153 research outputs found
Vector-meson contributions do not explain the rate and spectrum in K_L -> pi0 gamma gamma
We analyze the recent NA48 data for the reaction K_L -> pi0 gamma gamma with
and without the assumption of vector meson dominance (VMD). We find that the
data is well described by a three-parameter expression inspired by O(p^6)
chiral perturbation theory. We also find that it is impossible to fit the shape
of the decay distribution and the overall rate simultaneously if one imposes
the VMD constraints on the three parameters. We comment on the different fits
and their implications for the CP-conserving component of the decay K_L -> pi0
e+ e-.Comment: Version accepted for publication on Phys. Rev. D. 19 pages, LaTeX, 8
figures, uses epsf.st
K_S\rightarrow \gamma\gamma , K_L\rightarrow\pi^0\gamma\gamma$ and Unitarity
Agreement between the experimental value and the number predicted via a one-loop chiral perturbation
theory calculation has been cited as a success for the latter. On the other
hand the one-loop prediction for the closely related process has been found to be a factor three below the experimental
value. Using the inputs of unitarity and dispersion relations, we demonstrate
the importance of higher order loop effects to both of these processes.Comment: 20 pages (4 figures available on request), UMHEP-39
Phenomenological Aspects of Isospin Violation in the Nuclear Force
Phenomenological Lagrangians and dimensional power counting are used to
assess isospin violation in the nucleon-nucleon force. The coupling
constants (including the Goldberger-Treiman discrepancy), charge-symmetry
breaking, and meson-mixing models are examined. A one-loop analysis of the
isospin-violating coupling constants is performed using chiral
perturbation theory. Meson-mixing models and the He - H mass difference
are also discussed in the context of naturalness.Comment: 10 pages, latex, 1 figure -- To appear in Physics Letters B --
epsfig.sty require
Resonance saturation for four-nucleon operators
In the modern description of nuclear forces based on chiral effective field
theory, four-nucleon operators with unknown coupling constants appear. These
couplings can be fixed by a fit to the low partial waves of neutron-proton
scattering. We show that the so determined numerical values can be understood
on the basis of phenomenological one-boson-exchange models. We also extract
these values from various modern high accuracy nucleon-nucleon potentials and
demonstrate their consistency and remarkable agreement with the values in the
chiral effective field theory approach. This paves the way for estimating the
low-energy constants of operators with more nucleon fields and/or external
probes.Comment: 16 pp, REVTeX, 3 figure
Aspects of Nucleon Compton Scattering
We consider the spin-averaged nucleon forward Compton scattering amplitude in
heavy baryon chiral perturbation theory including all terms to order . The chiral prediction for the spin-averaged forward Compton scattering
amplitude is in good agreement with the data for photon energies MeV. We also evaluate the nucleon electric and magnetic Compton
polarizabilities to this order and discuss the uncertainties of the various
counter terms entering the chiral expansion of these quantities.Comment: 17 pp, TeX, 7 figures available from the authors, preprint CRN-93/5
Charge asymmetries in e+e- -> pi+ pi- gamma at the phi resonance
We consider the forward-backward pion charge asymmetry for the
e^{+}e^{-}\to\pi^{+}\pi^{-}\gamma process. At tree level we consider
bremsstrahlung and double resonance contributions. Although the latter
contribution is formally sub-leading, it is enhanced at low dipion invariant
mass due to resonant effects. We consider also four alternative models
to describe the final state radiation at the loop level: Resonance Chiral
Perturbation Theory, Unitarized Chiral Perturbation Theory, Kaon Loop Model and
Linear Sigma Model. The last three models yield results compatible with
experimental data. The Kaon Loop Model requires an energy dependent phase to
achieve the agreement.Comment: Revised version, conclusions changed, 11 pages, 8 figs, version to
appear in PL
Deconstructing 1S0 nucleon-nucleon scattering
A distorted-wave method is used to analyse nucleon-nucleon scattering in the
1S0 channel. Effects of one-pion exchange are removed from the empirical phase
shift to all orders by using a modified effective-range expansion. Two-pion
