Publications (143)541.42 Total impact
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ABSTRACT: Weinberg's asymptotic safety scenario provides an elegant mechanism to construct a quantum theory of gravity within the framework of quantum field theory based on a nonGau{\ss}ian fixed point of the renormalization group flow. In this work we report novel evidence for the validity of this scenario, using functional renormalization group techniques to determine the renormalization group flow of the EinsteinHilbert action supplemented by the twoloop counterterm found by Goroff and Sagnotti. The resulting system of beta functions comprises three scaledependent coupling constants and exhibits a nonGau{\ss}ian fixed point which constitutes the natural extension of the one found at the level of the EinsteinHilbert action. The fixed point exhibits two ultraviolet attractive and one repulsive direction supporting a lowdimensional UVcritical hypersurface. Our result vanquishes the longstanding criticism that asymptotic safety will not survive once a "proper perturbative counterterm" is included in the projection space.  [Show abstract] [Hide abstract]
ABSTRACT: We develop the worldline formalism for computations of composite operators such as the fluctuation induced energymomentum tensor. As an example, we use a fluctuating real scalar field subject to Dirichlet boundary conditions. The resulting worldline representation can be evaluated by worldline MonteCarlo methods in continuous spacetime. We benchmark this worldline numerical algorithm with the aid of analytically accessible singleplate and parallelplate Casimir configurations, providing a detailed analysis of statistical and systematic errors. The method generalizes straightforwardly to arbitrary Casimir geometries and general background potentials.  [Show abstract] [Hide abstract]
ABSTRACT: We critically examine the gauge, and fieldparametrization dependence of renormalization group flows in the vicinity of nonGau\ss{}ian fixed points in quantum gravity. While physical observables are independent of such calculational specifications, the construction of quantum gravity field theories typically relies on offshell quantities such as $\beta$ functions and generating functionals and thus face potential stability issues with regard to such generalized parametrizations. We analyze a twoparameter class of covariant gauge conditions, the role of momentumdependent field rescalings and a class of field parametrizations. Using the product of Newton and cosmological constant as an indicator, the principle of minimum sensitivity identifies stationary points in this parametrization space which show a remarkable insensitivity to the parametrization. In the most insensitive cases, the quantized gravity system exhibits a nonGau\ss{}ian UV stable fixed point, lending further support to asymptotically free quantum gravity. One of the stationary points facilitates an analytical determination of the quantum gravity phase diagram and features ultraviolet and infrared complete RG trajectories with a classical regime. 
Article: Critical Schwinger pair production
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ABSTRACT: We investigate Schwinger pair production in spatially inhomogeneous electric backgrounds. A critical point for the onset of pair production can be approached by fields that marginally provide sufficient electrostatic energy for an offshell longrange electronpositron fluctuation to become a real pair. Close to this critical point, we observe features of universality which are analogous to continuous phase transitions in critical phenomena with the pairproduction rate serving as an order parameter: electric backgrounds can be subdivided into universality classes and the onset of pair production exhibits characteristic scaling laws. An appropriate design of the electric background field can interpolate between powerlaw scaling, essential BKTtype scaling and a powerlaw scaling with log corrections. The corresponding critical exponents only depend on the largescale features of the electric background, whereas the microscopic details of the background play the role of irrelevant perturbations not affecting criticality.  [Show abstract] [Hide abstract]
ABSTRACT: Birefringence is one of the fascinating properties of the vacuum of quantum electrodynamics (QED) in strong electromagnetic fields. The scattering of linearly polarized incident probe photons into a perpendicularly polarized mode provides a distinct signature of the optical activity of the quantum vacuum and thus offers an excellent opportunity for a precision test of nonlinear QED. Precision tests require accurate predictions and thus a theoretical framework that is capable of taking the detailed experimental geometry into account. We derive analytical solutions for vacuum birefringence which include the spatiotemporal field structure of a strong optical pump laser field and an xray probe. We show that the angular distribution of the scattered photons depends strongly on the interaction geometry and find that scattering of the perpendicularly polarized scattered photons out of the cone of the incident probe xray beam is the key to making the phenomenon experimentally accessible with the current generation of FEL/highfield laser facilities.  [Show abstract] [Hide abstract]
