Searcharxiv⌕ Search

arXiv subjects

Herbert Neuberger

Publications and source records attributed to Herbert Neuberger.

At least 55 records · Page 3Linked to original sources
arXiv2000

Noncompact chiral U(1) gauge theories on the lattice

A new, adiabatic phase choice is adopted for the overlap in the case of an infinite volume, noncompact abelian chiral gauge theory. This gauge choice obeys the same symmetries as the Brillouin-Wigner (BW) phase choice, and, in addition, produces a Wess-Zumino functional that is linear in the gauge variables on the lattice. As a result, there are no gauge violations on the trivial orbit in all theories, consistent and covariant anomalies are simply related and Berry's curvature now appears as a Schwinger term. The adiabatic phase choice can be further improved to produce a perfect phase choice, with a lattice Wess-Zumino functional that is just as simple as the one in continuum. When perturbative anomalies cancel, gauge invariance in the fermionic sector is fully restored. The lattice effective action describing an anomalous abelian gauge theory has an explicit form, close to one analyzed in the past in a perturbative continuum framework.

hep-lat↗
arXiv1999

The overlap lattice Dirac operator and dynamical fermions

I show how to avoid a two level nested conjugate gradient procedure in the context of Hybrid Monte Carlo with the overlap fermionic action. The resulting procedure is quite similar to Hybrid Monte Carlo with domain wall fermions, but is more flexible and therefore has some potential worth exploring.

hep-lat↗
arXiv1999

Mathematical aspects of chiral gauge theories on the lattice

For two decades it was believed that chiral symmetries cannot be realized in lattice field theory but this has changed now. Highlights of these new developments will be presented with emphasis on the mathematical structure of the so called ``overlap''.

hep-th↗
arXiv1998

Minimizing storage in implementations of the overlap lattice-Dirac operator

The overlap lattice-Dirac operator contains the sign function $ε(H)$. Recent practical implementations replace $ε(H)$ by a ratio of polynomials, $H P_n (H^2)/Q_n (H^2)$, and require storage of $2n+2$ large vectors. Here I show that one can use only 4 large vectors at the cost of executing the core conjugate algorithm twice. The slow-down might be less than by a factor of 2, depending on the architecture of the computer one uses.

hep-lat↗
arXiv1998

Lattice Chirality

The external fermion propagator and the internal fermion propagator in the overlap are given by different matrices. A generic problem (formulated by Pelissetto) faced by all chiral, non-local, propagators of Rebbi type is avoided in this manner. Nussinov-Weingarten-Witten mass inequalities are exactly preserved. It is sketched how to obtain simple lattice chiral Yukawa models and simple expressions for covariant currents. Going beyond my oral presentation, I have added to the write-up several comments on Niedermayer's talk. His transparencies are available on the internet.

hep-lat↗
arXiv1998

Witten's SU(2) anomaly on the lattice

Witten's anomaly for SU(2) with a single I=1/2 Weyl fermion in four dimension is shown to be reproduced by the lattice overlap. The mechanism is based on Berry's phase, and on the analyticity of the matrix $H$ by which the overlap is defined.

hep-lat↗
arXiv1998

A practical implementation of the Overlap-Dirac operator

A practical implementation of the Overlap-Dirac operator ${{1+γ_5ε(H)}\over 2}$ is presented. The implementation exploits the sparseness of $H$ and does not require full storage. A simple application to parity invariant three dimensional SU(2) gauge theory is carried out to establish that zero modes related to topology are exactly reproduced on the lattice.

hep-lat↗
arXiv1998

Explicitly real form of the Wilson-Dirac matrix for SU(2)

The Wilson-Dirac matrix for SU(2) with I=1/2 fermions is written in an explicitly real form. The basis change relating it to the conventional form in which the matrix has complex entries is also given. Some applications are presented.

hep-lat↗
arXiv1997

Exactly massless quarks on the lattice

It is suggested that the fermion determinant for a vector-like gauge theory with strictly massless quarks can be represented on the lattice as $\det{{1+V}\over 2}$, where $V=X(X^\dagger X)^{-1/2}$ and $X$ is the Wilson-Dirac lattice operator with a negative mass term. There is no undesired doubling and no need for any fine tuning. Several other appealing features of the formula are pointed out.

hep-lat↗
arXiv1997

Vector like gauge theories with almost massless fermions on the lattice

A truncation of the overlap (domain wall fermions) is studied and a criterion for reliability of the approximation is obtained by comparison to the exact overlap formula describing massless quarks. We also present a truncated version of regularized, pure gauge, supersymmetric models. The mechanism for generating almost masslessness is shown to be a generalized see-saw which can also be viewed as a version of Froggatt-Nielsen's method for obtaining natural large mass hierarchies. Viewed in this way the mechanism preserving the mass hierarchy naturally avoids preserving even approximately axial U(1). The new insights into the source of the mass hierarchy suggest ways to increase the efficiency of numerical simulations of QCD employing the truncated overlap.

hep-lat↗
arXiv1997

The overlap passes a chiral dynamical test in two dimensions

A certain U(1) model in 2 dimensions, describing four right handed unit charged Weyl fermions interacting with one doubly charged left handed Weyl fermion, is exactly soluble and has massless Majorana-Weyl composites. Instanton induced fermion number violation is essential for 't Hooft anomaly consistency. The associated 't Hooft vertex can be analytically computed in the continuum, including finite size corrections. This number is reproduced in a dynamical simulation employing the overlap.

hep-lat↗
arXiv1997

Monte Carlo evaluation of a fermion number violating observable in 2D

We describe in some detail a computer evaluation of a 't Hooft vertex in a two dimensional model using the overlap. The computer result agrees with the known exact continuum value, and in this sense our work is a first successful fully dynamical simulation of a chiral gauge theory on the lattice. We add some new data to numbers obtained earlier and provide a selfcontained description which should make it easy for others to reproduce and follow up on our work.

hep-lat↗
arXiv1996

Overlap for 2D chiral U(1) models

The overlap formulation is applied to an anomaly free combination of chiral fermions coupled to U(1) gauge fields on a 2D torus. Evidence is presented that gauge averaging the overlap phases in these models produces correct continuum results.

hep-lat↗
arXiv1996

Anomaly free U(1) chiral gauge theories on a two dimensional torus

We consider anomaly free combinations of chiral fermions coupled to $U(1)$ gauge fields on a 2D torus first in the continuum and then on the lattice in the overlap formulation. Both in the continuum and on the lattice, when the background consists of sufficiently large constant gauge potentials the action induced by the fermions varies significantly under certain singular gauge transformations. ``Ruling away'' such discontinuities cannot be justified in the continuum framework and does not naturally fit on the lattice. Complete gauge invariance in the continuum can be restored in some models by choosing special boundary conditions for the fermions. Evidence is presented that gauge averaging the overlap phases in these models produces correct continuum results.

hep-th↗
arXiv1996

Overlap formulation of Majorana--Weyl fermions

An overlap method for regularizing Majorana--Weyl fermions interacting with gauge fields is presented. A mod(2) index is introduced in relation to the anomalous violation of a discrete global chiral symmetry. Most of the paper is restricted to 2 dimensions but generalizations to 2+8k dimensions should be straightforward.

hep-th↗
arXiv1995

A Lecture on Chiral Fermions

This is an informal and approximate transcription of a talk presented at the DESY workshop, September 27--29, 1995. The basic message is that real and long overdue progress is taking place on the problem of regulating non--perturbatively chiral gauge theories. Several approaches are reviewed with emphasis on the overlap and some of the questions raised about it. No claim for completeness or objectivity is made.

hep-lat↗