31 research outputs found
Wegner-Houghton equation and derivative expansion
We study the derivative expansion for the effective action in the framework
of the Exact Renormalization Group for a single component scalar theory. By
truncating the expansion to the first two terms, the potential and the
kinetic coefficient , our analysis suggests that a set of coupled
differential equations for these two functions can be established under certain
smoothness conditions for the background field and that sharp and smooth
cut-off give the same result. In addition we find that, differently from the
case of the potential, a further expansion is needed to obtain the differential
equation for , according to the relative weight between the kinetic and
the potential terms. As a result, two different approximations to the
equation are obtained. Finally a numerical analysis of the coupled equations
for and is performed at the non-gaussian fixed point in
dimensions to determine the anomalous dimension of the field.Comment: 15 pages, 3 figure
Noncommutative massive Thirring model in three-dimensional spacetime
We evaluate the noncommutative Chern-Simons action induced by fermions
interacting with an Abelian gauge field in a noncommutative massive Thirring
model in (2+1)-dimensional spacetime. This calculation is performed in the
Dirac and Majorana representations. We observe that in Majorana representation
when goes to zero we do not have induced Chern-Simons term in the
dimensional regularization scheme.Comment: Accepted to Phys. Rev. D; 9 pages, Revtex4, no figures, references
added, minor improvements, Eq.31 correcte
Nonequilibrium relaxation in neutral BCS superconductors: Ginzburg-Landau approach with Landau damping in real time
We present a field-theoretical method to obtain consistently the equations of
motion for small amplitude fluctuations of the order parameter directly in real
time for a homogeneous, neutral BCS superconductor. This method allows to study
the nonequilibrium relaxation of the order parameter as an initial value
problem. We obtain the Ward identities and the effective actions for small
phase the amplitude fluctuations to one-loop order. Focusing on the
long-wavelength, low-frequency limit near the critical point, we obtain the
time-dependent Ginzburg-Landau effective action to one-loop order, which is
nonlocal as a consequence of Landau damping. The nonequilibrium relaxation of
the phase and amplitude fluctuations is studied directly in real time. The
long-wavelength phase fluctuation (Bogoliubov-Anderson-Goldstone mode) is
overdamped by Landau damping and the relaxation time scale diverges at the
critical point, revealing critical slowing down.Comment: 31 pages 14 figs, revised version, to appear in Phys. Rev.
Effective Action for QED with Fermion Self-Interaction in D=2 and D=3 Dimensions
In this work we discuss the effect of the quartic fermion self-interaction of
Thirring type in QED in D=2 and D=3 dimensions. This is done through the
computation of the effective action up to quadratic terms in the photon field.
We analyze the corresponding nonlocal photon propagators nonperturbatively in %
\frac{k}{m}, where k is the photon momentum and m the fermion mass. The poles
of the propagators were determined numerically by using the Mathematica
software. In D=2 there is always a massless pole whereas for strong enough
Thirring coupling a massive pole may appear . For D=3 there are three regions
in parameters space. We may have one or two massive poles or even no pole at
all. The inter-quark static potential is computed analytically in D=2. We
notice that the Thirring interaction contributes with a screening term to the
confining linear potential of massive QED_{2}. In D=3 the static potential must
be calculated numerically. The screening nature of the massive QED
prevails at any distance, indicating that this is a universal feature of % D=3
electromagnetic interaction. Our results become exact for an infinite number of
fermion flavors.Comment: Latex, 13 pages, 3 figure
Effect of tensor couplings in a relativistic Hartree approach for finite nuclei
The relativistic Hartree approach describing the bound states of both
nucleons and anti-nucleons in finite nuclei has been extended to include tensor
couplings for the - and -meson. After readjusting the parameters
of the model to the properties of spherical nuclei, the effect of
tensor-coupling terms rises the spin-orbit force by a factor of 2, while a
large effective nucleon mass sustains. The overall
nucleon spectra of shell-model states are improved evidently. The predicted
anti-nucleon spectra in the vacuum are deepened about 20 -- 30 MeV.Comment: 31 pages, 4 postscript figures include
Do we live in the universe successively dominated by matter and antimatter?
We wonder if a cyclic universe may be dominated alternatively by matter and
antimatter. Such a scenario demands a mechanism for transformation of matter to
antimatter (or antimatter to matter) during the final stage of a big crunch. By
giving an example, we have shown that in principle such a mechanism is
possible. Our mechanism is based on a hypothetical repulsion between matter and
antimatter, existing at least deep inside the horizon of a black hole. When
universe is reduced to a supermassive black hole of a small size, a very strong
field of the conjectured force might create (through a Schwinger type
mechanism) particle-antiparticle pairs from the quantum vacuum. The amount of
antimatter created from the vacuum is equal to the decrease of mass of the
black hole and violently repelled from it. When the size of the black hole is
sufficiently small, the creation of antimatter may become so fast, that matter
of our Universe might be transformed to antimatter in a fraction of second.
Such a fast conversion of matter into antimatter may look as a Big Bang. Our
mechanism prevents a singularity; a new cycle might start with an initial size
more than 30 orders of magnitude greater than the Planck length, suggesting
that there is no need for inflationary scenario in Cosmology. In addition,
there is no need to invoke CP violation for explanation of matter-antimatter
asymmetry. Simply, our present day Universe is dominated by matter, because the
previous universe was dominated by antimatter
Progress in Tourism Management: from the geography of tourism to geographies of tourism - A review
This Progress in Tourism Management paper seeks to review the development of geographical contributions to the study of tourism over the last decade. Given the limited number of surveys of geography published in academic journals since the 1970s, it is particularly timely to question and debate where the subject has evolved to, the current debates and issues facing those who work within the subject and where the subject will evolve in the next five years. The paper is structured around a number of distinct themes to emerge from the research activity of geographers, which is deliberately selective in its coverage due to the constraints of space, but focuses on: explaining spatialities; tourism planning and places; development and its discontents; tourism as an 'applied' area of research, and future prospects
Synthetic approaches to artificial photosynthesis: general discussion
Metals in Catalysis, Biomimetics & Inorganic Material