[Show abstract][Hide abstract] ABSTRACT: We use a Green's function approach in order to develop a method for
calculating the tunneling rate between radiatively-generated non-degenerate
vacua. We apply this to a model that exhibits spontaneous symmetry breaking via
the Coleman-Weinberg mechanism, where we determine the self-consistent
tunneling configuration and illustrate the impact of gradient effects that
arise from accounting for the underlying space-time inhomogeneity.
[Show abstract][Hide abstract] ABSTRACT: We propose a novel method of evaluating the effective action, wherein the
physical one- and two-point functions are obtained in the limit of
non-vanishing external sources. We illustrate the self-consistency of this
method by recovering the usual 2PI effective action due to Cornwall, Jackiw and
Tomboulis, differing only by the fact that the saddle-point 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 non-perturbatively far away from
one-another. A pertinent example is the decay of false vacua in
radiatively-generated 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 two-particle-point-irreducible (2PPI)
effective action of Coppens and Verschelde. Finally, in the spirit of the
symmetry-improved 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 Hartree-Fock approximation to be re-organized, such that the
Goldstone boson remains massless algebraically in the symmetry-broken phase and
we obtain the correct second-order 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 thin-wall 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.
Physical Review D 01/2015; 91(10):105021. DOI:10.1103/PhysRevD.91.105021 · 4.64 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We consider a massive scalar field with quartic self-interaction
$\lambda/4!\,\phi^4$ 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
(Closed-Time-Path) 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 non-perturbative 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.
Physical Review D 12/2014; 91(6). DOI:10.1103/PhysRevD.91.063520 · 4.64 Impact Factor
[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 right-handed neutrinos generate the light neutrino
masses through the see-saw 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 right-handed 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.
Nuclear Physics B 11/2014; 896. DOI:10.1016/j.nuclphysb.2015.04.017 · 3.93 Impact Factor
[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 late-time limit
$\varepsilon=X\sin(2\varphi)/(X^2+\sin^2\varphi)$, where
$X=8\pi\Delta/(|Y_1|^2+|Y_2|^2)$, $\Delta=4(M_1-M_2)/(M_1+M_2)$,
$\varphi=\arg(Y_2/Y_1)$, 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 late-time
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 late-time decay asymmetry can be applied
throughout parameter space.
Journal of Cosmology and Astroparticle Physics 06/2014; 2014(09). DOI:10.1088/1475-7516/2014/09/033 · 5.81 Impact Factor
[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 freeze-out 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 non-leptonic 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.
Journal of Cosmology and Astroparticle Physics 04/2014; 2014(10). DOI:10.1088/1475-7516/2014/10/012 · 5.81 Impact Factor
[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 type-I 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 mass-scale.
Physical Review D 01/2014; 90(6). DOI:10.1103/PhysRevD.90.063522 · 4.64 Impact Factor
[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 Closed-Time-Path approach, we calculate the infrared corrections to
the two-point function of \phi to 2-loop 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.
Physical Review D 10/2013; 89(6). DOI:10.1103/PhysRevD.89.063506 · 4.64 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: By including all leading quantum-statistical 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 wave-function diagrams as well as all different possible
kinematic cuts that nominally lead to CP-violating 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.
Nuclear Physics B 07/2013; 882(1). DOI:10.1016/j.nuclphysb.2014.02.012 · 3.93 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Using the Closed-Time-Path approach, we perform a systematic leading
order calculation of the relaxation rate of flavour correlations of
left-handed 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 Higgs-boson 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
top-quark 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 right-handed neutrinos, a calculation that relies on the same
methods. We confirm most details of earlier results, but find a
substantially larger contribution from the t-channel exchange of
fermions.
Nuclear Physics B 03/2013; 877(1). DOI:10.1016/j.nuclphysb.2013.08.020 · 3.93 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The production and decay rate of massive sterile neutrinos at finite
temperature receives next-to-leading order corrections from the gauge
interactions of lepton and Higgs doublets. Using the Closed-Time-Path 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 tree-level rates from 1<->3 and 2<->2 processes to vertex and
wave-function 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 non-relativistic approximations, such that the full
quantum-statistical 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.
Journal of High Energy Physics 02/2013; 2013(4). DOI:10.1007/JHEP04(2013)099 · 6.11 Impact Factor
[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 right-handed neutrinos at finite temperature in the early
Universe. When calculating the flavour-mixing correlations, we account for the
effects of Yukawa as well as of gauge interactions. We compare the freeze-out
asymmetry from lepton-doublet mixing to the standard contributions from the
mixing and direct decays of right-handed neutrinos. The asymmetry from lepton
mixing is considerably large when the mass ratio between the right-handed
neutrinos is of order of a few, while it becomes Maxwell-suppressed for larger
hierarchies. For an intermediate range between the case of degenerate
right-handed neutrinos (resonant Leptogenesis) and the hierarchical case,
lepton mixing can yield the main contribution to the lepton asymmetry.
Nuclear Physics B 10/2012; 868(3). DOI:10.1016/j.nuclphysb.2012.11.021 · 3.93 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: For Leptogenesis based on the type-I seesaw mechanism, we present a
systematic calculation of lepton-number violating and purely flavoured
asymmetries within nonequilibrium Quantum Field Theory. We show that sterile
neutrinos with non-degenerate 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 non-degenerate sterile neutrinos derived for
Leptogenesis from out-of-equilibrium decays do not apply to flavoured
Leptogenesis with GeV-scale neutrinos.
Journal of High Energy Physics 06/2012; 2013(3). DOI:10.1007/JHEP03(2013)096 · 6.11 Impact Factor
[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 self-interactions 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
Closed-Time-Path methods. From the Kadanoff-Baym 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 constant-temperature 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.
Nuclear Physics B 12/2011; 861(1). DOI:10.1016/j.nuclphysb.2012.03.009 · 3.93 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We derive solutions to the Schwinger-Dyson equations on the Closed-Time-Path
for a scalar field in the limit where backreaction is neglected. In Wigner
space, the two-point Wightman functions have the curious property that the
equilibrium component has a finite width, while the out-of 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 finite-width. 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 zero-width
behaviour and the usual notion in kinetic theory, that the out-of-equilibrium
contributions have a finite width as well, is achieved. This result may also be
viewed as a generalisation of the fluctuation-dissipation relation to
out-of-equilibrium systems with negligible backreaction.
Annals of Physics 08/2011; 327(3). DOI:10.1016/j.aop.2011.10.005 · 2.10 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Self-energies 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 steady-state 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 self-consistent 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 two-point
correlator are naturally self-regulated.
[Show abstract][Hide abstract] ABSTRACT: We use present cosmological data from the cosmic microwave background, large-scale structure and deuterium at high redshifts to constrain supersymmetric F- and D-term 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 D-term model is ruled out at high significance. F-term models are also in tension with the data. We also discuss possible non-minimal 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 "lepton-mediated electroweak baryogenesis" scenario in which the baryon asymmetry is induced mainly through the presence of a left-handed 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 CP-violating phases, the baryon asymmetry has opposite sign compared to that calculated using established formulae. Comment: 26 pages, 5 figures