arXiv subjects

Francesco Knechtli

Publications and source records attributed to Francesco Knechtli.

At least 91 records · Page 5Linked to original sources

arXiv2012

Mean-Field Gauge Interactions in Five Dimensions II. The Orbifold

We study Gauge-Higgs Unification in five dimensions on the lattice by means of the mean-field expansion. We formulate it for the case of an SU(2) pure gauge theory and orbifold boundary conditions along the extra dimension, which explicitly break the gauge symmetry to U(1) on the boundaries. Our main result is that the gauge boson mass computed from the static potential along four-dimensional hyperplanes is nonzero implying spontaneous symmetry breaking. This observation supports earlier data from Monte Carlo simulations [12].

hep-lat↗

arXiv2012

The strange quark mass and Lambda parameter of two flavor QCD

We complete the non-perturbative calculations of the strange quark mass and the Lambda parameter in two flavor QCD by the ALPHA collaboration. The missing lattice scale is determined via the kaon decay constant, for whose chiral extrapolation complementary strategies are compared. We also give a value for the scale r_0 in physical units as well as an improved determination of the renormalization constant Z_A.

hep-lat↗

arXiv2012

The shape of the static potential with dynamical fermions

We present the analysis of the static potential extracted from Wilson loops measured on CLS ensembles generated with Wilson gauge action and Nf=2 flavors of O(a) improved Wilson quarks at three different lattice spacings and a range of quark masses. The shape of the static potential at distances well below the string breaking region is studied in terms of renormalized couplings derived from the static force and its derivative. We comment on the (im)possibility of extracting the Lambda parameter at our smallest lattice spacing a=0.05 fm. Finally we give an update on the scale determination through r0.

hep-lat↗

arXiv2011

Accuracy of Symmetric Partitioned Runge-Kutta Methods for Differential Equations on Lie-Groups

Computer simulations in QCD are based on the discretization of the theory on a Euclidean lattice. To compute the mean value of an observable, usually the Hybrid Monte Carlo method is applied. Here equations of motion, derived from an Hamiltonian, have to be solved numerically. Commonly the Leapfrog (Stoermer-Verlet) method or splitting methods with multiple timescales à la Sexton-Weingarten are used to integrate the dynamical system, defined on a Lie group. Here we formulate time-reversible higher order integrators based on implicit partitioned Runge-Kutta schemes and show that they allow for larger step-sizes than the Leapfrog method. Since these methods are based on an infinite series of exponential functions, we concentrate on the truncation of this series with respect to the global error and accuracy. Finally, we see that the global error of a SPRK scheme is always even such that a convergence order of one is gained for methods with odd convergence order.

hep-lat↗

arXiv2011

Fermions as global correction in lattice QCD

The fermion determinant is a highly non-local object and its logarithm is an extensive quantity. For these reasons it is widely believed that the determinant cannot be treated in acceptance steps of gauge link configurations that differ in a large fraction of the links. However, for exact factorisations of the determinant that separate the ultraviolet from the infra-red modes of the Dirac operator it is known that the latter show less variation under changes of the gauge field compared to the former. Using a factorisation based on recursive domain decomposition allows for a hierarchical algorithm that starts with pure gauge updates of the links within the domains and ends after a number of filters with a global acceptance step. We find that the global acceptance rate is high on moderate lattice sizes. Whether this type of algorithm can help in curing the problem of critical slowing down is presently under study.

hep-lat↗

arXiv2011

Dimensional reduction from five-dimensional gauge theories

We study the phase diagram of five-dimensional SU(2) gauge theories on anisotropic lattices with periodic boundary conditions. We locate a line of first order bulk phase transitions and second order phase transitions related to breaking of the center along one direction. A reduction to four dimensions occurs through compactification of one dimension but not only. By choosing a lattice spacing in the extra dimension larger than in the other dimensions, we find hints that the hyperplanes orthogonal to the extra dimension decouple from each other. Our analysis is based on measurements of Polyakov loops, the static potential extracted from Wilson loops and renormalized couplings defined through the static force and its derivative.

hep-lat↗

arXiv2011

The Lattice Mean-Field Approximation of Gauge-Higgs Unification on the Orbifold

A possible extension of the Standard Model of elementary particles is Gauge-Higgs unification, where the Higgs field is identified with (some of) the extra dimensional components of a five-dimensional gauge field. In this scenario there is evidence for the potential and the mass of the Higgs field to be finite. Here we show the behavior of the static potential of a five-dimensional SU(2) lattice gauge theory with orbifold boundary conditions. The potentials are computed within the mean-field approximation including first order corrections.

hep-lat↗

arXiv2011

On the phase structure of five-dimensional SU(2) gauge theories with anisotropic couplings

The phase diagram of five-dimensional SU(2) gauge theories is explored using Monte Carlo simulations of the theory discretized on a Euclidean lattice using the Wilson plaquette action and periodic boundary conditions. We simulate anisotropic gauge couplings which correspond to different lattice spacings a_4 in the four dimensions and a_5 along the extra dimension. In particular we study the case where a_5>a_4. We identify a line of first order phase transitions which separate the confined from the deconfined phase. We perform simulations in large volume at the bulk phase transition staying in the confined vacuum. The static potential measured in the hyperplanes orthogonal to the extra dimension hint at dimensional reduction. We also locate and analyze second order phase transitions related to breaking of the center along one direction.