ABSTRACT: We investigate a class of relativistic fermion theories in 2<d<4 spacetime dimensions with continuous chiral U(Nf)xU(Nf) symmetry. This includes a number of wellstudied models, e.g., of GrossNeveu and Thirring type, in a unified framework. Within the limit of pointlike interactions, the RG flow of couplings reveals a network of interacting fixed points, each of which defines a universality class. A subset of fixed points are "critical fixed points" with one RG relevant direction being candidates for critical points of secondorder phase transitions. Identifying invariant hyperplanes of the RG flow and classifying their attractive/repulsive properties, we find evidence for emergent higher chiral symmetries as a function of Nf. For the case of the Thirring model, we discover a new critical flavor number that separates the RG stable largeNf regime from an intermediateNf regime in which symmetrybreaking perturbations become RG relevant. This new critical flavor number has to be distinguished from the chiralcritical flavor number, below which the Thirring model is expected to allow spontaneous chiral symmetry breaking, and its existence offers a resolution to the discrepancy between previous results obtained in the continuum and the lattice Thirring models. Moreover, we find indications for a new feature of universality: details of the critical behavior can depend on additional "spectator symmetries" that remain intact across the phase transition. Implications for the physics of interacting fermions on the honeycomb lattice, for which our theory space provides a simple model, are given.  [Show abstract] [Hide abstract]
ABSTRACT: We construct asymptotically free renormalization group trajectories for the generic nonabelian Higgs model in fourdimensional spacetime. These ultravioletcomplete trajectories become visible by generalizing the renormalization/boundary conditions in the definition of the correlation functions of the theory. We identify a candidate threeparameter family of renormalization group trajectories interconnecting the asymptotically free ultraviolet regime with a Higgs phase in the lowenergy limit. We provide estimates of their lowenergy properties in the light of a possible application to the standard model Higgs sector. Finally, we find a twoparameter subclass of asymptotically free ColemanWeinbergtype trajectories that do not suffer from a naturalness problem.  [Show abstract] [Hide abstract]
ABSTRACT: The spinbase invariant formalism of Dirac fermions in curved space maintains the essential symmetries of general covariance as well as similarity transformations of the Clifford algebra. We emphasize the advantages of the spinbase invariant formalism both from a conceptual as well as from a practical viewpoint. This suggests that local spinbase invariance should be added to the list of (effective) properties of (quantum) gravity theories. We find support for this viewpoint by the explicit construction of a global realization of the Clifford algebra on a 2sphere which is impossible in the spinbase noninvariant vielbein formalism.  [Show abstract] [Hide abstract]
ABSTRACT: In view of the measured Higgs mass of 125 GeV, the perturbative renormalization group evolution of the Standard Model suggests that our Higgs vacuum might not be stable. We connect the usual perturbative approach and the functional renormalization group which allows for a straightforward inclusion of higherdimensional operators in the presence of an ultraviolet cutoff. In the latter framework we study vacuum stability in the presence of higherdimensional operators. We find that their presence can have a sizable influence on the maximum ultraviolet scale of the Standard Model and the existence of instabilities. Finally, we discuss how such operators can be generated in specific models and study the relation between the instability scale of the potential and the scale of new physics required to avoid instabilities.  [Show abstract] [Hide abstract]
