17,602 research outputs found
Investigations into the BFKL Mechanism with a Running QCD Coupling
We present approximations of varying degree of sophistication to the integral
equations for the (gluon) structure functions of a hadron (``the partonic flux
factor'') in a model valid in the Leading Log Approximation with a running
coupling constant. The results are all of the BFKL-type, i.e. a power in the
Bjorken variable x_B^{-\lambda} with the parameter \lambda determined from the
size \alpha_0 of the ``effective'' running coupling \bar{\alpha}\equiv
3\alpha_s/\pi= \alpha_0/\log(k_{\perp}^2) and varying depending upon the
treatment of the transverse momentum pole. We also consider the implications
for the transverse momentum (k_{\perp}) fluctuations along the emission chains
and we obtain an exponential falloff in the relevant \kappa\equiv
\log(k_{\perp}^2)-variable, i.e. an inverse power (k_{\perp}^2)^{-(2+\lambda)}
with the same parameter \lambda. This is different from the BFKL-result for a
fixed coupling, where the distributions are Gaussian in the \kappa-variable
with a width as in a Brownian motion determined by ``the length'' of the
emission chains, i.e. \log(1/x_B). The results are verified by a realistic
Monte Carlo simulation and we provide a simple physics motivation for the
change.Comment: 24 pages, 10 supplementary files, submitted to Physical Review
Resource effective control of Elymus repens
Preliminary results show that there is room for improvement within existing control methods of couch grass (Elymus repens (L.) Gould). It may be possible to reduce the number of stubble cultivations during autumn by timing the treatment, and to reduce the cultivation depth by using a goose foot cultivator (5 cm) instead of a disc cultivator (10 cm), without sacrificing couch grass control efficiency. The first year of the experiment, the use of a goose foot cultivator resulted in less nitrogen leaching than cultivation by disc. A reduced number of stubble cultivations potentially reduces nutrient loss, fuel consumption and the workload of the farmer.
Our experiments with cover crops to control couch grass in cereals has yet to prove significant effects on couch grass control, but cover crops combined with goose foot hoeing did reduce nitrogen leaching by more than a third compared to cultivation by disc. Further data is necessary to see if the system can be used to effectively control couch grass without significant yield losses. Regardless, it can reduce nitrogen leaching and potentially provide other ecosystem services, e.g. control weeds other than couch grass
Enhancement of singly and multiply strangeness in p-Pb and Pb-Pb collisions at 158A GeV/c
The idea that the reduction of the strange quark suppression in string
fragmentation leads to the enhancement of strange particle yield in
nucleus-nucleus collisions is applied to study the singly and multiply strange
particle production in p-Pb and Pb-Pb collisions at 158A GeV/c. In this
mechanism the strange quark suppression factor is related to the effective
string tension, which increases in turn with the increase of the energy, of the
centrality and of the mass of colliding system. The WA97 observation that the
strange particle enhancement increases with the increasing of centrality and of
strange quark content in multiply strange particles in Pb-Pb collisions with
respect to p-Pb collisions was accounted reasonably.Comment: 8 pages, 3 PostScript figures, in Latex form. submitted to PR
A detailed study of quasinormal frequencies of the Kerr black hole
We compute the quasinormal frequencies of the Kerr black hole using a
continued fraction method. The continued fraction method first proposed by
Leaver is still the only known method stable and accurate for the numerical
determination of the Kerr quasinormal frequencies. We numerically obtain not
only the slowly but also the rapidly damped quasinormal frequencies and analyze
the peculiar behavior of these frequencies at the Kerr limit. We also calculate
the algebraically special frequency first identified by Chandrasekhar and
confirm that it coincide with the quasinormal frequency only at the
Schwarzschild limit.Comment: REVTEX, 15 pages, 7 eps figure
Quark-Gluon-Plasma Formation at SPS Energies?
By colliding ultrarelativistic ions, one achieves presently energy densities
close to the critical value, concerning the formation of a quark-gluon-plasma.
This indicates the importance of fluctuations and the necessity to go beyond
the investigation of average events. Therefore, we introduce a percolation
approach to model the final stage ( fm/c) of ion-ion collisions, the
initial stage being treated by well-established methods, based on strings and
Pomerons. The percolation approach amounts to finding high density domains, and
treating them as quark-matter droplets. In this way, we have a {\bf realistic,
microscopic, and Monte--Carlo based model which allows for the formation of
quark matter.} We find that even at SPS energies large quark-matter droplets
are formed -- at a low rate though. In other words: large quark-matter droplets
are formed due to geometrical fluctuation, but not in the average event.Comment: 7 Pages, HD-TVP-94-6 (1 uuencoded figure
Time-resolved extinction rates of stochastic populations
Extinction of a long-lived isolated stochastic population can be described as
an exponentially slow decay of quasi-stationary probability distribution of the
population size. We address extinction of a population in a two-population
system in the case when the population turnover -- renewal and removal -- is
much slower than all other processes. In this case there is a time scale
separation in the system which enables one to introduce a short-time
quasi-stationary extinction rate W_1 and a long-time quasi-stationary
extinction rate W_2, and develop a time-dependent theory of the transition
between the two rates. It is shown that W_1 and W_2 coincide with the
extinction rates when the population turnover is absent, and present but very
slow, respectively. The exponentially large disparity between the two rates
reflects fragility of the extinction rate in the population dynamics without
turnover.Comment: 8 pages, 4 figure
On ``hyperboloidal'' Cauchy data for vacuum Einstein equations and obstructions to smoothness of ``null infinity''
Various works have suggested that the Bondi--Sachs--Penrose decay conditions
on the gravitational field at null infinity are not generally representative of
asymptotically flat space--times. We have made a detailed analysis of the
constraint equations for ``asymptotically hyperboloidal'' initial data and find
that log terms arise generically in asymptotic expansions. These terms are
absent in the corresponding Bondi--Sachs--Penrose expansions, and can be
related to explicit geometric quantities. We have nevertheless shown that there
exists a large class of ``non--generic'' solutions of the constraint equations,
the evolution of which leads to space--times satisfying the
Bondi--Sachs--Penrose smoothness conditions.Comment: 8 pages, revtex styl
Decay widths of large-spin mesons from the non-critical string/gauge duality
In this paper, we use the non-critical string/gauge duality to calculate the
decay widths of large-spin mesons. Since it is believed that the string theory
of QCD is not a ten dimensional theory, we expect that the non-critical
versions of ten dimensional black hole backgrounds lead to better results than
the critical ones. For this purpose we concentrate on the confining theories
and consider two different six dimensional black hole backgrounds. We choose
the near extremal AdS6 model and the near extremal KM model to compute the
decay widths of large-spin mesons. Then, we present our results from these two
non-critical backgrounds and compare them together with those from the critical
models and experimental data.Comment: 21 pages and 3 figure
R-mode oscillations and rocket effect in rotating superfluid neutron stars. I. Formalism
We derive the hydrodynamical equations of r-mode oscillations in neutron
stars in presence of a novel damping mechanism related to particle number
changing processes. The change in the number densities of the various species
leads to new dissipative terms in the equations which are responsible of the
{\it rocket effect}. We employ a two-fluid model, with one fluid consisting of
the charged components, while the second fluid consists of superfluid neutrons.
We consider two different kind of r-mode oscillations, one associated with
comoving displacements, and the second one associated with countermoving, out
of phase, displacements.Comment: 10 page
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