349 research outputs found
Lefschetz thimble structure in one-dimensional lattice Thirring model at finite density
We investigate Lefschetz thimble structure of the complexified
path-integration in the one-dimensional lattice massive Thirring model with
finite chemical potential. The lattice model is formulated with staggered
fermions and a compact auxiliary vector boson (a link field), and the whole set
of the critical points (the complex saddle points) are sorted out, where each
critical point turns out to be in a one-to-one correspondence with a singular
point of the effective action (or a zero point of the fermion determinant). For
a subset of critical point solutions in the uniform-field subspace, we examine
the upward and downward cycles and the Stokes phenomenon with varying the
chemical potential, and we identify the intersection numbers to determine the
thimbles contributing to the path-integration of the partition function. We
show that the original integration path becomes equivalent to a single
Lefschetz thimble at small and large chemical potentials, while in the
crossover region multi thimbles must contribute to the path integration.
Finally, reducing the model to a uniform field space, we study the relative
importance of multiple thimble contributions and their behavior toward
continuum and low-temperature limits quantitatively, and see how the rapid
crossover behavior is recovered by adding the multi thimble contributions at
low temperatures. Those findings will be useful for performing Monte-Carlo
simulations on the Lefschetz thimbles.Comment: 32 pages, 14 figures (typo etc. corrected
Four-dimensional lattice chiral gauge theories with anomalous fermion content
In continuum field theory, it has been discussed that chiral gauge theories
with Weyl fermions in anomalous gauge representations (anomalous gauge
theories) can consistently be quantized, provided that some of gauge bosons are
permitted to acquire mass. Such theories in four dimensions are inevitablly
non-renormalizable and must be regarded as a low-energy effective theory with a
finite ultraviolet (UV) cutoff. In this paper, we present a lattice framework
which enables one to study such theories in a non-perturbative level. By
introducing bare mass terms of gauge bosons that impose ``smoothness'' on the
link field, we explicitly construct a consistent fermion integration measure in
a lattice formulation based on the Ginsparg-Wilson (GW) relation. This
framework may be used to determine in a non-perturbative level an upper bound
on the UV cutoff in low-energy effective theories with anomalous fermion
content. By further introducing the St\"uckelberg or Wess-Zumino (WZ) scalar
field, this framework provides also a lattice definition of a non-linear sigma
model with the Wess-Zumino-Witten (WZW) term.Comment: 18 pages, the final version to appear in JHE
Schroedinger functional formalism with domain-wall fermion
Finite volume renormalization scheme is one of the most fascinating scheme
for non-perturbative renormalization on lattice.
By using the step scaling function one can follow running of renormalized
quantities with reasonable cost.
It has been established the Schroedinger functional is very convenient to
define a field theory in a finite volume for the renormalization scheme.
The Schroedinger functional, which is characterized by a
Dirichlet boundary condition in temporal direction, is well defined and works
well for the Yang-Mills theory and QCD with the Wilson fermion.
However one easily runs into difficulties if one sets the same sort of the
Dirichlet boundary condition for the overlap Dirac operator or the domain-wall
fermion.
In this paper we propose an orbifolding projection procedure to impose the
Schroedinger functional Dirichlet boundary condition on the domain-wall
fermion.Comment: 32 page
Exact Chiral Symmetry on the Lattice
Developments during the last eight years have refuted the folklore that
chiral symmetries cannot be preserved on the lattice. The mechanism that
permits chiral symmetry to coexist with the lattice is quite general and may
work in Nature as well. The reconciliation between chiral symmetry and the
lattice is likely to revolutionize the field of numerical QCD.Comment: 30 pages, LaTeX, reference adde
First-order restoration of SU(Nf) x SU(Nf) chiral symmetry with large Nf and Electroweak phase transition
It has been argued by Pisarski and Wilczek that finite temperature
restoration of the chiral symmetry SU(Nf) x SU(Nf) is first-order for Nf >=3.
This type of chiral symmetry with a large Nf may appear in the Higgs sector if
one considers models such as walking technicolor theories. We examine the
first-order restoration of the chiral symmetry from the point of view of the
electroweak phase transition. The strength of the transition is estimated in
SU(2) x U(1) gauged linear sigma model by means of the finite temperature
effective potential at one-loop with the ring improvement. Even if the mass of
the neutral scalar boson corresponding to the Higgs boson is larger than 114
GeV, the first-order transition can be strong enough for the electroweak
baryogenesis, as long as the extra massive scalar bosons (required for the
linear realization) are kept heavier than the neutral scalar boson. Explicit
symmetry breaking terms reduce the strength of the first-order transition, but
the transition can remain strongly first-order even when the masses of pseudo
Nambu-Goldstone bosons become as large as the current lower bound of direct
search experiments.Comment: 18 pages, 18 figures, minor corrections, references adde
A practical implementation of the Overlap-Dirac operator
A practical implementation of the Overlap-Dirac operator
is presented. The implementation exploits
the sparseness of 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.Comment: Y-axis label in figure correcte
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