24,335 research outputs found
String-derived renormalization of Yang-Mills theory
We review the application of bosonic string techniques to the calculation of
renormalization constants and effective actions in Yang-Mills theory. We
display the multiloop string formulas needed to compute Yang-Mills amplitudes,
and we discuss how the renormalizations of proper vertices can be extracted in
the field theory limit. We show how string techniques lead to the background
field method in field theory, and indicate how the gauge invariance of the
multiloop effective action can be inferred form the string formalism.
(Proceedings of the 29th International Symposium on the Theory of Elementary
Particles, Buckow (Germany), Aug.-Sept. 1995. Preprint DFTT 04/96)Comment: 11 pages. Latex, uses espcrc2.sty. Proceedings Buckow '9
Intrinsic colors and ages of extremely red elliptical galaxies at high redshift
In order to know the formation epoch of the oldest elliptical galaxies as a
function of mass and observed redshift, a statistical analysis for 333
extremely red objects (EROs) classified as old galaxies (OGs) at 0.8<z<2.3 is
carried out. Once we get M_V and (B-V) at rest for each galaxy, we calculate
the average variation of this intrinsic color with redshift and derive the
average age through a synthesis model (the code for the calculation of the age
has been made publicly available). The average gradient of the (B-V) color at
rest of EROs/OGs is 0.07-0.10 Gyr^{-1} for a fixed luminosity. The stars in
these extremely red elliptical galaxies were formed when the Universe was ~2
Gyr old on average. We have not found a significant enough dependence on the
observed redshift and stellar mass: dt_{formation}/dt_{observed}=-0.46+/-0.32,
dt_{formation}/(d log_10 M_*)=-0.81+/-0.98 Gyr. This fits a scenario in which
the stellar formation of the objects that we denominate as EROs-OGs is more
intense at higher redshifts, at which the stellar populations of the most
massive galaxies form earlier than or at the same time as less massive
galaxies.Comment: accepted to be published in A
String techniques for the calculation of renormalization constants in field theory
We describe a set of methods to calculate gauge theory renormalization
constants from string theory, all based on a consistent prescription to
continue off shell open bosonic string amplitudes. We prove the consistency of
our prescription by explicitly evaluating the renormalizations of the two,
three and four-gluon amplitudes, and showing that they obey the appropriate
Ward identities. The field theory limit thus performed corresponds to the
background field method in Feynman gauge. We identify precisely the regions in
string moduli space that correspond to different classes of Feynman diagrams,
and in particular we show how to isolate contributions to the effective action.
Ultraviolet divergent terms are then encoded in a single string integral over
the modular parameter . Finally, we derive a multiloop expression for the
effective action by computing the partition function of an open bosonic string
interacting with an external non-abelian background gauge field.Comment: 54 pages, Latex, uses FEYNMAN.te
Local polynomial regression for circular predictors
We consider local smoothing of datasets where the design space is the d-dimensional (d >= 1) torus and the response variable is real-valued. Our purpose is to extend least squares local polynomial fitting to this situation. We give both theoretical and empirical results
Many-Electron Systems with Constrained Current
A formulation for transport in an inhomogeneous, interacting electron gas is
described. Electronic current is induced by a constraint condition imposed as a
vector Lagrange multiplier. Constrained minimization of the total energy
functional on the manifold of an arbitrary constant current leads to a
many-electron Schroedinger equation with a complex, momentum-dependent
potential. Constant current Hartree-Fock and Kohn-Sham approximations are
formulated within the method and application to transport for quantum wires is
developed. No appeal is made to near equilibrium conditions or other
approximations allowing development of a general ab initio electronic transport
formulation
On boosting kernel regression
In this paper we propose a simple multistep regression smoother which is constructed in an iterative manner, by learning the Nadaraya-Watson estimator with L-2 boosting. We find, in both theoretical analysis and simulation experiments, that the bias converges exponentially fast. and the variance diverges exponentially slow. The first boosting step is analysed in more detail, giving asymptotic expressions as functions of the smoothing parameter, and relationships with previous work are explored. Practical performance is illustrated by both simulated and real data
Searching for pulsed emission from XTE J0929-314 at high radio frequencies
The aim of this work is to search for radio signals in the quiescent phase of
accreting millisecond X-ray pulsars, in this way giving an ultimate proof of
the recycling model, thereby unambiguously establishing that accreting
millisecond X-ray pulsars are the progenitors of radio millisecond pulsars.
To overcome the possible free-free absorption caused by matter surrounding
accreting millisecond X-ray pulsars in their quiescence phase, we performed the
observations at high frequencies. Making use of particularly precise orbital
and spin parameters obtained from X-ray observations, we carried out a deep
search for radio-pulsed emission from the accreting millisecond X-ray pulsar
XTE J0929-314 in three steps, correcting for the effect of the dispersion due
to the interstellar medium, eliminating the orbital motions effects, and
finally folding the time series.
No radio pulsation is present in the analyzed data down to a limit of 68
microJy at 6.4 GHz and 26 microJy at 8.5 GHz.
We discuss several mechanisms that could prevent the detection, concluding
that beaming factor and intrinsic low luminosity are the most likely
explanations.Comment: 7 pages, 4 figures. Accepted for publication in Astronomy &
Astrophysic
Current-Voltage Curves for Molecular Junctions Computed Using All-Electron Basis Sets
We present current-voltage (I-V) curves computed using all-electron basis
sets on the conducting molecule. The all-electron results are very similar to
previous results obtained using effective core potentials (ECP). A hybrid
integration scheme is used that keeps the all-electron calculations cost
competitive with respect to the ECP calculations. By neglecting the coupling of
states to the contacts below a fixed energy cutoff, the density matrix for the
core electrons can be evaluated analytically. The full density matrix is formed
by adding this core contribution to the valence part that is evaluated
numerically. Expanding the definition of the core in the all-electron
calculations significantly reduces the computational effort and, up to biases
of about 2 V, the results are very similar to those obtained using more
rigorous approaches. The convergence of the I-V curves and transmission
coefficients with respect to basis set is discussed. The addition of diffuse
functions is critical in approaching basis set completeness
Kernel density estimation on the torus
Kernel density estimation for multivariate, circular data has been formulated only when the sample space is the sphere, but theory for the torus would also be useful. For data lying on a d-dimensional torus (d >= 1), we discuss kernel estimation of a density, its mixed partial derivatives, and their squared functionals. We introduce a specific class of product kernels whose order is suitably defined in such a way to obtain L-2-risk formulas whose structure can be compared to their Euclidean counterparts. Our kernels are based on circular densities; however, we also discuss smaller bias estimation involving negative kernels which are functions of circular densities. Practical rules for selecting the smoothing degree, based on cross-validation, bootstrap and plug-in ideas are derived. Moreover, we provide specific results on the use of kernels based on the von Mises density. Finally, real-data examples and simulation studies illustrate the findings
- …