arXiv subjects
Publications and source records attributed to Ch. Hoelbling.
We rebut the claim by Nakamura and Schierholz [1] that the mass of a potential axion needs to be no less than ~230MeV pointing out errors in both their analytic argument and numerical simulations.
We perform a comparative study of the infrared properties of overlap and staggered fermions in QCD. We observe that the infrared spectrum of the APE/HYP improved staggered Dirac operator develops a four-fold near-degeneracy and is in quantitative agreement with the infrared spectrum of the overlap operator. The near-degeneracy allows us to identify the zero modes of the staggered operator and we find that the number of zero modes is in line with the topological index of the overlap operator.
We study Fermi fields defined on tori in the presence of gauge backgrounds carrying non-trivial topology. We show that 2k dimensional field space can alternatively be described by fields over a k-dimensional space. This dual description is particularly natural when the background is uniform and abelian. The reduction in number of dimensions carries over to the lattice. The lattice ultraviolet regularization induces an infrared regularization of the lower dimensional representations. We focus on k=1,2.
We present a high-statistic numerical study of the free energy of a monopole-antimonopole pair in pure SU(2) theory. We find that the monopole-antimonopole interaction potential exhibits a screened behavior, as one would expect in presence of a monopole condensate. Screening occurs both in the low-temperature, confining phase of the theory, and in the high-temperature deconfined phase, with no evidence of a discontinuity of the screening mass across the transition. The mass of the object responsible for the screening at low temperature is approximately twice the established value for the lightest glueball, indicating a prevalent coupling to glueball excitations. At high temperature, the screening mass increases. We contrast the behavior of the quantum system with that of the corresponding classical system, where the monopole-antimonopole potential is of the Coulomb type.
We discuss new approaches to the numerical implementation of Neuberger's operator for lattice fermions and the possible use of block spin transformations.
We present the results of a study of the free energy of a monopole pair in pure SU(2) theory at finite temperature, both below and above the deconfinement tran sition. We find a Yukawa potential between monopoles in both phases. At low temp erature, the screening mass is compatible with the lightest glueball mass. At hi gh temperature, we observe an increased screening mass with no apparent disconti nuity at the phase transition.
We induce an external ${Z}_2$ monopole-antimonopole pair in an SU(2) lattice gauge system and measure its free energy as a way to probe the vacuum structure. We discuss the motivation and computational methodology of the investigation and illustrate our preliminary results.
We study some aspects of the ${\cal O}(a)$ improved Sheikholeslami-Wohlert (SW) action for the lattice Schwinger model. We find some improvement concerning the distribution of eigenvalues of the Dirac operator and of the masses but little or no improvement for rotational invariance of correlators or dispersion relations.
We perform a model study on the 2-flavor lattice Schwinger model using standard Wilson and ${\cal O}(a)$ improved Sheikholeslami-Wohlert (SW) action. We find, that the phase diagram is altered, the critical line shifted closer towards its continuum value $κ_c=0.25$. We find no improvement in the rotation invariance of meson propagators; the scaling of the Schwinger mass is considerably improved, high momentum states are not. The additional cost of $\approx 30 %$ CPU-time is highly justified when calculating masses, but not for high momentum observables.
We study the 2D Ising model on three different types of lattices that are topologically equivalent to spheres. The geometrical shapes are reminiscent of the surface of a pillow, a 3D cube and a sphere, respectively. Systems of volumes ranging up to O($10^5$) sites are simulated and finite size scaling is analyzed. The partition function zeros and the values of various cumulants at their respective peak positions are determined and they agree with the scaling behavior expected from universality with the Onsager solution on the torus ($ν=1$). For the pseudocritical values of the coupling we find significant anomalies indicating a shift exponent $\neq 1$ for sphere-like lattice topology.
We present results for 2D and 4D systems on lattices with topology homotopic to the surface of a (hyper) sphere $S^2$ or $S^4$. Finite size scaling is studied in situations with phase transitions of first and second order type. The Ising and Potts models exhibit the expected behaviour; for the 4D pure gauge $U(1)$ theory we find consistent scaling indicative of a second order phase transition with critical exponent $ν\simeq 0.36(1)$.
Two-particle correlation functions are calculated for bosons emitted from a localized thermal source (the ``glow'' of a ``hot spot''). In contrast to existing work, non-equilibrium effects up to first order in gradients of the particle distribution function are taken into account. The spectral width of the bosons is shown to be an important quantity: If it is too small, they do not equilibrate locally and therefore strongly increase the measured correlation radius. In memoriam of Eugene Wigner and Hiroomi Umezawa.
Within a field theoretical formalism suited to treat inhomogeneous hot quantum systems, we derive the two-particle correlation function for particles having a spectral width gamma in the region of their emission. We find, that this correlation function measures the radius R0 of the thermal source only in case gamma*R0 > 1.