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Georg Bergner

Publications and source records attributed to Georg Bergner.

At least 91 records · Page 5Linked to original sources
arXiv2015

Recent results from SU(2) with one adjoint Dirac fermion

We present some results for SU(2) with one adjoint Dirac flavour from lattice studies. Data for the spectroscopy, the static potential, topological charge, and the anomalous dimension of the fermionic condensate are included. Our findings are found to be in- consistent with conventional confining behaviour, instead pointing tentatively towards a theory lying within or very near the onset of the conformal window, with an anomalous dimension of the fermionic condensate of almost 1. Implications of these findings on the building of models of strongly-interacting dynamics beyond the standard model are discussed.

hep-lat↗
arXiv2015

Numerical corrections to the strong coupling effective Polyakov-line action for finite T Yang-Mills theory

We consider a three-dimensional effective theory of Polyakov lines derived previously from lattice Yang-Mills theory and QCD by means of a resummed strong coupling expansion. The effective theory is useful for investigations of the phase structure, with a sign problem mild enough to allow simulations also at finite density. In this work we present a numerical method to determine improved values for the effective couplings directly from correlators of the 4d Yang-Mills theory. For values of the gauge coupling up to the vicinity of the phase transition, the dominant short range effective coupling are well described by their corresponding strong coupling series. We provide numerical results also for the longer range interactions, Polyakov lines in higher representations as well as four-point interactions, and discuss the growing significance of non-local contributions as the lattice gets finer. Within this approach the critical Yang-Mills coupling $β_c$ is reproduced to better than one percent from a one-coupling effective theory on $N_τ=4$ lattices while up to five couplings are needed on $N_τ=8$ for the same accuracy.

hep-lat↗
arXiv2014

The infrared regime of SU(2) with one adjoint Dirac flavour

SU(2) gauge theory with one Dirac flavour in the adjoint representation is investigated on a lattice. Initial results for the gluonic and mesonic spectrum, static potential from Wilson and Polyakov loops, and the anomalous dimension of the fermionic condensate from the Dirac mode number are presented. The results found are not consistent with conventional confining behaviour, instead tentatively pointing towards a theory lying within or very near the onset of the conformal window, with the anomalous dimension of the fermionic condensate in the range $0.9 \lesssim γ_* \lesssim 0.95$. The implications of our work for building a viable theory of strongly interacting dynamics beyond the standard model are discussed.

hep-lat↗
arXiv2014

Influence of topology on the scale setting

Recently a new method to set the scale in lattice gauge theories, based on the gradient flow generated by the Wilson action, has been proposed, and the systematic errors of the new scales t0 and w0 have been investigated by various groups. The Wilson flow provides also an interesting alternative smoothing procedure in particular useful for the measurement of the topological charge as a pure gluonic observable. We show the viability of this method for N=1 supersymmetric Yang-Mills theory by analysing the configurations produced by the DESY-Muenster collaboration. For increasing flow time the topological charge quickly approaches near-integer values. The topological susceptibility has been measured for different fermion masses and its value is observed to approach zero in the chiral limit. Finally, the relation between the scale defined by the Wilson flow and the topological charge has been investigated, demonstrating a correlation between these two quantities.

hep-lat↗
arXiv2013

Effective lattice Polyakov loop theory vs. full SU(3) Yang-Mills at finite temperature

A three-dimensional effective theory of Polyakov loops has recently been derived from Wilson's Yang-Mills lattice action by means of a strong coupling expansion. It is valid in the confined phase up to the deconfinement phase transition, for which it predicts the correct order and gives quantitative estimates for the critical coupling. In this work we study its predictive power for further observables like correlation functions and the equation of state. We find that the effective theory correctly reproduces qualitative features and symmetries of the full theory as the continuum is approached. Regarding quantitative predictions, we identify two classes of observables by numerical comparison as well as analytic calculations: correlation functions and their associated mass scales cannot be described accurately from a truncated effective theory, due to its inherently non-local nature involving long-range couplings. On the other hand, phase transitions and bulk thermodynamic quantities are accurately reproduced by the leading local part of the effective theory. In particular, the effective theory description is numerically superior when computing the equation of state at low temperatures or the properties of the phase transition.

