169 research outputs found
Higher Order Processes in Electromagnetic Production of Electron Positron Pairs in Relativistic Heavy Ion Collisions
We study higher-order effects in the electromagnetic production of
electron-positron pairs in relativistic heavy ion collisions. Treating the
field of the heavy ions as an external field and neglecting the interaction
among electrons and positrons, we show that the -pair creation amplitude is
the antisymmetrised product of one-pair creation amplitudes and the vacuum
amplitude. Neglecting contributions coming from exchange terms, we show that
the total probability for pairs is approximately a Poisson distribution. We
investigate further the structure of the reduced one-pair amplitude,
concentrating especially on multiple-particle corrections. We calculate the
first of these corrections in second order Magnus theory based on our previous
result in second-order Born approximation for impact parameter zero.
Explicit calculations show that the total probability is increased up to 10 \%
by this correction for realistic collider parameters. The calculations can also
be used to confirm the use of the Poisson distribution for the total
probability.Comment: 29 pages RevTeX and 12 uuencoded figures (compressed postscript
On the Testing of Seismicity Models
Recently a likelihood-based methodology has been developed by the
Collaboratory for the Study of Earthquake Predictability (CSEP) with a view to
testing and ranking seismicity models. We analyze this approach from the
standpoint of possible applications to hazard analysis. We arrive at the
conclusion that model testing can be made more efficient by focusing on some
integral characteristics of the seismicity distribution. This is achieved
either in the likelihood framework but with economical and physically
reasonable coarsening of the phase space or by choosing a suitable measure of
closeness between empirical and model seismicity rate in this space.Comment: To appear at Acta Geophysic
Coulomb Effects on Electromagnetic Pair Production in Ultrarelativistic Heavy-Ion Collisions
We discuss the implications of the eikonal amplitude on the pair production
probability in ultrarelativistic heavy-ion transits. In this context the
Weizs\"acker-Williams method is shown to be exact in the ultrarelativistic
limit, irrespective of the produced particles' mass. A new equivalent
single-photon distribution is derived which correctly accounts for the Coulomb
distortions. As an immediate application, consequences for unitarity violation
in photo-dissociation processes in peripheral heavy-ion encounters are
discussed.Comment: 13 pages, 4 .eps figure
Fate of biosolids trace metals in a dryland wheat agroecosystem
Biosolids land application for beneficial reuse applies varying
amounts of trace metals to soils. Measuring plant-available or
total soil metals is typically performed to ensure environmental
protection, but these techniques do not quantify which
soil phases play important roles in terms of metal release or
attenuation. This study assessed the distribution of Cd, Cr,
Cu, Mo, Ni, Pb, and Zn associated with soluble/exchangeable,
specifically adsorbed/carbonate-bound, amorphous Mn
hydroxyoxide-bound, amorphous Fe hydroxyoxideâbound,
organically complexed, and residual inorganic phases. Biosolids
were applied every 2 yr from 1982 to 2002 (except in 1998)
at rates of 0, 6.7, 13.4, 26.8, and 40.3 dry Mg biosolids ha?1
to 3.6- by 17.1-m plots. In 2003, 0- to 20-cm and 20- to
60-cm soil depths were collected and subjected to 4 mol L?1
HNO3 digestion and sequential extraction. Trace metals were
concentrated in the 0- to 20-cm depth, with no significant
observable downward movement using 4 mol L?1 HNO3 or
sequential extraction. The sequential extraction showed nearly
all measurable Cd present in relatively mobile forms and Cr,
Cu, Mo, Ni, Pb, and Zn present in more resistant phases.
Biosolids application did not affect Cd or Cr fractionation
but did increase relatively immobile Cu, Mo, and Zn phases
and relatively mobile Cu, Ni, and Pb pools. The mobile
phases have not contributed to significant downward metal
movement. Long-term, repeated biosolids applications at rates
considered several times greater than agronomic levels should
not significantly contribute to downward metal transport and
ground water contamination for soils under similar climatic
conditions, agronomic practices, and histories
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