Publications (129)279.82 Total impact
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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.07/2014;  [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.06/2014;  [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.04/2014;  [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.Physical Review Letters 02/2014; 112(5):050402. · 7.73 Impact Factor  [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.11/2013; 89(8).  [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.10/2013; 89(6).  [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.08/2013; 89(4).  [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.06/2013;  [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.Annalen der Physik 06/2013; 525(6). · 1.51 Impact Factor  [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.New Journal of Physics 05/2013; 15(8). · 4.06 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Catalyzed symmetry breaking arises from a parametric enhancement of critical fluctuations independently of the coupling strength. Symmetrybreaking fermionic longrange fluctuations exhibit such an enhancement on negatively curved spaces, as is known from meanfield studies. We study gravitational catalysis from the viewpoint of the functional renormalization group using the 3d GrossNeveu model as a specific example. We observe gravitational catalysis towards a phase of broken discrete chiral symmetry both on a maximally symmetric (AdS) and on a purely spatially curved manifold for constant negative curvature (Lobachevsky plane). The resulting picture for gravitational catalysis obtained from the renormalization flow is closely related to that of magnetic catalysis. As an application, we estimate the curvature required for subcritical systems of finite length to acquire a gravitionally catalyzed gap.Physical review D: Particles and fields 03/2013; 87(10).  Physical review D: Particles and fields 03/2013; 87(6).
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ABSTRACT: We study the phase diagram of the GrossNeveu model in d=2+1 spacetime dimensions in the plane spanned by temperature and the number of massless fermion flavors. We use a functional renormalization group approach based on a nonperturbative derivative expansion that accounts for fermionic as well as composite bosonic fluctuations. We map out the phase boundary separating the ordered massive lowtemperature phase from the disordered hightemperature phase. The phases are separated by a secondorder phase transition in the 2d Ising universality class. We determine the size of the Ginzburg region and show that it scales to zero for large $\Nf$ following a powerlaw, in agreement with large$\Nf$ and lattice studies. We also study the regimes of local order above as well as the classical regime below the critical temperature.Journal of Physics A Mathematical and Theoretical 12/2012; 46(28). · 1.77 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Ever since Schwinger published his influential paper [J. Schwinger, Phys. Rev. 82, 664 (1951)], the maxim that there can be no pair creation in a plane wave has been often cited. We advance an analysis that indicates that in any real situation, where thermal effects are present, in a single planewave field, even in the limit of zero frequency (a constant crossed field), thermal photons can seed pair creation. Interestingly, the pairproduction rate is found to depend nonperturbatively on both the amplitude of the constant crossed field and on the temperature.Physical review D: Particles and fields 12/2012; 86(12).  [Show abstract] [Hide abstract]
ABSTRACT: We show that magnetic fields significantly enhance a new tunneling mechanism in quantum field theories with photons coupling to fermionic minicharged particles (MCPs). We propose a dedicated laboratory experiment of the lightshiningthroughwalls type that can explore a parameter regime comparable to and even beyond the best modelindependent cosmological bounds. With presentday technology, such an experiment is particularly sensitive to MCPs with masses in and below the meV regime as suggested by newphysics extensions of the standard model.Physical Review Letters 09/2012; 109(13):131802. · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We investigate chiral symmetry breaking in the (2+1)dimensional Thirring model as a function of the coupling as well as the Dirac flavor number Nf with the aid of the functional renormalization group. For small enough flavor number Nf < Nfc, the model exhibits a chiral quantum phase transition for sufficiently large coupling. We compute the critical exponents of this second order transition as well as the fermionic and bosonic mass spectrum inside the broken phase within a nexttoleading order derivative expansion. We also determine the quantum critical behavior of the manyflavor transition which arises due to a competition between vector and chiralscalar channel and which is of second order as well. Due to the problem of competing channels, our results rely crucially on the RG technique of dynamical bosonization. For the critical flavor number, we find Nfc ~ 5.1 with an estimated systematic error of approximately one flavor.Physical review D: Particles and fields 08/2012; 86(10).  [Show abstract] [Hide abstract]
ABSTRACT: We study the renormalization flow of axion electrodynamics, concentrating on the nonperturbative running of the axionphoton coupling and the mass of the axion (like) particle. Due to a nonrenormalization property of the axionphoton vertex, the renormalization flow is controlled by photon and axion anomalous dimensions. As a consequence, momentumindependent axion selfinteractions are not induced by photon fluctuations. The nonperturbative flow towards the ultraviolet exhibits a Landaupoletype behavior, implying that the system has a scale of maximum UV extension and that the renormalized axionphoton coupling in the deep infrared is bounded from above. Even though gauge invariance guarantees that photon fluctuations do not decouple in the infrared, the renormalized couplings remain finite even in the deep infrared and even for massless axions. Within our truncation, we also observe the existence of an exceptional RG trajectory, which is extendable to arbitrarily high scales, without being governed by a UV fixed point.Physical review D: Particles and fields 07/2012; 86(12).  [Show abstract] [Hide abstract]
ABSTRACT: Integrating out virtual quantum fluctuations in an originally local quantum field theory results in an effective theory which is nonlocal. In this Letter we argue that tunneling of the 3rd kind  where particles traverse a barrier by splitting into a pair of virtual particles which recombine only after a finite distance  provides a direct test of this nonlocality. We sketch a quantumoptical setup to test this effect, and investigate observable effects in a simple toy model.EPL (Europhysics Letters) 04/2012; 101(6). · 2.26 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We show that magnetic fields have the potential to significantly enhance a recently proposed lightshiningthroughwalls scenario in quantumfield theories with photons coupling to minicharged particles. Suggesting a dedicated laboratory experiment, we demonstrate that this particular tunneling scenario could provide access to a parameter regime competitive with the currently best direct laboratory limits on minicharged fermions below the $\mathrm{meV}$ regime. With present day technology, such an experiment has the potential to even overcome the best modelindependent cosmological bounds on minicharged fermions with masses below $\mathcal{O} (10^{4}) \mathrm{eV}$.Physical review D: Particles and fields 03/2012; 87(2).  [Show abstract] [Hide abstract]
ABSTRACT: We discuss Casimir phenomena which are dominated by longrange fluctuations. A prime example is given by "geothermal" Casimir phenomena where thermal fluctuations in open Casimir geometries can induce significantly enhanced thermal corrections. We illustrate the underlying mechanism with the aid of the inclinedplates configuration, giving rise to enhanced powerlaw temperature dependences compared to the parallelplates case. In limiting cases, we find numerical evidence even for fractional power laws induced by longrange fluctuations. We demonstrate that thermal energy densities for open geometries are typically distributed over length scales of 1/T. As an important consequence, approximation methods for thermal corrections based on local energydensity estimates such as the proximityforce approximation are expected to become unreliable even at small surface separations.International Journal of Modern Physics A 01/2012; 25(11). · 1.13 Impact Factor
Publication Stats
3k  Citations  
279.82  Total Impact Points  
Top Journals
Institutions

2008–2014

FriedrichSchillerUniversity Jena
 Department of Theoretical Physics
Jena, Thuringia, Germany


2009–2011

KarlFranzensUniversität Graz
 Institute of Physics
Graz, Styria, Austria


2010

TRIUMF
Vancouver, British Columbia, Canada


2003–2010

Universität Heidelberg
 Institute of Theoretical Physics
Heidelberg, BadenWuerttemberg, Germany


2005

Heidelberger Institut für Theoretische Studien
Heidelburg, BadenWürttemberg, Germany


2001–2003

CERN
Genève, Geneva, Switzerland


1996–2002

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