5,765 research outputs found
A non-perturbative study of 4d U(1) non-commutative gauge theory -- the fate of one-loop instability
Recent perturbative studies show that in 4d non-commutative spaces, the
trivial (classically stable) vacuum of gauge theories becomes unstable at the
quantum level, unless one introduces sufficiently many fermionic degrees of
freedom. This is due to a negative IR-singular term in the one-loop effective
potential, which appears as a result of the UV/IR mixing. We study such a
system non-perturbatively in the case of pure U(1) gauge theory in four
dimensions, where two directions are non-commutative. Monte Carlo simulations
are performed after mapping the regularized theory onto a U(N) lattice gauge
theory in d=2. At intermediate coupling strength, we find a phase in which open
Wilson lines acquire non-zero vacuum expectation values, which implies the
spontaneous breakdown of translational invariance. In this phase, various
physical quantities obey clear scaling behaviors in the continuum limit with a
fixed non-commutativity parameter , which provides evidence for a
possible continuum theory. The extent of the dynamically generated space in the
non-commutative directions becomes finite in the above limit, and its
dependence on is evaluated explicitly. We also study the dispersion
relation. In the weak coupling symmetric phase, it involves a negative
IR-singular term, which is responsible for the observed phase transition. In
the broken phase, it reveals the existence of the Nambu-Goldstone mode
associated with the spontaneous symmetry breaking.Comment: 29 pages, 23 figures, references adde
The Fuzzy Disc
We introduce a finite dimensional matrix model approximation to the algebra
of functions on a disc based on noncommutative geometry. The algebra is a
subalgebra of the one characterizing the noncommutative plane with a * product
and depends on two parameters N and theta. It is composed of functions which
decay exponentially outside a disc. In the limit in which the size of the
matrices goes to infinity and the noncommutativity parameter goes to zero the
disc becomes sharper. We introduce a Laplacian defined on the whole algebra and
calculate its eigenvalues. We also calculate the two--points correlation
function for a free massless theory (Green's function). In both cases the
agreement with the exact result on the disc is very good already for relatively
small matrices. This opens up the possibility for the study of field theories
on the disc with nonperturbative methods. The model contains edge states, a
fact studied in a similar matrix model independently introduced by
Balachandran, Gupta and Kurkcuoglu.Comment: 17 pages, 8 figures, references added and correcte
Probing the fuzzy sphere regularisation in simulations of the 3d \lambda \phi^4 model
We regularise the 3d \lambda \phi^4 model by discretising the Euclidean time
and representing the spatial part on a fuzzy sphere. The latter involves a
truncated expansion of the field in spherical harmonics. This yields a
numerically tractable formulation, which constitutes an unconventional
alternative to the lattice. In contrast to the 2d version, the radius R plays
an independent r\^{o}le. We explore the phase diagram in terms of R and the
cutoff, as well as the parameters m^2 and \lambda. Thus we identify the phases
of disorder, uniform order and non-uniform order. We compare the result to the
phase diagrams of the 3d model on a non-commutative torus, and of the 2d model
on a fuzzy sphere. Our data at strong coupling reproduce accurately the
behaviour of a matrix chain, which corresponds to the c=1-model in string
theory. This observation enables a conjecture about the thermodynamic limit.Comment: 31 pages, 15 figure
Classical Gravity on Fuzzy Space-Time
A review is made of recent efforts to find relations between the commutation
relations which define a noncommutative geometry and the gravitational field
which remains as a shadow in the commutative limit.Comment: Lecture given at the 30th International Symposium Ahrenshoop on the
Theory of Elementary Particles, Buckow, Germany, August 27-31, 1996; 11 Pages
LaTe
Spectral geometry with a cut-off: topological and metric aspects
Inspired by regularization in quantum field theory, we study topological and
metric properties of spaces in which a cut-off is introduced. We work in the
framework of noncommutative geometry, and focus on Connes distance associated
to a spectral triple (A, H, D). A high momentum (short distance) cut-off is
implemented by the action of a projection P on the Dirac operator D and/or on
the algebra A. This action induces two new distances. We individuate conditions
making them equivalent to the original distance. We also study the
Gromov-Hausdorff limit of the set of truncated states, first for compact
quantum metric spaces in the sense of Rieffel, then for arbitrary spectral
triples. To this aim, we introduce a notion of "state with finite moment of
order 1" for noncommutative algebras. We then focus on the commutative case,
and show that the cut-off induces a minimal length between points, which is
infinite if P has finite rank. When P is a spectral projection of , we work
out an approximation of points by non-pure states that are at finite distance
from each other. On the circle, such approximations are given by Fejer
probability distributions. Finally we apply the results to Moyal plane and the
fuzzy sphere, obtained as Berezin quantization of the plane and the sphere
respectively.Comment: Reference added. Minor corrections. Published version. 38 pages, 2
figures. Journal of Geometry and Physics 201
The One-Plaquette Model Limit of NC Gauge Theory in 2D
It is found that noncommutative U(1) gauge field on the fuzzy sphere S^2_N is
equivalent in the quantum theory to a commutative 2-dimensional U(N) gauge
field on a lattice with two plaquettes in the axial gauge A_1=0. This quantum
equivalence holds in the fuzzy sphere-weak coupling phase in the limit of
infinite mass of the scalar normal component of the gauge field. The doubling
of plaquettes is a natural consequence of the model and it is reminiscent of
the usual doubling of points in Connes standard model. In the continuum large N
limit the plaquette variable W approaches the identity 1_{2N} and as a
consequence the model reduces to a simple matrix model which can be easily
solved. We compute the one-plaquette critical point and show that it agrees
with the observed value \bar{\alpha}_*=3.35. We compute the quantum effective
potential and the specific heat for U(1) gauge field on the fuzzy sphere
S^2_{N} in the 1/N expansion using this one-plaquette model. In particular the
specific heat per one degree of freedom was found to be equal to 1 in the fuzzy
sphere-weak coupling phase of the gauge field which agrees with the observed
value 1 seen in Monte Carlo simulation. This value of 1 comes precisely because
we have 2 plaquettes approximating the NC U(1) gauge field on the fuzzy sphere.Comment: 46 pages,4 figures,typos corrected,references adde
Fredkin Gates for Finite-valued Reversible and Conservative Logics
The basic principles and results of Conservative Logic introduced by Fredkin
and Toffoli on the basis of a seminal paper of Landauer are extended to
d-valued logics, with a special attention to three-valued logics. Different
approaches to d-valued logics are examined in order to determine some possible
universal sets of logic primitives. In particular, we consider the typical
connectives of Lukasiewicz and Godel logics, as well as Chang's MV-algebras. As
a result, some possible three-valued and d-valued universal gates are described
which realize a functionally complete set of fundamental connectives.Comment: 57 pages, 10 figures, 16 tables, 2 diagram
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