Bjorn Garbrecht

Technische Universität München, München, Bavaria, Germany

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Publications (32)135.99 Total impact

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    Bjorn Garbrecht, Peter Millington
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    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 · 4.86 Impact Factor
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    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 · 4.86 Impact Factor
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    Bjorn Garbrecht, Ignacio Izaguirre
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    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 · 3.95 Impact Factor
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    Bjorn Garbrecht, Florian Gautier, Juraj Klaric
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    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 · 5.88 Impact Factor
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    Bjorn Garbrecht, Pedro Schwaller
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    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 · 5.88 Impact Factor
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    Bjorn Garbrecht
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    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 · 4.86 Impact Factor
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    Bjorn Garbrecht, Gerasimos Rigopoulos, Yi Zhu
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    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 · 4.86 Impact Factor
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    Bjorn Garbrecht, Michael J. Ramsey-Musolf
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    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. DOI:10.1016/j.nuclphysb.2014.02.012 · 3.95 Impact Factor
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    Bjorn Garbrecht, Frank Glowna, Pedro Schwaller
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    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 · 3.95 Impact Factor
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    Bjorn Garbrecht, Frank Glowna, Matti Herranen
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    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 · 6.22 Impact Factor
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    Bjorn Garbrecht
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    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 · 3.95 Impact Factor
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    Marco Drewes, Bjorn Garbrecht
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    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 · 6.22 Impact Factor
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    Bjorn Garbrecht
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    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.
    Physical review D: Particles and fields 01/2012; 85(12). DOI:10.1103/PhysRevD.85.123509 · 4.86 Impact Factor
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    Bjorn Garbrecht, Matti Herranen
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    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 · 3.95 Impact Factor
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    Bjorn Garbrecht, Mathias Garny
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    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; DOI:10.1016/j.aop.2011.10.005 · 3.07 Impact Factor
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    Bjorn Garbrecht, Gerasimos Rigopoulos
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    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.
    Physical review D: Particles and fields 05/2011; 84(6). DOI:10.1103/PhysRevD.84.063516 · 4.86 Impact Factor
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    Richard Battye, Bjorn Garbrecht, Adam Moss
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    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
    Physical review D: Particles and fields 01/2010; 81(12). DOI:10.1103/PHYSREVD.81.123512 · 4.86 Impact Factor
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    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
    Physical review D: Particles and fields 05/2009; 81(6). DOI:10.1103/PhysRevD.81.063506 · 4.86 Impact Factor
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    ABSTRACT: This paper has been withdrawn by the authors. Comment: This paper has been withdrawn by the authors
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    Bjorn Garbrecht, Thomas Konstandin
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    ABSTRACT: We study thermalization by applying gradient expansion to the Kadanoff-Baym equations of the 2PI effective action to two-loop in a theory with Dirac fermions coupled to scalars. In addition to those chemical potentials which equilibrate in the on-shell limit, we identify modes which are conserved in this approximation, but which relax when off-shell effects are taken into account. This implies that chemical equilibration does not require higher loop contributions to the effective action and is compatible with the gradient expansion. We explicitly calculate the damping time-scales of both, on- and off-shell, chemical equilibration rates. It is shown that off-shell equilibration is suppressed by the thermal width of the particles in the plasma, which explains the separation of on- and off-shell chemical equilibration time-scales. Comment: 20 pages, 3 figures, published version
    Physical review D: Particles and fields 10/2008; 79(8). DOI:10.1103/PhysRevD.79.085003 · 4.86 Impact Factor

Publication Stats

445 Citations
135.99 Total Impact Points

Institutions

  • 2012–2014
    • Technische Universität München
      München, Bavaria, Germany
  • 2011–2012
    • RWTH Aachen University
      • Institut für Theoretische Teilchenphysik und Kosmologie (Theorie E)
      Aachen, North Rhine-Westphalia, Germany
  • 2007–2009
    • University of Wisconsin, Madison
      • Department of Physics
      Madison, MS, United States
  • 2008
    • Korea Institute for Advanced Study
      • School of Physics
      Sŏul, Seoul, South Korea
  • 2005–2007
    • The University of Manchester
      • School of Physics and Astronomy
      Manchester, ENG, United Kingdom
  • 2004
    • Universität Heidelberg
      • Institute of Theoretical Physics
      Heidelburg, Baden-Württemberg, Germany