ABSTRACT: We reconsider the recently proposed nonlinear QED effect of quantum reflection of photons off an inhomogeneous strongfield region. We present new results for strong fields varying both in space and time. While such configurations can give rise to new effects such as frequency mixing, estimated reflection rates based on previous onedimensional studies are corroborated. On a conceptual level, we critically reexamine the validity regime of the conventional locallyconstantfield approximation and identify kinematic configurations which can be treated reliably. Our results further underline the discovery potential of quantum reflection as a new signature of the nonlinearity of the quantum vacuum.  [Show abstract] [Hide abstract]
ABSTRACT: We study a chiral Yukawa model mimicking the Higgstopbottom sector of the standard model. We reanalyze the conventional arguments that relate a lower bound for the Higgs mass with vacuum stability in the light of exact results for the regularized fermion determinant as well as in the framework of the functional renormalization group. In both cases, we find no indication for vacuum instability nor metastability induced by topfluctuations if the cutoff is kept finite but arbitrary. A lower bound for the Higgs mass arises for the class of standard bare potentials of \phi^4 type from the requirement of a welldefined functional integral (i.e., stability of the bare potential). This consistency bound can however be relaxed considerably by more general forms of the bare potential without necessarily introducing new metastable minima.  [Show abstract] [Hide abstract]
ABSTRACT: We study the effect of laser photon merging, or equivalently high harmonic generation, in the quantum vacuum subject to inhomogeneous electromagnetic fields. Such a process is facilitated by the effective nonlinear couplings arising from charged particleantiparticle fluctuations in the quantum vacuum subject to strong electromagnetic fields. We derive explicit results for general kinematic and polarization configurations involving optical photons. Concentrating on merged photons in reflected channels which are preferable in experiments for reasons of noise suppression, we demonstrate that photon merging is typically dominated by the competing nonlinear process of quantum reflection, though appropriate polarization and signal filtering could specifically search for the merging process. As a byproduct, we devise a novel systematic expansion of the photon polarization tensor in plane wave fields.  [Show abstract] [Hide abstract]
ABSTRACT: We analyze the manyflavor phase diagram of quantum electrodynamics (QED) in 2+1 (Euclidean) spacetime dimensions. We compute the critical flavor number above which the theory is in the quasiconformal massless phase. For this, we study the renormalization group fixedpoint structure in the space of gauge interactions and pointlike fermionic selfinteractions, the latter of which are induced dynamically by fermionphoton interactions. We find that a reliable estimate of the critical flavor number crucially relies on a careful treatment of the Fierz ambiguity in the fermionic sector. Using a Fierzcomplete basis, our results indicate that the phase transition towards a chirallybroken phase occurring at small flavor numbers could be separated from the quasiconformal phase at larger flavor numbers, allowing for an intermediate phase which is dominated by fluctuations in a vector channel. If these interactions approach criticality, the intermediate phase could be characterized by a Lorentzbreaking vector condensate.  [Show abstract] [Hide abstract]
ABSTRACT: Electronpositron pair production in oscillating electric fields is investigated in the nonperturbative threshold regime. Accurate numerical solutions of quantum kinetic theory for corresponding observables are presented and analyzed in terms of a proposed model for an effective mass of electrons and positrons acquired within the given strong electric field. Although this effective mass cannot provide an exact description of the collective interaction of a charged particle with the strong field, physical observables are identified which carry direct and sensitive signatures of the effective mass.  [Show abstract] [Hide abstract]
ABSTRACT: We explore Schwinger pair production in rotating timedependent electric fields using the realtime DHW formalism. We determine the time evolution of the Wigner function as well as asymptotic particle distributions neglecting backreactions on the electric field. Whereas qualitative features can be understood in terms of effective Keldysh parameters, the field rotation leaves characteristic imprints in the momentum distribution that can be interpreted in terms of interference and multiphoton effects. These phenomena may seed characteristic features of QED cascades created in the antinodes of a highintensity standing wave laser field.  [Show abstract] [Hide abstract]
