6,514 research outputs found
Quantum properties of a non-Abelian gauge invariant action with a mass parameter
We continue the study of a local, gauge invariant Yang-Mills action
containing a mass parameter, which we constructed in a previous paper starting
from the nonlocal gauge invariant mass dimension two operator F_{\mu\nu}
(D^2)^{-1} F_{\mu\nu}. We return briefly to the renormalizability of the model,
which can be proven to all orders of perturbation theory by embedding it in a
more general model with a larger symmetry content. We point out the existence
of a nilpotent BRST symmetry. Although our action contains extra
(anti)commuting tensor fields and coupling constants, we prove that our model
in the limit of vanishing mass is equivalent with ordinary massless Yang-Mills
theories. The full theory is renormalized explicitly at two loops in the MSbar
scheme and all the renormalization group functions are presented. We end with
some comments on the potential relevance of this gauge model for the issue of a
dynamical gluon mass generation.Comment: 17 pages. v2: version accepted for publication in Phys.Rev.
Relativistic field theories in a magnetic background as noncommutative field theories
We study the connection of the dynamics in relativistic field theories in a
strong magnetic field with the dynamics of noncommutative field theories
(NCFT). As an example, the Nambu-Jona-Lasinio models in spatial dimensions are considered. We show that this connection is rather sophisticated.
In fact, the corresponding NCFT are different from the conventional ones
considered in the literature. In particular, the UV/IR mixing is absent in
these theories. The reason of that is an inner structure (i.e., dynamical
form-factors) of neutral composites which plays an important role in providing
consistency of the NCFT. An especially interesting case is that for a magnetic
field configuration with the maximal number of independent nonzero tensor
components. In that case, we show that the NCFT are finite for even and
their dynamics is quasi-(1+1)-dimensional for odd . For even , the NCFT
describe a confinement dynamics of charged particles. The difference between
the dynamics in strong magnetic backgrounds in field theories and that in
string theories is briefly discussed.Comment: 19 pages, REVTeX4, clarifications added, references added, to appear
in Phys. Rev.
Strongly Enhanced Current Densities in Superconducting Coated Conductors of YBa2Cu3O7-x + BaZrO3
There are numerous potential applications for superconducting tapes, based on
YBa2Cu3O7-x (YBCO) films coated onto metallic substrates. A long established
goal of more than 15 years has been to understand the magnetic flux pinning
mechanisms which allow films to maintain high current densities out to high
magnetic fields. In fact, films carry 1-2 orders of magnitude higher current
densities than any other form of the material. For this reason, the idea of
further improving pinning has received little attention. Now that
commercialisation of conductors is much closer, for both better performance and
lower fabrication costs, an important goal is to achieve enhanced pinning in a
practical way. In this work, we demonstrate a simple and industrially scaleable
route which yields a 1.5 to 5-fold improvement in the in-field current
densities of already-high-quality conductors
Enhanced Peculiar Velocities in Brane-Induced Gravity
The mounting evidence for anomalously large peculiar velocities in our
Universe presents a challenge for the LCDM paradigm. The recent estimates of
the large scale bulk flow by Watkins et al. are inconsistent at the nearly 3
sigma level with LCDM predictions. Meanwhile, Lee and Komatsu have recently
estimated that the occurrence of high-velocity merging systems such as the
Bullet Cluster (1E0657-57) is unlikely at a 6.5-5.8 sigma level, with an
estimated probability between 3.3x10^{-11} and 3.6x10^{-9} in LCDM cosmology.
We show that these anomalies are alleviated in a broad class of
infrared-modifed gravity theories, called brane-induced gravity, in which
gravity becomes higher-dimensional at ultra large distances. These theories
include additional scalar forces that enhance gravitational attraction and
therefore speed up structure formation at late times and on sufficiently large
scales. The peculiar velocities are enhanced by 24-34% compared to standard
gravity, with the maximal enhancement nearly consistent at the 2 sigma level
with bulk flow observations. The occurrence of the Bullet Cluster in these
theories is 10^4 times more probable than in LCDM cosmology.Comment: 15 pages, 6 figures. v2: added reference
Strong Pinning in High Temperature Superconductors
Detailed measurements of the critical current density jc of YBa2Cu3O7 films
grown by pulsed laser deposition reveal the increase of jc as function of the
filmthickness. Both this thickness dependence and the field dependence of the
critical current are consistently described using a generalization of the
theory of strong pinning of Ovchinnikov and Ivlev [Phys. Rev. B 43, 8024
(1991)]. From the model, we deduce values of the defect density (10^21 m^-3)
and the elementary pinning force, which are in good agreement with the
generally accepted values for Y2O3-inclusions. In the absence of clear evidence
that the critical current is determined by linear defects or modulations of the
film thickness, our model provides an alternative explanation for the rather
universal field dependence of the critical current density found in YBa2Cu3O7
films deposited by different methods.Comment: 11 pages; 8 Figures; Published Phys. Rev. B 66, 024523 (2002
The Erd\H{o}s-Ko-Rado theorem for twisted Grassmann graphs
We present a "modern" approach to the Erd\H{o}s-Ko-Rado theorem for
Q-polynomial distance-regular graphs and apply it to the twisted Grassmann
graphs discovered in 2005 by van Dam and Koolen.Comment: 5 page
Non-adaptive Measurement-based Quantum Computation and Multi-party Bell Inequalities
Quantum correlations exhibit behaviour that cannot be resolved with a local
hidden variable picture of the world. In quantum information, they are also
used as resources for information processing tasks, such as Measurement-based
Quantum Computation (MQC). In MQC, universal quantum computation can be
achieved via adaptive measurements on a suitable entangled resource state. In
this paper, we look at a version of MQC in which we remove the adaptivity of
measurements and aim to understand what computational abilities still remain in
the resource. We show that there are explicit connections between this model of
computation and the question of non-classicality in quantum correlations. We
demonstrate this by focussing on deterministic computation of Boolean
functions, in which natural generalisations of the Greenberger-Horne-Zeilinger
(GHZ) paradox emerge; we then explore probabilistic computation, via which
multipartite Bell Inequalities can be defined. We use this correspondence to
define families of multi-party Bell inequalities, which we show to have a
number of interesting contrasting properties.Comment: 13 pages, 4 figures, final version accepted for publicatio
Irreducible numerical semigroups having Toms decomposition
In this article we prove that if S is an irreducible numerical semigroup and S is generated by an interval or S has multiplicity 3 or 4, then it enjoys Toms decomposition. We also prove that if a numerical semigroup can be expressed as an expansion of a numerical semigroup generated by an interval, then it is irreducible and has Toms decomposition
On brane-induced gravity in warped backgrounds
We study whether modification of gravity at large distances is possible in
warped backgrounds with two branes and a brane-induced term localized on one of
the branes. We find that there are three large regions in the parameter space
where the theory is weakly coupled up to high energies. In one of these regions
gravity on the brane is four-dimensional at arbitrarily large distances, and
the induced Einstein term results merely in the renormalization of the 4d
Planck mass. In the other two regions the behavior of gravity changes at
ultra-large distances; however, radion becomes a ghost. In parts of these
regions, both branes have positive tensions, so the only reason for the
appearance of the ghost field is the brane-induced term. In between these three
regions, there are domains in the parameter space where gravity is strongly
coupled at phenomenologically unacceptable low energy scale.Comment: 12 pages, 2 fig, JHEP3 style required, typos correcte
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