hep-lat↗

arXiv2011

Symmetric Partitioned Runge-Kutta Methods for Differential Equations on Lie Groups

In this paper, we develop a higher order symmetric partitioned Runge-Kutta method for a coupled system of differential equations on Lie groups. We start with a discussion on partitioned Runge-Kutta methods on Lie groups of arbitrary order. As symmetry is not met for higher orders, we generalize the method to a symmetric partitioned Runge-Kutta (SPRK) scheme. Furthermore, we derive a set of coefficients for convergence order 4. The SPRK integration method can be used, for example, in simulations of quantum field theories. Finally, we compare the new SPRK scheme numerically with the Störmer-Verlet scheme, one of the state-of-the-art schemes used in this subject.

hep-lat↗

arXiv2011

Determination of the Static Potential with Dynamical Fermions

We present in detail a technique to extract the potential between a static quark and anti-quark pair from Wilson loops measured on dynamical configurations. This technique is based on HYP smearing and leads to an exponential improvement of the noise-to-signal ratio of Wilson loops. We explain why the correct continuum potential is obtained and show numerical evidence that the cut-off effects are small. We present precise results for the non-perturbative potential. As applications, we determine the scale r_0/a and study the shape of the static potential in the range of distances around r_0, where it can be compared with phenomenological potential models.

hep-lat↗

arXiv2011

Scale r_0 and the static potential from the CLS lattices

We report on the measurement of the static potential and the scale r_0 from HYP-smeared Wilson loops in two flavour QCD. We analyse the quark mass dependence of the potential and r_0 at three lattice spacings. We also compare the QCD static potential around distance r_0 with the static potential obtained from potential models.

hep-lat↗

arXiv2010

Dimensional reduction and confinement from five dimensions

We study non-perturbatively five-dimensional SU(2) gauge theories by means of the mean-field expansion on the lattice. On the anisotropic torus we show that a continuum limit can be defined where the anisotropy is a relevant parameter. The analysis of the static force supports the fact that the four-dimensional hyperplanes decouple from each other in the continuum limit. Clear signs of confinement are found in the static potential along the hyperplanes. We present first results from Monte Carlo simulations on the phase diagram.

hep-lat↗

arXiv2010

Mean-Field Gauge Interactions in Five Dimensions I. The Torus

We consider the lattice regularization of a five dimensional SU(2) gauge theory with periodic boundary conditions. We determine a consistent mean-field background and perform computations of various observables originating from fluctuations around this background. Our aim is to extract the properties of the system in regimes of its phase diagram where it seems to be in a dimensionally reduced state. Within the mean-field theory we establish the existence of a second order phase transition at finite value of the gauge coupling for anisotropy parameter less than one, where there is evidence for dimensional reduction.

hep-lat↗

arXiv2010

A new model for confinement

We propose a new approach towards the understanding of confinement. Starting from an anisotropic five dimensional pure gauge theory, we approach a second order phase transition where the system reduces dimensionally. Dimensional reduction is realized via localization of the gauge and scalar degrees of freedom on four dimensional branes. The gauge coupling deriving from the brane Wilson loop observable runs like an asymptotically free coupling at short distance, while it exhibits clear signs of string formation at long distance. The regularization used is the lattice. We take the continuum limit by keeping the ratio of the lattice spacing in the brane over the lattice spacing along the extra dimension constant and smaller than one.

hep-lat↗

arXiv2007

Higgs mechanism in five-dimensional gauge theories

Lattice simulations of five-dimensional gauge theories on an orbifold revealed that there is spontaneous symmetry breaking. Some of the extra-dimensional components of the gauge field play the role of a Higgs field and some of the four-dimensional components become massive gauge bosons. The effect is confirmed by computing the Coleman-Weinberg potential with a cutoff. We compare the results of this computation with the lattice data.

hep-lat↗

arXiv2007

The Higgs mechanism as a cut-off effect

We compute the Coleman-Weinberg potential with a finite cut-off for pure SU(2) and SU(3) five-dimensional gauge theories compactified on an interval. We show that besides the expected Coulomb phase located at and in the vicinity of the free infrared stable or "trivial" fixed point, the theory possesses also a Higgs phase. We compare the results from the potential computation with lattice data from simulations.

hep-ph↗

arXiv2005

Lattice computation of the strange quark mass in QCD

We present a determination of the strange quark mass using lattice QCD. Particular focus is put on the definition and renormalization of the mass. The latter is done non-perturbatively, using a recursive finite-size scaling technique. The hadronic regime of QCD, where the kaon mass is used as input of the calculation, is connected with the perturbative regime, where the strange quark mass can be translated into the MSbar scheme. A summary plot of the present lattice computations using dynamical (sea) quarks is included.

hep-ph↗

arXiv2005

Monte-Carlo simulation of the chiral Gross-Neveu model

We investigate the two flavor chiral Gross-Neveu model in the Schroedinger functional on the lattice. The procedure necessary to recover chiral symmetry in the continuum limit of this model with Wilson fermions is discussed. We introduce several useful observables and present a first demonstration of the feasibility of Monte-Carlo simulations in this model.

hep-lat↗