hep-lat↗
arXiv2013

Effective lattice Polyakov loop theory for finite temperature Yang-Mills

Effective Polyakov loop theories are a useful tool for an investigation of pure Yang-Mills theory and full QCD. A systematic derivation of the effective action can be done in a spatial strong coupling expansion. Quite accurate predictions for the deconfinement phase transition of Yang-Mills theory have been obtained in this approach. Besides the critical couplings, further observables can be measured in the effective theory. These provide additional tests for the reliability of the strong coupling approach and the truncation of the effective action. In this contribution we will present recent results for the free energy of the static quark-antiquark pair and the equation of state.

hep-lat↗
arXiv2013

First results for SU(2) Yang-Mills with one adjoint Dirac Fermion

We present a first exploratory study of SU(2) gauge theory with one Dirac flavour in the adjoint representation. We provide initial results for the spectroscopy and the anomalous dimension for the chiral condensate. Our investigation indicates that the theory is conformal or near-conformal, with an anomalous dimension of order one. A discussion of the relevance of these findings in relation to walking technicolor scenarios is also presented.

hep-lat↗
arXiv2013

N=1 supersymmetric Yang-Mills theory on the lattice

Numerical simulations of supersymmetric theories on the lattice are intricate and challenging with respect to their theoretical foundations and algorithmic realisation. Nevertheless, the simulations of a four-dimensional supersymmetric gauge theory have made considerable improvements over the recent years. In this contribution we summarise the results of our collaboration concerning the mass spectrum of this theory. The investigation of systematic errors allows now a more precise estimate concerning the expected formation of supersymmetric multiplets of the lightest particles. These multiplets contain flavour singlet mesons, glueballs, and an additional fermionic state.

hep-lat↗
arXiv2013

Towards the spectrum of low-lying particles in supersymmetric Yang-Mills theory

We present the current results of our simulations of N=1 supersymmetric Yang-Mills theory on a lattice. The masses of the gluino-glue particle, the a-eta-prime, the a-f0 meson, and the scalar glueball are obtained at finer lattice spacing than before, and extrapolations towards vanishing gluino mass are made. The calculations employ different levels of stout smearing. The statistical accuracy as well as the control of finite size effects and lattice artefacts are better than in previous investigations. Taking the statistical and systematic uncertainties into account, the extrapolations towards vanishing gluino mass of the masses of the fermionic and bosonic states in our present calculations are consistent with the formation of degenerate supermultiplets.

hep-lat↗
arXiv2013

Blocking-inspired supersymmetric actions: a status report

We provide a status report on the advances in blocking-inspired supersymmetric actions. This is done at the example of interacting supersymmetric quantum mechanics as well as the Wess-Zumino model. We investigate in particular the implications of a nontrivial realisation of translational symmetry on the lattice in this approach. We also discuss the locality of symmetry generators.

hep-lat↗
arXiv2012

The gluino-glue particle and relevant scales for the simulations of supersymmetric Yang-Mills theory

Supersymmetric Yang-Mills theory is in several respects different from QCD and pure Yang-Mills theory. Therefore, a reinvestigation of the scales, at which finite size effects and lattice artifacts become relevant, is necessary. Both, finite size effects and lattice artifacts, induce a breaking of supersymmetry. In view of the unexpected mass gap between bosonic and fermionic particles an estimation of these effects is essential.

hep-lat↗
arXiv2012

The gluino-glue particle and finite size effects in supersymmetric Yang-Mills theory