exchange is then subtracted in the distorted-wave Born approximation, with
matrix elements taken between scattering waves for the one-pion exchange
potential. The residual short-range interaction shows a very rapid energy
dependence for kinetic energies above about 100 MeV, suggesting that the
breakdown scale of the corresponding effective theory is only 270MeV. This may
signal the need to include the Delta resonance as an explicit degree of freedom
in order to describe scattering at these energies. An alternative strategy of
keeping the cutoff finite to reduce large, but finite, contributions from the
long-range forces is also discussed.Comment: 10 pages, 2 figures (introduction revised, references added; version
to appear in EPJA
Compton Scattering and the Spin Structure of the Nucleon at Low Energies
We analyze polarized Compton scattering which provides information on the
spin-structure of the nucleon. For scattering processes with photon energies up
to 100 MeV the spin-structure dependence can be encoded into four independent
parameters-the so-called spin-polarizabilities of the
nucleon, which we calculate within the framework of the "small scale expansion"
in SU(2) baryon chiral perturbation theory. Specific application is made to
"forward" and "backward" spin- polarizabilities.Comment: 8 pages revtex file, separation between pion-pole and regular
contributions detailed + minor wording changes, results and conclusions
unchange
Association of labor induction with offspring risk of autism spectrum disorders
IMPORTANCE: Induction of labor is a frequently performed obstetrical
intervention. It would thus be of great concern if reported associations between
labor induction and offspring risk of autism spectrum disorders (ASD) reflected
causal influence. OBJECTIVE: To assess the associations of labor induction with
ASD, comparing differentially exposed relatives (siblings and cousins discordant
for induction). DESIGN, SETTING, AND PARTICIPANTS: Follow-up of all live births
in Sweden between 1992 and 2005, defined in the Medical Birth Register. The
register was linked to population registers of familial relations, inpatient and
outpatient visits, and education records. Diagnoses of ASD were from 2001 through
2013, and data were analyzed in the 2015-2016 year. EXPOSURES: Induction of
labor. MAIN OUTCOMES AND MEASURES: Autism spectrum disorders identified by
diagnoses from inpatient and outpatient records between 2001 and 2013. Hazard
ratios (HRs) quantified the association between labor induction and offspring
ASD. In addition to considering a wide range of measured confounders, comparison
of exposure-discordant births to the same woman allowed additional control for
all unmeasured factors shared by siblings. RESULTS: The full cohort included
1362950 births, of which 22077 offspring (1.6%) were diagnosed with ASD by ages 8
years through 21 years. In conventional models of the full cohort, associations
between labor induction and offspring ASD were attenuated but remained
statistically significant after adjustment for measured potential confounders
(HR, 1.19; 95% CI, 1.13-1.24). When comparison was made within siblings whose
births were discordant with respect to induction, thus accounting for all
environmental and genetic factors shared by siblings, labor induction was no
longer associated with offspring ASD (HR, 0.99; 95% CI, 0.88-1.10). CONCLUSIONS
AND RELEVANCE: In this nationwide sample of live births we observed no
association between induction of labor and offspring ASD within sibling
comparison. Our findings suggest that concern for ASD should not factor into the
clinical decision about whether to induce labor.NonePublishe
Scalar meson dynamics in Chiral Perturbation Theory
A comparison of the linear sigma model (LM) and Chiral Perturbation
Theory (ChPT) predictions for pion and kaon dynamics is presented. Lowest and
next-to-leading order terms in the ChPT amplitudes are reproduced if one
restricts to scalar resonance exchange. Some low energy constants of the order
ChPT Lagrangian are fixed in terms of scalar meson masses. Present values
of these low energy constants are compatible with the LM dynamics. We
conclude that more accurate values would be most useful either to falsify the
LM or to show its capability to shed some light on the controversial
scalar physics.Comment: 9 pages, REVTeX 4.0. Final version accepted for publicatio
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