180 research outputs found
Ising exponents from the functional renormalisation group
We study the 3d Ising universality class using the functional renormalisation
group. With the help of background fields and a derivative expansion up to
fourth order we compute the leading index, the subleading symmetric and
anti-symmetric corrections to scaling, the anomalous dimension, the scaling
solution, and the eigenperturbations at criticality. We also study the
cross-correlations of scaling exponents, and their dependence on
dimensionality. We find a very good numerical convergence of the derivative
expansion, also in comparison with earlier findings. Evaluating the data from
all functional renormalisation group studies to date, we estimate the
systematic error which is found to be small and in good agreement with findings
from Monte Carlo simulations, \epsilon-expansion techniques, and resummed
perturbation theory.Comment: 24 pages, 3 figures, 7 table
Scaling of variables and the relation between noncommutative parameters in Noncommutative Quantum Mechanics
We consider Noncommutative Quantum Mechanics with phase space
noncommutativity. In particular, we show that a scaling of variables leaves the
noncommutative algebra invariant, so that only the self-consistent effective
parameters of the model are physically relevant. We also discuss the recently
proposed relation of direct proportionality between the noncommutative
parameters, showing that it has a limited applicability.Comment: Revtex4, 4 pages; version to match the published on
Renormalization-Group flow for the field strength in scalar self-interacting theories
We consider the Renormalization-Group coupled equations for the effective
potential V(\phi) and the field strength Z(\phi) in the spontaneously broken
phase as a function of the infrared cutoff momentum k. In the k \to 0 limit,
the numerical solution of the coupled equations, while consistent with the
expected convexity property of V(\phi), indicates a sharp peaking of Z(\phi)
close to the end points of the flatness region that define the physical
realization of the broken phase. This might represent further evidence in favor
of the non-trivial vacuum field renormalization effect already discovered with
variational methods.Comment: 10 pages, 3 Figures, version accepted for publication in Phys. Lett.
Comment on "Feynman Effective Classical Potential in the Schrodinger Formulation"
We comment on the paper "Feynman Effective Classical Potential in the
Schrodinger Formulation"[Phys. Rev. Lett. 81, 3303 (1998)]. We show that the
results in this paper about the time evolution of a wave packet in a double
well potential can be properly explained by resorting to a variational
principle for the effective action. A way to improve on these results is also
discussed.Comment: 1 page, 2eps figures, Revte
Spontaneous breaking of translational invariance in non-commutative lambda phi^4 theory in two dimensions
The spontaneous breaking of of translational invariance in non-commutative
self-interacting scalar field theory in two dimensions is investigated by
effective action techniques. The analysis confirms the existence of the stripe
phase, already observed in lattice simulations, due to the non-local nature of
the non-commutative dynamics.Comment: 7 pages, 2 figure
On the Vacuum Cherenkov Radiation in Noncommutative Electrodynamics and the Elusive Effects of Lorentz Violation
We show that in the framework of noncommutative classical electrodynamics
Cherenkov radiation is permitted in vacuum and we explicitly compute its
spectrum at first order in the noncommutative parameter. We discuss the
phenomenological impact of the merge of this new analysis with the old results
of the substantial modification to the spectrum of the synchrotron radiation
obtained in P.Castorina, A.Iorio and D.Zappala, Phys. Rev. D 69 (2004)065008.
We propose to consider the pulsars' radiation spectrum - due to its very strong
magnetic field - to investigate these Lorentz violating effects in
astrophysical phenomena.Comment: 6 pgs, latex file; published versio
Bendings of radio jets in BL Lacertae objects I: EVN and MERLIN observations
Several blazars, and BL Lac objects in particular, show a misalignment
between the jet orientation on parsec and kiloparsec scales. Some authors (i.e.
Conway & Murphy, 1993) have attempted to explain this behaviour invoking
helical jets for misalignment angles around 90\degr, showing how in this case
there are interesting implications for the understanding of the medium into
which the jet is expanding. By comparing sensitive VLA observations (Cassaro et
al., 1999) with images available in the literature for the BL Lac objects from
the 1-Jy Sample (Stickel et al., 1991), it is clear that there is a wide range
of misalignments between the initial jet direction and the kpc-scale jet, when
detected. We have carried out VLBI observations of these BL Lac objects, in
order to investigate the spatial evolution of the radio jets from few tens to
hundreds of mas, and to search for helical jets in this class of sources. We
present here the first dataset obtained from EVN+MERLIN observations at 5 GHz
for seven objects. From these observations we never have a clear detection of
helical jets, we only have a possible signature of their presence in 2 objects.
In only one of the sources with a misalignment angle around 90\degr the
presence of helical jets can be ruled out. This implies that it is not possible
to invoke helical jets to explain the morphology of all the sources showing a
misalignment of about 90\degr between the parsec and the kiloparsec scale
jets.Comment: 12 pages, 9 figures, latex, accepted by Astronomy & Astrophysic
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