65,058 research outputs found
Polyimides with pendant alkyl groups
The effect on selected polyimide properties when pendant alkyl groups were attached to the polymer backbone was investigated. A series of polymers were prepared using benzophenone tetracarboxylic acid dianhydride (BTDA) and seven different p-alkyl-m,p'-diaminobenzophenone monomers. The alkyl groups varied in length from C(1) (methyl) to C(9) (nonyl). The polyimide prepared from BTDA and m,p'-diaminobenzophenone was included as a control. All polymers were characterized by various chromatographic, spectroscopic, thermal, and mechanical techniques. Increasing the length of the pendant alkyl group resulted in a systematic decrease in glass transition temperature (Tg) for vacuum cured films. A 70 C decrease in Tg to 193 C was observed for the nonyl polymer compared to the Tg for the control. A corresponding systematic increase in Tg indicative of crosslinking, was observed for air cured films. Thermogravimetric analysis revealed a slight sacrifice in thermal stability with increasing alkyl length. No improvement in film toughness was observed
Analysis of the extraterrestrial life detection problem
Analysis of extraterrestrial life detection proble
Chiral extrapolation and physical insights
It has recently been established that finite-range regularisation in chiral
effective field theory enables the accurate extrapolation of modern lattice QCD
results to the chiral regime. We review some of the highlights of
extrapolations of quenched lattice QCD results, including spectroscopy and
magnetic moments. The resonance displays peculiar chiral features in
the quenched theory which can be exploited to demonstrate the presence of
significant chiral corrections.Comment: 6 pages, 5 figures, presented at LHP2003, Cairns, Australi
Hadron structure on the back of an envelope
In order to remove a little of the mysticism surrounding the issue of
strangeness in the nucleon, we present simple, physically transparent estimates
of both the strange magnetic moment and charge radius of the proton. Although
simple, the estimates are in quite good agreement with sophisticated
calculations using the latest input from lattice QCD. We further explore the
possible size of systematic uncertainties associated with charge symmetry
violation (CSV) in the recent precise determination of the strange magnetic
moment of the proton. We find that CSV acts to increase the error estimate by
0.003 \mu_N such that G_M^s = -0.046 +/- 0.022 \mu_N.Comment: 9 pages, 1 figure, Invited talk at First Workshop on Quark-Hadron
Duality and the Transition to pQCD, Frascati, June 6-8 200
Chiral Symmetry and the Intrinsic Structure of the Nucleon
Understanding hadron structure within the framework of QCD is an extremely
challenging problem. In order to solve it, it is vital that our thinking should
be guided by the best available insight. Our purpose here is to explain the
model independent consequences of the approximate chiral symmetry of QCD for
two famous results concerning the structure of the nucleon. We show that both
the apparent success of the constituent quark model in reproducing the ratio of
the proton to neutron magnetic moments and the apparent success of the Foldy
term in reproducing the observed charge radius of the neutron are coincidental.
That is, a relatively small change of the current quark mass would spoil both
results.Comment: RevTeX, 10 pages, 2 figure
Extrapolation of lattice QCD results beyond the power-counting regime
Resummation of the chiral expansion is necessary to make accurate contact
with current lattice simulation results of full QCD. Resummation techniques
including relativistic formulations of chiral effective field theory and
finite-range regularization (FRR) techniques are reviewed, with an emphasis on
using lattice simulation results to constrain the parameters of the chiral
expansion. We illustrate how the chiral extrapolation problem has been solved
and use FRR techniques to identify the power-counting regime (PCR) of chiral
perturbation theory. To fourth-order in the expansion at the 1% tolerance
level, we find 0 \le m_pi \le 0.18 GeV for the PCR, extending only a small
distance beyond the physical pion mass.Comment: 12 pages, 5 figures, plenary talk at BARYONS 2004, Paris, Oct. 25-2
Chiral extrapolations for nucleon magnetic moments
Lattice QCD simulations have made significant progress in the calculation of
nucleon electromagnetic form factors in the chiral regime in recent years. With
simulation results achieving pion masses of order ~180 MeV, there is an
apparent challenge as to how the physical regime is approached. By using
contemporary methods in chiral effective field theory, both the quark-mass and
finite-volume dependence of the isovector nucleon magnetic moment are carefully
examined. The extrapolation to the physical point yields a result that is
compatible with experiment, albeit with a combined statistical and systematic
uncertainty of 10%. The extrapolation shows a strong finite-volume dependence;
lattice sizes of L > 5 fm must be used to simulate results within 2% of the
infinite-volume result for the magnetic moment at the physical pion mass.Comment: 7 pages, 12 figures, 1 tabl
- …