176 research outputs found
Gaussian Effective Potential and Antiferromagnetism in the Hubbard Model
The Gaussian Effective Potential (GEP) is shown to be a useful variational
tool for the study of the magnetic properties of strongly correlated electronic
systems. The GEP is derived for a single band Hubbard model on a
two-dimensional bi-partite square lattice in the strong coupling regime. At
half-filling the antiferromagnetic order parameter emerges as the minimum of
the effective potential with an accuracy which improves over RPA calculations
and is very close to that achieved by Monte Carlo simulations. Extensions to
other magnetic systems are discussed.Comment: 9 pages, 3 figures; 1 figure removed; final discussion revised and a
new reference adde
Self-consistent variational approach to the minimal left-right symmetric model of electroweak interactions
The problem of mass generation is addressed by a Gaussian variational method
for the minimal left-right symmetric model of electroweak interactions. Without
any scalar bidoublet, the Gaussian effective potential is shown to have a
minimum for a broken symmetry vacuum with a finite expectation value for both
the scalar Higgs doublets. The symmetry is broken by the fermionic coupling
that destabilizes the symmetric vacuum, yielding a self consistent fermionic
mass. In this framework a light Higgs is only compatible with the existence of
a new high energy mass scale below 2 TeV.Comment: 5 pages, 3 figures. New comments added and typing errors in eq. 8 and
11 correcte
Non-perturbative effective model for the Higgs sector of the Standard Model
A non-perturbative effective model is derived for the Higgs sector of the
standard model, described by a simple scalar theory. The renormalized couplings
are determined by the derivatives of the Gaussian Effective Potential that are
known to be the sum of infinite bubble graphs contributing to the vertex
functions. A good agreement has been found with strong coupling lattice
simulations when a comparison can be made
Patterns of trading profiles at the Nordic Stock Exchange. A correlation-based approach
We investigate the trading behavior of Finnish individual investors trading
the stocks selected to compute the OMXH25 index in 2003 by tracking the
individual daily investment decisions. We verify that the set of investors is a
highly heterogeneous system under many aspects. We introduce a correlation
based method that is able to detect a hierarchical structure of the trading
profiles of heterogeneous individual investors. We verify that the detected
hierarchical structure is highly overlapping with the cluster structure
obtained with the approach of statistically validated networks when an
appropriate threshold of the hierarchical trees is used. We also show that the
combination of the correlation based method and of the statistically validated
method provides a way to expand the information about the clusters of investors
with similar trading profiles in a robust and reliable way.Comment: 25 pages, 8 figure
Backbone of credit relationships in the Japanese credit market
We detect the backbone of the weighted bipartite network of the Japanese
credit market relationships. The backbone is detected by adapting a general
method used in the investigation of weighted networks. With this approach we
detect a backbone that is statistically validated against a null hypothesis of
uniform diversification of loans for banks and firms. Our investigation is done
year by year and it covers more than thirty years during the period from 1980
to 2011. We relate some of our findings with economic events that have
characterized the Japanese credit market during the last years. The study of
the time evolution of the backbone allows us to detect changes occurred in
network size, fraction of credit explained, and attributes characterizing the
banks and the firms present in the backbone.Comment: 14 pages, 8 figure
Bank-firm credit network in Japan. An analysis of a bipartite network
We present an analysis of the credit market of Japan. The analysis is
performed by investigating the bipartite network of banks and firms which is
obtained by setting a link between a bank and a firm when a credit relationship
is present in a given time window. In our investigation we focus on a community
detection algorithm which is identifying communities composed by both banks and
firms. We show that the clusters obtained by directly working on the bipartite
network carry information about the networked nature of the Japanese credit
market. Our analysis is performed for each calendar year during the time period
from 1980 to 2011. Specifically, we obtain communities of banks and networks
for each of the 32 investigated years, and we introduce a method to track the
time evolution of these communities on a statistical basis. We then
characterize communities by detecting the simultaneous over-expression of
attributes of firms and banks. Specifically, we consider as attributes the
economic sector and the geographical location of firms and the type of banks.
In our 32 year long analysis we detect a persistence of the over-expression of
attributes of clusters of banks and firms together with a slow dynamics of
changes from some specific attributes to new ones. Our empirical observations
show that the credit market in Japan is a networked market where the type of
banks, geographical location of firms and banks and economic sector of the firm
play a role in shaping the credit relationships between banks and firms.Comment: 9 pages, 4 figures, 2 Table
Gaussian effective potential for the standard model SU(2)xU(1) electroweak theory
The Gaussian Effective Potential (GEP) is derived for the non-Abelian
SU(2)xU(1) gauge theory of electroweak interactions. First the problem of gauge
invariance is addressed in the Abelian U(1) theory, where an optimized GEP is
shown to be gauge invariant. The method is then extended to the full
non-Abelian gauge theory where, at variance with naive derivations, the GEP is
proven to be a genuine variational tool in any gauge. The role of ghosts is
discussed and the unitarity gauge is shown to be the only choice which allows
calculability without insertion of further approximations. The GEP for the
standard model is derived and its predictions are compared to the known
phenomenology, thus showing that the GEP provides an alternative
non-perturbative description of the known experimental data. By a consistent
renormalization of masses the full non-Abelian calculation confirms the
existence of a light Higgs boson in the non-perturbative strong coupling regime
of the Higgs sector
Long-term ecology of investors in a financial market
The cornerstone of modern finance is the efficient market hypothesis. Under this hypothesis all information available about a financial asset is immediately incorporated into its price dynamics by fully rational investors. In contrast to this hypothesis many studies have pointed out behavioral biases in investors. Recently it has become possible to access databases that track the trading decisions of investors. Studies of such databases have shown that investors acting in a financial market are highly heterogeneous among them, and that heterogeneity is a common characteristic of many financial markets. The article describes an empirical study of the daily trading decisions of all Finnish investors investing Nokia stock over a time period of 15 years. The investigation is performed by adapting and using methods and tools in network science. By investigating daily trading decisions, and by constructing the time-evolution of statistically validated networks of investors, clusters of investors\u2014and their time evolution\u2014 which are characterized by similar trading profiles are detected. These clusters are performing distinct trading decisions on time scales ranging from several months to twelve years. These empirical observations show the presence of an ecology of groups of investors characterized by different attributes and by various investment styles over many years. Some of the detected clusters present a persistent over-expression of specific investor categories. The study shows that the logarithm of the ratio of pairs of statistically validated trading decisions is different for different values of the market volatility. These findings suggest that an ecology of investors is present in financial markets and that groups of traders are always competing, adopting, using and eventually discarding new investment strategies. This adaptation process is observed over a multiplicity of time scales, and is compatible with several conclusions of behavioral finance and with the assumptions of the so-called adaptive market hypothesis
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