285 research outputs found
Lack of trinification in Z_3 orbifolds of the SO(32) heterotic string
We report results relating to the trinification scenario in some explicit
string constructions that contain SU(3)^3 as a gauge symmetry. These models are
obtained from symmetric Z_3 orbifolds of the SO(32) heterotic string with one
discrete Wilson line. We highlight the obstacles that were encountered: the
absence of the usual Higgs sector that would break SU(3)^3 \to SU(3)_c \times
SU(2)_L \times U(1)_Y; the presence of exotics that would generically befoul
gauge coupling unification and lead to fractionally-charged states in the low
energy spectrum.Comment: 1+7 pages, comments and refs adde
Supercurrent conservation in the lattice Wess-Zumino model with Ginsparg-Wilson fermions
We study supercurrent conservation for the four-dimensional Wess-Zumino model
formulated on the lattice. The formulation is one that has been discussed
several times, and uses Ginsparg-Wilson fermions of the overlap (Neuberger)
variety, together with an auxiliary fermion (plus superpartners), such that a
lattice version of U(1)_R symmetry is exactly preserved in the limit of
vanishing bare mass. We show that the almost naive supercurrent is conserved at
one loop. By contrast we find that this is not true for Wilson fermions and a
canonical scalar action. We provide nonperturbative evidence for the
nonconservation of the supercurrent in Monte Carlo simulations.Comment: 19 pages, 5 figure
Vortex Structure vs. Monopole Dominance in Abelian-Projected Gauge Theory
We find that Polyakov lines, computed in abelian-projected SU(2) lattice
gauge theory in the confined phase, have finite expectation values for lines
corresponding to two units of the abelian electric charge. This means that the
abelian-projected lattice has at most Z(2), rather than U(1), global symmetry.
We also find a severe breakdown of the monopole dominance approximation, as
well as positivity, in this charge-2 case. These results imply that the
abelian-projected lattice is not adequately represented by a monopole Coulomb
gas; the data is, however, consistent with a center vortex structure. Further
evidence is provided, in lattice Monte Carlo simulations, for collimation of
confining color-magnetic flux into vortices.Comment: 37 pages, including 23 eps figures, Latex2
Warped Domain Wall Fermions
We consider Kaplan's domain wall fermions in the presence of an Anti-de
Sitter (AdS) background in the extra dimension. Just as in the flat space case,
in a completely vector-like gauge theory defined after discretizing this extra
dimension, the spectrum contains a very light charged fermion whose chiral
components are localized at the ends of the extra dimensional interval. The
component on the IR boundary of the AdS space can be given a large mass by
coupling it to a neutral fermion via the Higgs mechanism. In this theory, gauge
invariance can be restored either by taking the limit of infinite proper length
of the extra dimension or by reducing the AdS curvature radius towards zero. In
the latter case, the Kaluza-Klein modes stay heavy and the resulting classical
theory approaches a chiral gauge theory, as we verify numerically. Potential
difficulties for this approach could arise from the coupling of the
longitudinal mode of the light gauge boson, which has to be treated
non-perturbatively
The phase structure of a chirally invariant lattice Higgs-Yukawa model for small and for large values of the Yukawa coupling constant
We consider a chirally invariant lattice Higgs-Yukawa model based on the
Neuberger overlap operator. As a first step towards the eventual determination
of Higgs mass bounds we study the phase diagram of the model analytically in
the large Nf-limit. We present an expression for the effective potential at
tree-level in the regime of small Yukawa and quartic coupling constants and
determine the order of the phase transitions. In the case of strong Yukawa
couplings the model effectively becomes an O(4)-symmetric non-linear
sigma-model for all values of the quartic coupling constant. This leads to the
existence of a symmetric phase also in the regime of large values of the Yukawa
coupling constant. On finite and small lattices, however, strong finite volume
effects prevent the expectation value of the Higgs field from vanishing thus
obscuring the existence of the symmetric phase at strong Yukawa couplings.Comment: 21 pages, 6 figures, added reference
RG inspired Machine Learning for lattice field theory
Machine learning has been a fast growing field of research in several areas
dealing with large datasets. We report recent attempts to use Renormalization
Group (RG) ideas in the context of machine learning. We examine coarse graining
procedures for perceptron models designed to identify the digits of the MNIST
data. We discuss the correspondence between principal components analysis (PCA)
and RG flows across the transition for worm configurations of the 2D Ising
model. Preliminary results regarding the logarithmic divergence of the leading
PCA eigenvalue were presented at the conference and have been improved after.
More generally, we discuss the relationship between PCA and observables in
Monte Carlo simulations and the possibility of reduction of the number of
learning parameters in supervised learning based on RG inspired hierarchical
ansatzes.Comment: Talk given by Yannick Meurice at the conference Lattice 2017,
Granada, Spai
"Light from chaos" in two dimensions
We perform a Monte-Carlo study of the lattice two-dimensional gauged
XY-model. Our results confirm the strong-coupling expansion arguments that for
sufficiently small values of the spin-spin coupling the ``gauge symmetry
breaking" terms decouple and the long-distance physics is that of the unbroken
pure gauge theory. We find no evidence for the existence, conjectured earlier,
of massless states near a critical value of the spin-spin coupling. We comment
on recent remarks in the literature on the use of gauged XY-models in proposed
constructions of chiral lattice gauge theories.Comment: 6 pages, 7 figure
Deconstruction and other approaches to supersymmetric lattice field theories
This report contains both a review of recent approaches to supersymmetric
lattice field theories and some new results on the deconstruction approach. The
essential reason for the complex phase problem of the fermion determinant is
shown to be derivative interactions that are not present in the continuum.
These irrelevant operators violate the self-conjugacy of the fermion action
that is present in the continuum. It is explained why this complex phase
problem does not disappear in the continuum limit. The fermion determinant
suppression of various branches of the classical moduli space is explored, and
found to be supportive of previous claims regarding the continuum limit.Comment: 70 page
Bulk fields in AdS_5 from probe D7 branes
We relate bulk fields in Randall-Sundrum AdS_5 phenomenological models to the
world-volume fields of probe D7 branes in the Klebanov-Witten background of
type IIB string theory. The string constructions are described by AdS_5 X
T^{1,1} in their near-horizon geometry, with T^{1,1} a 5d compact internal
manifold that yields N=1 supersymmetry in the dual 4d gauge theory. The
effective 5d Lagrangian description derived from the explicit string
construction leads to additional features that are not usually encountered in
phenomenological model building.Comment: 23 pages, 4 figures; v2: references added; v3: equations simplified,
typos correcte
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