ABSTRACT: We study a formulation of Dirac fermions in curved spacetime that respects general coordinate invariance as well as invariance under local spinbase transformations. The natural variables for this formulation are spacetimedependent Dirac matrices subject to the Cliffordalgebra constraint. In particular, a coframe, i.e. vierbein field is not required. The corresponding affine spin connection consists of a canonical part that is completely fixed in terms of the Dirac matrices and a free part that can be interpreted as spin torsion. A general variation of the Dirac matrices naturally induces a spinorial Lie derivative which coincides with the known KosmannLie derivative in the absence of torsion. Using this formulation for building a field theory of quantized gravity and matter fields, we show that it suffices to quantize the metric and the matter fields. This observation is of particular relevance for field theory approaches to quantum gravity, as it can serve for a purely metricbased quantization scheme for gravity even in the presence of fermions.  [Show abstract] [Hide abstract]
ABSTRACT: We study the functional renormalization group flow of a HiggsYukawa toy model mimicking the topHiggs sector of the standard model. This approach allows for treating arbitrary bare couplings. For the class of standard bare potentials of \phi^4type at a given ultraviolet cutoff, we show that a finite infrared Higgs mass range emerges naturally from the renormalization group flow itself. Higgs masses outside the resulting bounds cannot be connected to any conceivable set of bare parameters in this standardmodel \phi^4 class. By contrast, more general bare potentials allow to diminish the lower bound considerably. We identify a simple renormalization group mechanism for this depletion of the lower bound. If active also in the full standard model, Higgs masses smaller than the conventional infrared window do not necessarily require new physics at low scales or give rise to instability problems.  [Show abstract] [Hide abstract]
ABSTRACT: We investigate chiral HiggsYukawa models with a nonabelian gauged lefthanded sector reminiscent to a subsector of the standard model. We discover a new weakcoupling fixedpoint behavior that allows for ultraviolet complete RG trajectories which can be connected with a conventional longrange infrared behavior in the Higgs phase. This nontrivial ultraviolet behavior is characterized by asymptotic freedom in all interaction couplings, but a quasi conformal behavior in all masslike parameters. The stable microscopic scalar potential asymptotically approaches flatness in the ultraviolet, however, with a nonvanishing minimum increasing inversely proportional to the asymptotically free gauge coupling. This gives rise to nonperturbative  though weakcoupling  threshold effects which induce ultraviolet stability along a line of fixed points. Despite the weakcoupling properties, the system exhibits nonGaussian features which are distinctly different from its standard perturbative counterpart: e.g., on a branch of the line of fixed points, we find linear instead of quadratically running renormalization constants. Whereas the Fermi constant and the top mass are naturally of the same order of magnitude, our model generically allows for light Higgs boson masses. Realistic mass ratios are related to particular RG trajectories with a "walking" midmomentum regime.  [Show abstract] [Hide abstract]
ABSTRACT: Standard Model extensions often predict lowmass and very weakly interacting particles, such as the axion. A number of smallscale experiments at the intensity/precision frontier are actively searching for these elusive particles, complementing searches for physics beyond the Standard Model at colliders. Whilst a next generation of experiments will give access to a huge unexplored parameter space, a discovery would have a tremendous impact on our understanding of fundamental physics.  [Show abstract] [Hide abstract]
ABSTRACT: We show that photons subject to a spatially inhomogeneous electromagnetic field can experience quantum reflection. Based on this observation, we propose quantum reflection as a novel means to probe the nonlinearity of the quantum vacuum in the presence of strong electromagnetic fields.
Publication Stats
5k  Citations  
541.42  Total Impact Points  
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Institutions

20082015

Friedrich Schiller University Jena
 Department of Theoretical Physics
Jena, Thuringia, Germany 
Universitätsklinikum Jena
Jena, Thuringia, Germany


20032011

Universität Heidelberg
 Institute of Theoretical Physics
Heidelburg, BadenWürttemberg, Germany


2005

Heidelberger Institut für Theoretische Studien
Heidelburg, BadenWürttemberg, Germany


20012003

CERN
Genève, Geneva, Switzerland 
Technische Universität Dresden
 Institut für theoretische Physik
Dresden, Saxony, Germany


19962001

University of Tuebingen
 Institute of Physical and Theoretical Chemistry
Tübingen, BadenWürttemberg, Germany
