339 research outputs found
Exact Supersymmetry on the Lattice
We discuss the possibility of representing supersymmetry exactly in a lattice
discretized system. In particular, we construct a perfect supersymmetric action
for the Wess-Zumino model.Comment: 9 pages, LaTex, no figure
Prospects for perfect actions
The fixed-point (FP) action in QCD, although it is local and determined by
classical equations, is difficult to parametrize well and is expensive to
simulate. But the stake is high: the FP action has scale invariant instanton
solutions, has no topological artifacts, satisfies the index theorem on the
lattice, does not allow exceptional configurations, requires no tuning to get
the pion massless and is expected to reduce the cut-off effects significantly.
An overview is given including a discussion on tests in Yang-Mills theory, QCD
and spin and gauge models.Comment: 6 pages, Late
On the Isomorphic Description of Chiral Symmetry Breaking by Non-Unitary Lie Groups
It is well-known that chiral symmetry breaking (SB) in QCD with
light quark flavours can be described by orthogonal groups as , due to local isomorphisms. Here we discuss the question how specific
this property is. We consider generalised forms of SB involving an
arbitrary number of light flavours of continuum or lattice fermions, in various
representations. We search systematically for isomorphic descriptions by
non-unitary, compact Lie groups. It turns out that there are a few alternative
options in terms of orthogonal groups, while we did not find any description
entirely based on symplectic or exceptional Lie groups. If we adapt such an
alternative as the symmetry breaking pattern for a generalised Higgs mechanism,
we may consider a Higgs particle composed of bound fermions and trace back the
mass generation to SB. In fact, some of the patterns that we encounter
appear in technicolour models. In particular if one observes a Higgs mechanism
that can be expressed in terms of orthogonal groups, we specify in which cases
it could also represent some kind of SB of techniquarks.Comment: 18 pages, to appear in Int. J. Mod. Phys.
Exploring Topology Conserving Gauge Actions for Lattice QCD
We explore gauge actions for lattice QCD, which are constructed such that the
occurrence of small plaquette values is strongly suppressed. By choosing strong
bare gauge couplings we arrive at values for the physical lattice spacings of
O(0.1 fm). Such gauge actions tend to confine the Monte Carlo history to a
single topological sector. This topological stability facilitates the
collection of a large set of configurations in a specific sector, which is
profitable for numerical studies in the epsilon-regime. The suppression of
small plaquette values is also expected to be favourable for simulations with
dynamical quarks. We use a local Hybrid Monte Carlo algorithm to simulate such
actions, and we present numerical results for the static potential, the
physical scale, the topological stability and the kernel condition number of
the overlap Dirac operator. In addition we discuss the question of reflection
positivity for a class of such gauge actions.Comment: 28 pages, 8 figure
The First VLBI Image of the Young, Oxygen-Rich Supernova Remnant in NGC 4449
We report on sensitive 1.4-GHz VLBI radio observations of the unusually
luminous supernova remnant SNR 4449-1 in the galaxy NGC 4449, which gave us the
first well-resolved image of this object. The remnant's radio morphology
consists of two approximately parallel bright ridges, suggesting similarities
to the barrel shape seen for many older Galactic supernova remnants or possibly
to SN 1987A. The angular extent of the remnant is 65 x 40 mas, corresponding to
(3.7 x 2.3) x 10^{18} (D/3.8 Mpc) cm. We also present a new, high
signal-to-noise optical spectrum. By comparing the remnant's linear size to the
maximum velocities measured from optical lines, as well as using constraints
from historical images, we conclude that the supernova explosion occurred
between ~1905 and 1961, likely around 1940. The age of the remnant is therefore
likely ~70 yr. We find that SNR 4449-1's shock wave is likely still interacting
with the circumstellar rather than interstellar medium.Comment: 7 pages, Accepted for publication in MNRA
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
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