1,259 research outputs found
Casimir dependence of transverse distribution of pairs produced from a strong constant chromo-electric background field
The transverse distribution of gluon and quark-antiquark pairs produced from
a strong constant chromo-electric field depends on two gauge invariant
quantities, and , as shown earlier in
[G.C. Nayak and P. van Nieuwenhuizen, Phys. Rev. D 71, 125001 (2005)] for
gluons and in [G.C. Nayak, Phys. Rev. D 72, 125010 (2005)] for quarks. Here, we
discuss the explicit dependence of the distribution on the second Casimir
invariant, C_2, and show the dependence is at most a 15% effect.Comment: 5 fig
Measuring x-ray polarization in the presence of systematic effects: Known background
The prospects for accomplishing x-ray polarization measurements of
astronomical sources have grown in recent years, after a hiatus of more than 37
years. Unfortunately, accompanying this long hiatus has been some confusion
over the statistical uncertainties associated with x-ray polarization
measurements of these sources. We have initiated a program to perform the
detailed calculations that will offer insights into the uncertainties
associated with x-ray polarization measurements. Here we describe a
mathematical formalism for determining the 1- and 2-parameter errors in the
magnitude and position angle of x-ray (linear) polarization in the presence of
a (polarized or unpolarized) background. We further review relevant
statistics-including clearly distinguishing between the Minimum Detectable
Polarization (MDP) and the accuracy of a polarization measurement.Comment: 12 pages, 4 figures, for SPIE conference proceeding
On understanding the figures of merit for detection and measurement of x-ray polarization
The prospects for accomplishing X-ray polarization measurements appear to
have grown in recent years after a more than 35-year hiatus. Unfortunately,
this long hiatus has brought with it some confusion over the statistical
uncertainties associated with polarization measurements of astronomical
sources. The heart of this confusion stems from a misunderstanding (or
potential misunderstanding) of a standard figure of merit-the minimum
detectable polarization (MDP)-that one of us introduced many years ago. We
review the relevant statistics, and quantify the differences between the MDP
and the uncertainty of an actual polarization measurement. We discuss the
implications for future missions.Comment: 5 pages, 2 figures, to be presented at SPIE conference 7732 (paper
13), corrected typo
Quantum Electro and Chromodynamics treated by Thompson's heuristic approach
In this work we apply Thompson's method (of the dimensions and scales) to
study some features of the Quantum Electro and Chromodynamics. This heuristic
method can be considered as a simple and alternative way to the Renormalisation
Group (R.G.) approach and when applied to QED-lagrangian is able to obtain in a
first approximation both the running coupling constant behavior of alpha(mu)
and the mass m(mu).The calculations are evaluated just at d_c=4, where d_c is
the upper critical dimension of the problem, so that we obtain the logarithmic
behavior both for the coupling alpha and the excess of mass Delta m on the
energy scale mu. Although our results are well-known in the vast literature of
field theories,it seems that one of the advantages of Thompson's method, beyond
its simplicity is that it is able to extract directly from QED-lagrangian the
physical (finite) behavior of alpha(mu) and m(mu), bypassing hard problems of
divergences which normally appear in the conventional renormalisation schemes
applied to field theories like QED. Quantum Chromodynamics (QCD) is also
treated by the present method in order to obtain the quark condensate value.
Besides this, the method is also able to evaluate the vacuum pressure at the
boundary of the nucleon. This is done by assumming a step function behavior for
the running coupling constant of the QCD, which fits nicely to some quantities
related to the strong interaction evaluated through the MIT-bag model.Comment: RevTex, 25 pages, no figure
Methods of optimizing X-ray optical prescriptions for wide-field applications
We are working on the development of a method for optimizing wide-field X-ray
telescope mirror prescriptions, including polynomial coefficients, mirror shell
relative displacements, and (assuming 4 focal plane detectors) detector
placement along the optical axis and detector tilt. With our methods, we hope
to reduce number of Monte-Carlo ray traces required to search the
multi-dimensional design parameter space, and to lessen the complexity of
finding the optimum design parameters in that space. Regarding higher order
polynomial terms as small perturbations of an underlying Wolter I optic design,
we begin by using the results of Monte-Carlo ray traces to devise trial
analytic functions, for an individual Wolter I mirror shell, that can be used
to represent the spatial resolution on an arbitrary focal surface. We then
introduce a notation and tools for Monte-Carlo ray tracing of a polynomial
mirror shell prescription which permits the polynomial coefficients to remain
symbolic. In principle, given a set of parameters defining the underlying
Wolter I optics, a single set of Monte-Carlo ray traces are then sufficient to
determine the polymonial coefficients through the solution of a large set of
linear equations in the symbolic coefficients. We describe the present status
of this development effort.Comment: 14 pages, to be presented at SPIE conference 7732 (paper 93
X-ray Polarimetry: a new window on the high energy sky
Polarimetry is widely considered a powerful observational technique in X-ray
astronomy, useful to enhance our understanding of the emission mechanisms,
geometry and magnetic field arrangement of many compact objects. However, the
lack of suitable sensitive instrumentation in the X-ray energy band has been
the limiting factor for its development in the last three decades. Up to now,
polarization measurements have been made exclusively with Bragg diffraction at
45 degrees or Compton scattering at 90 degrees and the only unambiguous
detection of X-ray polarization has been obtained for one of the brightest
object in the X-ray sky, the Crab Nebula. Only recently, with the development
of a new class of high sensitivity imaging detectors, the possibility to
exploit the photoemission process to measure the photon polarization has become
a reality. We will report on the performance of an imaging X-ray polarimeter
based on photoelectric effect. The device derives the polarization information
from the track of the photoelectrons imaged by a finely subdivided Gas Pixel
Detector. It has a great sensitivity even with telescopes of modest area and
can perform simultaneously good imaging, moderate spectroscopy and high rate
timing. Being truly 2D it is non-dispersive and does not require any rotation.
This device is included in the scientific payload of many proposals of
satellite mission which have the potential to unveil polarimetry also in X-rays
in a few years.Comment: Accepted for publication by NIMA. Proceeding of the 1st International
Conference on "Frontiers in Diagnostic Technologies", November 25-27 2009,
Frascati (Italy). 11 pages, 4 figures, 1 table
Singularity-Free Electrodynamics for Point Charges and Dipoles: Classical Model for Electron Self-Energy and Spin
It is shown how point charges and point dipoles with finite self-energies can
be accomodated into classical electrodynamics. The key idea is the introduction
of constitutive relations for the electromagnetic vacuum, which actually
mirrors the physical reality of vacuum polarization. Our results reduce to
conventional electrodynamics for scales large compared to the classical
electron radius cm. A classical simulation for a
structureless electron is proposed, with the appropriate values of mass, spin
and magnetic moment.Comment: 3 page
Measurements with the Chandra X-Ray Observatory's flight contamination monitor
NASA's Chandra X-ray Observatory includes a Flight Contamination Monitor
(FCM), a system of 16 radioactive calibration sources mounted to the inside of
the Observatory's forward contamination cover. The purpose of the FCM is to
verify the ground-to-orbit transfer of the Chandra flux scale, through
comparison of data acquired during the ground calibration with those obtained
in orbit, immediately prior to opening the Observatory's sun-shade door. Here
we report results of these measurements, which place limits on the change in
mirror--detector system response and, hence, on any accumulation of molecular
contamination on the mirrors' iridium-coated surfaces.Comment: 7pages,8figures,for SPIE 4012, paper 7
- …