1,764 research outputs found
Building simulated queries for known-item topics: an analysis using six european languages
There has been increased interest in the use of simulated queries for evaluation and estimation purposes in Information Retrieval. However, there are still many unaddressed issues regarding their usage and impact on evaluation because their quality, in terms of retrieval performance, is unlike real queries. In this paper, we focus on methods for building simulated known-item topics and explore their quality against real known-item topics. Using existing generation models as our starting point, we explore factors which may influence the generation of the known-item topic. Informed by this detailed analysis (on six European languages) we propose a model with improved document and term selection properties, showing that simulated known-item topics can be generated that are comparable to real known-item topics. This is a significant step towards validating the potential usefulness of simulated queries: for evaluation purposes, and because building models of querying behavior provides a deeper insight into the querying process so that better retrieval mechanisms can be developed to support the user
Discrete Symmetry Enhancement in Nonabelian Models and the Existence of Asymptotic Freedom
We study the universality between a discrete spin model with icosahedral
symmetry and the O(3) model in two dimensions. For this purpose we study
numerically the renormalized two-point functions of the spin field and the four
point coupling constant. We find that those quantities seem to have the same
continuum limits in the two models. This has far reaching consequences, because
the icosahedron model is not asymptotically free in the sense that the coupling
constant proposed by L"uscher, Weisz and Wolff [1] does not approach zero in
the short distance limit. By universality this then also applies to the O(3)
model, contrary to the predictions of perturbation theory.Comment: 18 pages, 8 figures Color coding in Fig. 5 changed to improve
visibilit
An ideal toy model for confining, walking and conformal gauge theories: the O(3) sigma model with theta-term
A toy model is proposed for four dimensional non-abelian gauge theories
coupled to a large number of fermionic degrees of freedom. As the number of
flavors is varied the gauge theory may be confining, walking or conformal. The
toy model mimicking this feature is the two dimensional O(3) sigma model with a
theta-term. For all theta the model is asymptotically free. For small theta the
model is confining in the infra red, for theta = pi the model has a non-trivial
infra red fixed point and consequently for theta slightly below pi the coupling
walks. The first step in investigating the notoriously difficult systematic
effects of the gauge theory in the toy model is to establish non-perturbatively
that the theta parameter is actually a relevant coupling. This is done by
showing that there exist quantities that are entirely given by the total
topological charge and are well defined in the continuum limit and are
non-zero, despite the fact that the topological susceptibility is divergent.
More precisely it is established that the differences of connected correlation
functions of the topological charge (the cumulants) are finite and non-zero and
consequently there is only a single divergent parameter in Z(theta) but
otherwise it is finite. This divergent constant can be removed by an
appropriate counter term rendering the theory completely finite even at theta >
0.Comment: 9 pages, 2 figures, minor modification, references adde
Perturbative versus Non-perturbative QFT -- Lessons from the O(3) NLS Model
The two-point functions of the energy-momentum tensor and the Noether current
are used to probe the O(3) nonlinear sigma model in an energy range below 10^4
in units of the mass gap . We argue that the form factor approach, with the
form factor series trunctated at the 6-particle level, provides an almost exact
solution of the model in this energy range. The onset of the (2-loop)
perturbative regime is found to occur only at energies around .Comment: 13 pages LaTex, 4 PostScript figures; version published in Physics
Letters
The two dimensional XY model at the transition temperature: A high precision Monte Carlo study
We study the classical XY (plane rotator) model at the Kosterlitz-Thouless
phase transition. We simulate the model using the single cluster algorithm on
square lattices of a linear size up to L=2048.We derive the finite size
behaviour of the second moment correlation length over the lattice size
xi_{2nd}/L at the transition temperature. This new prediction and the analogous
one for the helicity modulus are confronted with our Monte Carlo data. This way
beta_{KT}=1.1199 is confirmed as inverse transition temperature. Finally we
address the puzzle of logarithmic corrections of the magnetic susceptibility
chi at the transition temperature.Comment: Monte Carlo results for xi/L in table 1 and 2 corrected. Due to a
programming error,these numbers were wrong by about a factor 1+1/L^2.
Correspondingly the fits with L_min=64 and 128 given in table 5 and 6 are
changed by little.The central results of the paper are not affected. Wrong
sign in eq.(52) corrected. Appendix extende
A Scaling Hypothesis for the Spectral Densities in the O(3) Nonlinear Sigma-Model
A scaling hypothesis for the n-particle spectral densities of the O(3)
nonlinear sigma-model is described. It states that for large particle numbers
the n-particle spectral densities are ``self-similar'' in being basically
rescaled copies of a universal shape function. This can be viewed as a
2-dimensional, but non-perturbative analogue of the KNO scaling in QCD.
Promoted to a working hypothesis, it allows one to compute the two point
functions at ``all'' energy or length scales. In addition, the values of two
non-perturbative constants (needed for a parameter-free matching of the
perturbative and the non-perturbative regime) are determined exactly.Comment: 9 Pages, Latex, 3 Postscript Figure
Drastic Reduction of Cutoff Effects in 2-d Lattice O(N) Models
We investigate the cutoff effects in 2-d lattice O(N) models for a variety of
lattice actions, and we identify a class of very simple actions for which the
lattice artifacts are extremely small. One action agrees with the standard
action, except that it constrains neighboring spins to a maximal relative angle
delta. We fix delta by demanding that a particular value of the step scaling
function agrees with its continuum result already on a rather coarse lattice.
Remarkably, the cutoff effects of the entire step scaling function are then
reduced to the per mille level. This also applies to the theta-vacuum effects
of the step scaling function in the 2-d O(3) model. The cutoff effects of other
physical observables including the renormalized coupling and the mass in the
isotensor channel are also reduced drastically. Another choice, the mixed
action, which combines the standard quadratic with an appropriately tuned large
quartic term, also has extremely small cutoff effects. The size of cutoff
effects is also investigated analytically in 1-d and at N = infinity in 2-d.Comment: 39 pages, 18 figure
The Intrinsic Coupling in Integrable Quantum Field Theories
The intrinsic 4-point coupling, defined in terms of a truncated 4-point
function at zero momentum, provides a well-established measure for the
interaction strength of a QFT. We show that this coupling can be computed
non-perturbatively and to high accuracy from the form factors of an
(integrable) QFT. The technique is illustrated and tested with the Ising model,
the XY-model and the O(3) nonlinear sigma-model. The results are compared to
those from high precision lattice simulations.Comment: 69 pages, Late
Broad expertise retrieval in sparse data environments
Expertise retrieval has been largely unexplored on data other than the W3C collection. At the same time, many intranets of universities and other knowledge-intensive organisations offer examples of relatively small but clean multilingual expertise data, covering broad ranges of expertise areas. We first present two main expertise retrieval tasks, along with a set of baseline approaches based on generative language modeling, aimed at finding expertise relations between topics and people. For our experimental evaluation, we introduce (and release) a new test set based on a crawl of a university site. Using this test set, we conduct two series of experiments. The first is aimed at determining the effectiveness of baseline expertise retrieval methods applied to the new test set. The second is aimed at assessing refined models that exploit characteristic features of the new test set, such as the organizational structure of the university, and the hierarchical structure of the topics in the test set. Expertise retrieval models are shown to be robust with respect to environments smaller than the W3C collection, and current techniques appear to be generalizable to other settings
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