13,364 research outputs found
Bounds on Spin-Dependent Lorentz Violation From Inverse Compton Observations
Some of the best bounds on possible Lorentz violation in the electron sector
come from observations of high-energy astrophysical phenomena. Using
measurements of TeV inverse Compton radiation from a number of sources, we
place the first bounds--at the 10^(-15) level--on seven of the electron d
coefficients.Comment: 10 page
Gauge Invariance and the Pauli-Villars Regulator in Lorentz- and CPT-Violating Electrodynamics
We examine the nonperturbative structure of the radiatively induced
Chern-Simons term in a Lorentz- and CPT-violating modification of QED. Although
the coefficient of the induced Chern-Simons term is in general undetermined,
the nonperturbative theory appears to generate a definite value. However, the
CPT-even radiative corrections in this same formulation of the theory generally
break gauge invariance. We show that gauge invariance may yet be preserved
through the use of a Pauli-Villars regulator, and, contrary to earlier
expectations, this regulator does not necessarily give rise to a vanishing
Chern-Simons term. Instead, two possible values of the Chern-Simons coefficient
are allowed, one zero and one nonzero. This formulation of the theory therefore
allows the coefficient to vanish naturally, in agreement with experimental
observations.Comment: 8 page
Laboratory Bounds on Electron Lorentz Violation
Violations of Lorentz boost symmetry in the electron and photon sectors can
be constrained by studying several different high-energy phenomenon. Although
they may not lead to the strongest bounds numerically, measurements made in
terrestrial laboratories produce the most reliable results. Laboratory bounds
can be based on observations of synchrotron radiation, as well as the observed
absences of vacuum Cerenkov radiation. Using measurements of synchrotron energy
losses at LEP and the survival of TeV photons, we place new bounds on the three
electron Lorentz violation coefficients c_(TJ), at the 3 x 10^(-13) to 6 x
10^(-15) levels.Comment: 18 page
Synchrotron and Inverse Compton Constraints on Lorentz Violations for Electrons
We present a method for constraining Lorentz violation in the electron
sector, based on observations of the photons emitted by high-energy
astrophysical sources. The most important Lorentz-violating operators at the
relevant energies are parameterized by a tensor c^{nu mu) with nine independent
components. If c is nonvanishing, then there may be either a maximum electron
velocity less than the speed of light or a maximum energy for subluminal
electrons; both these quantities will generally depend on the direction of an
electron's motion. From synchrotron radiation, we may infer a lower bound on
the maximum velocity, and from inverse Compton emission, a lower bound on the
maximum subluminal energy. With observational data for both these types of
emission from multiple celestial sources, we may then place bounds on all nine
of the coefficients that make up c. The most stringent bound, on a certain
combination of the coefficients, is at the 6 x 10^(-20) level, and bounds on
the coefficients individually range from the 7 x 10^(-15) level to the 2 x
10^(-17) level. For most of the coefficients, these are the most precise bounds
available, and with newly available data, we can already improve over previous
bounds obtained by the same methods.Comment: 28 page
Lorentz Violation and Synchrotron Radiation
We consider the radiation emitted by an ultrarelativistic charged particle
moving in a magnetic field, in the presence of an additional Lorentz-violating
interaction. In contrast with prior work, we treat a form of Lorentz violation
that is represented by a renormalizable operator. Neglecting the radiative
reaction force, the particle's trajectory can be determined exactly. The
resulting orbit is generally noncircular and does not lie in the place
perpendicular to the magnetic field. We do not consider any Lorentz violation
in the electromagnetic sector, so the radiation from the accelerated charge can
be determined by standard means, and the radiation spectrum will exhibit a
Lorentz-violating directional dependence. Using data on emission from the Crab
nebula, we can set a bound on a particular combination of Lorentz-violating
coefficients at the level.Comment: 14 page
Back-translation for discovering distant protein homologies
Frameshift mutations in protein-coding DNA sequences produce a drastic change
in the resulting protein sequence, which prevents classic protein alignment
methods from revealing the proteins' common origin. Moreover, when a large
number of substitutions are additionally involved in the divergence, the
homology detection becomes difficult even at the DNA level. To cope with this
situation, we propose a novel method to infer distant homology relations of two
proteins, that accounts for frameshift and point mutations that may have
affected the coding sequences. We design a dynamic programming alignment
algorithm over memory-efficient graph representations of the complete set of
putative DNA sequences of each protein, with the goal of determining the two
putative DNA sequences which have the best scoring alignment under a powerful
scoring system designed to reflect the most probable evolutionary process. This
allows us to uncover evolutionary information that is not captured by
traditional alignment methods, which is confirmed by biologically significant
examples.Comment: The 9th International Workshop in Algorithms in Bioinformatics
(WABI), Philadelphia : \'Etats-Unis d'Am\'erique (2009
Clustering with shallow trees
We propose a new method for hierarchical clustering based on the optimisation
of a cost function over trees of limited depth, and we derive a
message--passing method that allows to solve it efficiently. The method and
algorithm can be interpreted as a natural interpolation between two well-known
approaches, namely single linkage and the recently presented Affinity
Propagation. We analyze with this general scheme three biological/medical
structured datasets (human population based on genetic information, proteins
based on sequences and verbal autopsies) and show that the interpolation
technique provides new insight.Comment: 11 pages, 7 figure
CPT and Lorentz violation as signatures for Planck-scale physics
In recent years, the breakdown of spacetime symmetries has been identified as
a promising research field in the context of Planck-scale phenomenology. For
example, various theoretical approaches to the quantum-gravity problem are
known to accommodate minute violations of CPT invariance. This talk covers
various topics within this research area. In particular, some mechanisms for
spacetime-symmetry breaking as well as the Standard-Model Extension (SME) test
framework will be reviewed; the connection between CPT and Lorentz invariance
in quantum field theory will be exposed; and various experimental CPT tests
with emphasis on matter--antimatter comparisons will be discussed.Comment: 6 page
Parallel approach to sliding window sums
Sliding window sums are widely used in bioinformatics applications, including
sequence assembly, k-mer generation, hashing and compression. New vector
algorithms which utilize the advanced vector extension (AVX) instructions
available on modern processors, or the parallel compute units on GPUs and
FPGAs, would provide a significant performance boost for the bioinformatics
applications. We develop a generic vectorized sliding sum algorithm with
speedup for window size w and number of processors P is O(P/w) for a generic
sliding sum. For a sum with commutative operator the speedup is improved to
O(P/log(w)). When applied to the genomic application of minimizer based k-mer
table generation using AVX instructions, we obtain a speedup of over 5X.Comment: 10 pages, 5 figure
Non-local on-shell field redefinition for the SME
This work instigates a study of non-local field mappings within the Lorentz-
and CPT-violating Standard-Model Extension (SME). An example of such a mapping
is constructed explicitly, and the conditions for the existence of its inverse
are investigated. It is demonstrated that the associated field redefinition can
remove b-type Lorentz violation from free SME fermions in certain situations.
These results are employed to obtain explicit expressions for the corresponding
Lorentz-breaking momentum-space eigenspinors and their orthogonality relations.Comment: 12 pages, REVTeX
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