4,140 research outputs found
Papapetrou Energy-Momentum Tensor for Chern-Simons Modified Gravity
We construct a conserved, symmetric energy-momentum (pseudo-)tensor for
Chern-Simons modified gravity, thus demonstrating that the theory is Lorentz
invariant. The tensor is discussed in relation to other gravitational
energy-momentum tensors and analyzed for the Schwarzschild, Reissner-Nordstrom,
and FRW solutions. To our knowledge this is the first confirmation that the
Reissner-Nordstrom and FRW metrics are solutions of the modified theory.Comment: 8 pages; typos corrected, references fixed, some calculations
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Analytic lymph node number establishes staging accuracy by occult tumor burden in colorectal cancer.
BACKGROUND AND OBJECTIVES: Recurrence in lymph node-negative (pN0) colorectal cancer suggests the presence of undetected occult metastases. Occult tumor burden in nodes estimated by GUCY2C RT-qPCR predicts risk of disease recurrence. This study explored the impact of the number of nodes analyzed by RT-qPCR (analytic) on the prognostic utility of occult tumor burden.
METHODS: Lymph nodes (range: 2-159) from 282 prospectively enrolled pN0 colorectal cancer patients, followed for a median of 24 months (range: 2-63), were analyzed by GUCY2C RT-qPCR. Prognostic risk categorization defined using occult tumor burden was the primary outcome measure. Association of prognostic variables and risk category were defined by multivariable polytomous and semi-parametric polytomous logistic regression.
RESULTS: Occult tumor burden stratified this pN0 cohort into categories of low (60%; recurrence rate (RR) = 2.3% [95% CI 0.1-4.5%]), intermediate (31%; RR = 33.3% [23.7-44.1%]), and high (9%; RR = 68.0% [46.5-85.1%], P \u3c 0.001) risk of recurrence. Beyond race and T stage, the number of analytic nodes was an independent marker of risk category (P \u3c 0.001). When \u3e12 nodes were analyzed, occult tumor burden almost completely resolved prognostic risk classification of pN0 patients.
CONCLUSIONS: The prognostic utility of occult tumor burden assessed by GUCY2C RT-qPCR is dependent on the number of analytic lymph nodes
Collinearity, convergence and cancelling infrared divergences
The Lee-Nauenberg theorem is a fundamental quantum mechanical result which
provides the standard theoretical response to the problem of collinear and
infrared divergences. Its argument, that the divergences due to massless
charged particles can be removed by summing over degenerate states, has been
successfully applied to systems with final state degeneracies such as LEP
processes. If there are massless particles in both the initial and final
states, as will be the case at the LHC, the theorem requires the incorporation
of disconnected diagrams which produce connected interference effects at the
level of the cross-section. However, this aspect of the theory has never been
fully tested in the calculation of a cross-section. We show through explicit
examples that in such cases the theorem introduces a divergent series of
diagrams and hence fails to cancel the infrared divergences. It is also
demonstrated that the widespread practice of treating soft infrared divergences
by the Bloch-Nordsieck method and handling collinear divergences by the
Lee-Nauenberg method is not consistent in such cases.Comment: 29 pages, 17 figure
Constraining the Metallicity of the Low Density Lyman-alpha Forest Using OVI Absorption
We search for OVI absorption in a Keck HIRES spectrum of the z=3.62 quasar
Q1422+231. Comparison of CIV measurements to cosmological simulations shows
that \lya forest absorbers with N_HI > 10^{14.5} have [C/H]~=-2.5, for the UV
background spectrum of Haardt & Madau (HM). Lower column density absorption
arises in lower density gas, where OVI is the most sensitive metal tracer.
Since OVI lines lie at wavelengths contaminated by Lyman series absorption, we
interpret our Q1422 results by comparing to artificial spectra drawn from an
SPH simulation of a Lambda-dominated CDM model. A search for deep, narrow
features in Q1422 yields only a few candidate OVI lines, statistically
consistent with the number in artificial spectra with no metals; spectra
generated with the HM background and [O/H] >= -2.5 predict too many narrow
lines. However, applying the optical depth ratio technique of Songaila (1998),
we DO find significant OVI associated with CIV systems; matching Q1422 requires
[O/C]~=+0.5, implying [O/H]~=-2.0. Taken together these results imply that (a)
the metallicity in the low density IGM is at least a factor of three below that
in the overdense regions where CIV absorption is detectable, and (b) oxygen is
overabundant in these regions, consistent with the enrichment pattern of old
halo stars. If the UV background is heavily truncated above 4 Ry, an
implausibly high oxygen overabundance ([O/C]>+2) is required by the data; thus
a majority of the volume of the universe must have undergone helium
reionization by z=3.(Abridged)Comment: Submitted to ApJ, 48 pp including 14 ps figures, uses aaspp4.st
InP homojunction solar cell performance on the LIPS 3 flight experiment
Performance data for the NASA Lewis Research Center indium phosphide n+p homojunction solar cell module on the LIPS 3 Flight Experiment is presented. The objective of the experiment is to measure the performance of InP cells in the natural radiation environment of the 1100 km altitude, 60+ deg inclination orbit. Analysis of flight data indicates that the performance of the four cells throughout the first year is near expected values. No degradation in short-circuit current was seen, as was expected from radiation tolerance studies of similar cells. Details of the cell structure and flight module design are discussed. The results of the temperature dependency and radiation tolerance studies necessary for normalization and analysis of the data are included
Gravitational waves from an early matter era
We investigate the generation of gravitational waves due to the gravitational
instability of primordial density perturbations in an early matter-dominated
era which could be detectable by experiments such as LIGO and LISA. We use
relativistic perturbation theory to give analytic estimates of the tensor
perturbations generated at second order by linear density perturbations. We
find that large enhancement factors with respect to the naive second-order
estimate are possible due to the growth of density perturbations on sub-Hubble
scales. However very large enhancement factors coincide with a breakdown of
linear theory for density perturbations on small scales. To produce a
primordial gravitational wave background that would be detectable with LIGO or
LISA from density perturbations in the linear regime requires primordial
comoving curvature perturbations on small scales of order 0.02 for Advanced
LIGO or 0.005 for LISA, otherwise numerical calculations of the non-linear
evolution on sub-Hubble scales are required.Comment: 23 pages, 2 figure
Radiative Transitions in Charmonium from Lattice QCD
Radiative transitions between charmonium states offer an insight into the
internal structure of heavy-quark bound states within QCD. We compute, for the
first time within lattice QCD, the transition form-factors of various
multipolarities between the lightest few charmonium states. In addition, we
compute the experimentally unobservable, but physically interesting vector
form-factors of the and .
To this end we apply an ambitious combination of lattice techniques,
computing three-point functions with heavy domain wall fermions on an
anisotropic lattice within the quenched approximation. With an anisotropy
at we find a reasonable gross spectrum and a
hyperfine splitting , which compares favourably with
other improved actions.
In general, after extrapolation of lattice data at non-zero to the
photopoint, our results agree within errors with all well measured experimental
values. Furthermore, results are compared with the expectations of simple quark
models where we find that many features are in agreement; beyond this we
propose the possibility of constraining such models using our extracted values
of physically unobservable quantities such as the quadrupole moment.
We conclude that our methods are successful and propose to apply them to the
problem of radiative transitions involving hybrid mesons, with the eventual
goal of predicting hybrid meson photoproduction rates at the GlueX experiment.Comment: modified version as publishe
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