogenesis from the oscillations of sterile neutrinos and the more commonly studied scenarios from decays and inverse decays. We explain why lower mass bounds for nondegenerate sterile neutrinos derived for Leptogenesis from outofequilibrium decays do not apply to flavoured Leptogenesis with GeVscale neutrinos.  [Show abstract] [Hide abstract]
ABSTRACT: Leptogenesis may be induced by the mixing of extra Higgs doublets with experimentally accessible masses. This mechanism relies on diagrammatic cuts that are kinematically forbidden in the vacuum but contribute at finite temperature. A resonant enhancement of the asymmetry occurs generically provided the dimensionless Yukawa and selfinteractions are suppressed compared to those of the Standard Model Higgs field. This is in contrast to typical scenarios of Resonant Leptogenesis, where the asymmetry is enhanced by imposing a degeneracy of singlet neutrino masses.  [Show abstract] [Hide abstract]
ABSTRACT: We describe mixing scalar particles and Majorana fermions using ClosedTimePath methods. From the KadanoffBaym equations, we obtain the charge asymmetry, that is generated from decays and inverse decays of the mixing particles. Within one single formalism, we thereby treat Leptogenesis from oscillations and recover as well the standard results for the asymmetry in Resonant Leptogenesis, which apply when the oscillation frequency is much larger than the decay rate. Analytic solutions for two mixing neutral particles in a constanttemperature background illustrate our results qualitatively. We also perform the modification of the kinetic equations that is necessary in order to take account of the expansion of the Universe and the washout of the asymmetry.  [Show abstract] [Hide abstract]
ABSTRACT: We derive solutions to the SchwingerDyson equations on the ClosedTimePath for a scalar field in the limit where backreaction is neglected. In Wigner space, the twopoint Wightman functions have the curious property that the equilibrium component has a finite width, while the outof equilibrium component has zero width. This feature is confirmed in a numerical simulation for scalar field theory with quartic interactions. When substituting these solutions into the collision term, we observe that an expansion including terms of all orders in gradients leads to an effective finitewidth. Besides, we observe no breakdown of perturbation theory, that is sometimes associated with pinch singularities. The effective width is identical with the width of the equilibrium component. Therefore, reconciliation between the zerowidth behaviour and the usual notion in kinetic theory, that the outofequilibrium contributions have a finite width as well, is achieved. This result may also be viewed as a generalisation of the fluctuationdissipation relation to outofequilibrium systems with negligible backreaction.  [Show abstract] [Hide abstract]
ABSTRACT: Selfenergies of a minimally coupled scalar field with quartic and trilinear interactions are calculated in a de Sitter background, using a position space propagator. For quartic interactions, we recover earlier results for the seagull diagram, namely that it contributes an effective mass for the scalar field at leading order in the infrared enhancement in a steadystate de Sitter background. We further show that the sunset diagram also contributes to this effective mass and argue that these two contributions are sufficient in order to determine a selfconsistent dynamical mass. In addition, trilinear interactions also induce a dynamical mass for the scalar field which we calculate. Since an interacting scalar field in de Sitter acquires a dynamical mass through these loop corrections, the infrared divergences of the twopoint correlator are naturally selfregulated.  [Show abstract] [Hide abstract]
ABSTRACT: We use present cosmological data from the cosmic microwave background, largescale structure and deuterium at high redshifts to constrain supersymmetric F and Dterm hybrid inflation scenarios including possible contributions to the CMB anisotropies from cosmic strings. Using two different realizations of the cosmic string spectrum, we find that the minimal version of the Dterm model is ruled out at high significance. Fterm models are also in tension with the data. We also discuss possible nonminimal variants of the models. Comment: 4 pages, 2 figures  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the impact of the tau and bottom Yukawa couplings on the transport dynamics for electroweak baryogenesis in supersymmetric extensions of the Standard Model. Although it has generally been assumed in the literature that all Yukawa interactions except those involving the top quark are negligible, we find that the tau and bottom Yukawa interaction rates are too fast to be neglected. We identify an illustrative "leptonmediated electroweak baryogenesis" scenario in which the baryon asymmetry is induced mainly through the presence of a lefthanded leptonic charge. We derive analytic formulae for the computation of the baryon asymmetry that, in light of these effects, are qualitatively different from those in the established literature. In this scenario, for fixed CPviolating phases, the baryon asymmetry has opposite sign compared to that calculated using established formulae. Comment: 26 pages, 5 figures
Publication Stats
560  Citations  
137.59  Total Impact Points  
Top Journals
Institutions

20132015

Technische Universität München
München, Bavaria, Germany


20112012

RWTH Aachen University
 Institut für Theoretische Teilchenphysik und Kosmologie (Theorie E)
Aachen, North RhineWestphalia, Germany


20072009

University of Wisconsin, Madison
 Department of Physics
Madison, MS, United States


2008

Korea Institute for Advanced Study
 School of Physics
Sŏul, Seoul, South Korea


20052007

The University of Manchester
 School of Physics and Astronomy
Manchester, ENG, United Kingdom


2004

Universität Heidelberg
 Institute of Theoretical Physics
Heidelburg, BadenWürttemberg, Germany