The spectrum of particles in supersymmetric Yang-Mills theory is expected to contain a spin 1/2 bound state of gluons and gluinos, the gluino-glue particle. We study the mass of this particle in softly broken supersymmetric Yang-Mills theory on a lattice by means of numerical simulations. The main focus is the estimation of finite size effects. We extrapolate the mass first to the infinite volume and then to the limit of a vanishing gluino mass. The results indicate that finite size effects are tolerable on lattices of moderate size, and that remaining deviations from supersymmetry are probably due to finite lattice spacing effects.

hep-lat↗
arXiv2011

Supersymmetric Yang-Mills theory: a step towards the continuum

The spectrum of supersymmetric Yang-Mills theory presented so far shows an unexpected gap between the bosonic and fermionic masses. This finding was in contradiction with the basic requirements of supersymmetry. In this work we will present new results indicating that the mass gap is reduced at a smaller lattice spacing. Hence lattice artifacts are the most likely explanation for it. These new results have been obtained at a larger beta value and on a larger lattice.

hep-lat↗
arXiv2010

The two dimensional N=(2,2) Wess-Zumino Model in the Functional Renormalization Group Approach

We study the supersymmetric N=(2,2) Wess-Zumino model in two dimensions with the functional renormalization group. At leading order in the supercovariant derivative expansion we recover the nonrenormalization theorem which states that the superpotential has no running couplings. Beyond leading order the renormalization of the bare mass is caused by a momentum dependent wave function renormalization. To deal with the partial differential equations we have developed a numerical toolbox called FlowPy. For weak couplings the quantum corrections to the bare mass found in lattice simulations is reproduced with high accuracy. But in the regime with intermediate couplings higher-order-operators that are not constrained by the nonrenormalization theorem yield the dominating contribution to the renormalized mass.

hep-th↗
arXiv2010

Supersymmetry on the lattice and the status of the Super-Yang-Mills simulations

Supersymmetry (SUSY) and supersymmetric field theories are an interesting topic for numerical lattice simulations. Similar to the chiral symmetry there is also no local realization of (interacting) supersymmetry on the lattice. I briefly review the basic reasons for the breaking of supersymmetry. One attempt to solve the problem uses a Ginsparg-Wilson relation for supersymmetry. However, apart from the free theory, a solution of this relation has so far not been found. For supersymmetric Yang-Mills (SYM) theory a fine-tuning of the bare gluino mass is enough to arrive at a supersymmetric continuum limit. The last part of this work contains a short status report of recent SYM simulations.

hep-lat↗
arXiv2009

Flow Equation for Supersymmetric Quantum Mechanics

We study supersymmetric quantum mechanics with the functional RG formulated in terms of an exact and manifestly off-shell supersymmetric flow equation for the effective action. We solve the flow equation nonperturbatively in a systematic super-covariant derivative expansion and concentrate on systems with unbroken supersymmetry. Already at next-to-leading order, the energy of the first excited state for convex potentials is accurately determined within a 1% error for a wide range of couplings including deeply nonperturbative regimes.

hep-th↗
arXiv2008

Generalising the Ginsparg-Wilson relation: Lattice Supersymmetry from Blocking Transformations

The Ginsparg-Wilson relation is extended to interacting field theories with general linear symmetries. Our relation encodes the remnant of the original symmetry in terms of the blocked fields and guides the construction of invariant lattice actions. We apply this approach in the case of lattice supersymmetry. An additional constraint has to be satisfied because of the appearance of a derivative operator in the symmetry transformations. The solution of this constraint leads to non-local SLAC-type derivatives. We investigate the corresponding kinetic operators on the lattice within an exact solution of supersymmetric quantum mechanics. These solutions - analogues of the overlap operator for supersymmetry - can be made local through a specific choice of the blocking kernel. We show that the corresponding relation allows for local lattice symmetry operators as well as local lattice actions. We argue that for interacting theories the lattice action is polynomial in the fields only under special circumstances, which is exemplified within an exact solution.

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