3,249 research outputs found
Pore fluid constraints on deep ocean temperature and salinity during the Last Glacial Maximum
Pore water records of δ^(18)O and [Cl] from ODP Site 1063A on the Bermuda Rise constrain the change in seawater δ^(18)O and salinity from the Last Glacial Maximum (LGM) to the Holocene to be 0.75±0.05‰ and 2.5±0.1% respectively. Coupled with a measured benthic foraminiferal δ^(18)O change, this result means that bottom waters were 4.6±0.8°C cooler than the Holocene at the LGM and therefore at or near the seawater freezing point. Coupled δ^(18)O and chlorinity results give an extrapolated mean ocean LGM to Holocene change in δ^(18)O of 0.95±0.09‰. These data also constrain the past southern source deep‐water salinity to be 35.76±0.04 psu, which is within error of the mean deep ocean value for this time
alpha^2 corrections to parapositronium decay: a detailed description
We present details of our recent calculation of alpha^2 corrections to the
parapositronium decay into two photons. These corrections are rather small and
our final result for the parapositronium lifetime agrees well with the most
recent measurement. Implications for orthopositronium decays are briefly
discussed.Comment: 18 pages, late
Recoil corrections in the hydrogen isoelectronic sequence
A version of the Bethe-Salpeter equation appropriate for calculating recoil
corrections in highly charged hydrogenlike ions is presented. The nucleus is
treated as a scalar particle of charge Z, and the electron treated
relativistically. The known recoil corrections of order are
derived in both this formalism and in NRQED
The "recoil" correction of order to hyperfine splitting of positronium ground state
The "recoil" correction of order to the hyperfine splitting of
positronium ground state was found. The formalism employed is based on the
noncovariant perturbation theory in QED. Equation for two-particle component of
full (many-body) wave function is used, in which effective Hamiltonian depends
on the energy of a system. The effective Hamiltonian is not restricted to the
nonrelativistic region, so there is no need in any regularization. To evaluate
integrals over loop momenta, they are divided into "hard" and "soft" parts,
coming from large and small momenta respectively. Soft contributions were found
analytically, and hard ones are evaluated by numerical integration. Some soft
terms due to the retardation cancel each other. To calculate the "hard"
contributions, a great number of noncovariant graphs is replaced by only a few
covariant ones. The hard contribution was found in two ways. The first way is
to evaluate contributions of separate graphs, using the Coulomb gauge. The
second one is to calculate full hard contribution as a whole using the Feynman
gauge. The final result for the "recoil" correction is 0.381(6) m\al^6 and
agrees with those of previous papers. Diagram-to-diagram comparison with the
revised results of Adkins&Sapirstein was done. All the results agree, so the
"recoil" correction is now firmly established. This means a considerable
disagreement with the experimental data.Comment: 28 pages, latex including latex figure
The Single Photon Annihilation Contributions to the Positronium Hyperfine Splitting to Order
The single photon annihilation contributions for the positronium ground state
hyperfine splitting are calculated analytically to order using
NRQED. Based on intuitive physical arguments the same result can also be
determined by a trivial calculation using results from existing literature. Our
result completes the hyperfine splitting calculation to order . We
compare the theoretical prediction with the most recent experimental
measurement.Comment: 8 pages, latex, two eps figures include
The role of ocean cooling in setting glacial southern source bottom water salinity
At the Last Glacial Maximum (LGM), the salinity contrast between northern source deep water and southern source bottom water was reversed with respect to the contrast today. Additionally, Glacial Southern Source Bottom Water (GSSBW) was saltier than Antarctic Bottom Water (AABW), over and above the difference implied by the mean sea level change. This study examines to what extent cold temperatures, through their effect on ice formation and melting, could have caused these differences. Computational sensitivity experiments using a coupled ice shelf cavity–sea ice–ocean model are performed in a Weddell Sea domain, as a representative case study for bottom water formation originating from Antarctic continental shelves. Ocean temperatures at the domain open boundaries are systematically lowered to determine the sensitivity of Weddell Sea water mass properties to a range of cool ocean temperatures. The steady state salinities differ between experiments due to temperature-induced responses of ice shelf and sea ice melting and freezing, evaporation and open boundary fluxes. The results of the experiments indicate that reduced ocean temperature can explain up to 30% of the salinity difference between GSSBW and AABW, primarily due to decreased ice shelf melting. The smallest and most exposed ice shelves, which abut narrow continental shelves, have the greatest sensitivity to the ocean temperature changes, suggesting that at the LGM there could have been a shift in geographical site dominance in bottom water formation. More sea ice is formed and exported in the cold ocean experiments, but the effect of this on salinity is negated by an equal magnitude reduction in evaporation
Dynamical description of vesicle growth and shape change
We systematize and extend the description of vesicle growth and shape change
using linear nonequilibrium thermodynamics. By restricting the study to shape
changes from spheres to axisymmetric ellipsoids, we are able to give a
consistent formulation which includes the lateral tension of the vesicle
membrane. This allows us to generalize and correct a previous calculation. Our
present calculations suggest that, for small growing vesicles, a prolate
ellipsoidal shape should be favored over oblate ellipsoids, whereas for large
growing vesicles oblates should be favored over prolates. The validity of this
prediction is examined in the light of the various assumptions made in its
derivation.Comment: 6 page
Precision Study of Positronium: Testing Bound State QED Theory
As an unstable light pure leptonic system, positronium is a very specific
probe atom to test bound state QED. In contrast to ordinary QED for free
leptons, the bound state QED theory is not so well understood and bound state
approaches deserve highly accurate tests. We present a brief overview of
precision studies of positronium paying special attention to uncertainties of
theory as well as comparison of theory and experiment. We also consider in
detail advantages and disadvantages of positronium tests compared to other QED
experiments.Comment: A talk presented at Workshop on Positronium Physics (ETH Zurich, May
30-31, 2003
Stability and Representation Dependence of the Quantum Skyrmion
A constructive realization of Skyrme's conjecture that an effective pion mass
``may arise as a self consistent quantal effect'' based on an ab initio quantum
treatment of the Skyrme model is presented. In this quantum mechanical Skyrme
model the spectrum of states with , which appears in the collective
quantization, terminates without any infinite tower of unphysical states. The
termination point depends on the model parameters and the dimension of the
SU(2) representation. Representations, in which the nucleon and
resonance are the only stable states, exist. The model is developed for both
irreducible and reducible representations of general dimension. States with
spin larger than 1/2 are shown to be deformed. The representation dependence of
the baryon observables is illustrated numerically.Comment: 19 pages, Late
Order \alpha^2 corrections to the decay rate of orthopositronium
Order \alpha^2 corrections to the decay rate of orthopositronium are
calculated in the framework of nonrelativistic QED. The resulting contribution
is found to be in significant disagreement with one set of experimental
measurements though another experiment is in agreement with theory.Comment: 7 pages, 1 figur
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