739 research outputs found
Responses of soybean ancestral lines and cultivars to simulated acid rain
During 1985, 1986, and 1987, field and greenhouse experiments were conducted on four ancestral lines and two cultivars from each of the northern and southern gene pools. The ancestral lines have contributed approximately 80% of the genes to their respective gene pools. Representatives from the northern germplasm collection included Mandarin, Manchu, AK Harrow, Richland, \u27Amsoy 71\u27, and \u27Williams 82\u27. The southern germplasm collection was represented by CNS, S 100, PI 54610, Tokyo, \u27Lee 74\u27, and \u27Essex\u27. Spray-to-wet applications of simulated acid rain (SAR) were applied at three acidity levels (pH 2.8, 3.2, and 4.3). Field experiments were grown under optimum and sub-optimum soil conditions (pH and fertility) at two diverse locations to determine if soybean response to SAR was affected by edaphic factors. In general, SAR had no effect on seed yield. In cases where yield was affected, there were more positive than negative effects. There was no consistent trend with respect to optimum vs sub-optimum soil conditions. Seed weight was affected by SAR for some lines but was not necessarily related to yield. In general, photosynthesis, transpiration, and stomatal frequency were not affected by SAR. Early stages of plant growth (leaf and stem weight) in the greenhouse was affected by SAR, but measurements taken on the same characteristics during later growth stages were not significant. These results indicate that soybean cultivars in general will not be detrimentally affected by acid rain based on the response of their ancestral lines and evaluations of selected cultivars derived from them
Multiple parton interactions in high-density QCD matter
Multiple interactions of quarks and gluons in high-energy heavy-ion
collisions may give rise to interesting phemomena of color charges propagating
in high-density QCD matter. We study the dynamics of multi-parton systems
produced in nucleus-nucleus collisions at energies corresponding the the CERN
SPS and the future BNL RHIC experiments. Due to the complexity of the
multi-particle dynamics we choose to employ the parton cascade model in order
to simulate the development of multiple parton scatterings and associated
stimulated emision processes. Our results indicate a non-linear increase with
nuclear mass A of, e.g., parton multiplicity, energy density, strangeness, and
contrast a linear A-scaling as in Glauber-type approaches. If multiple
interactions are suppressed and only single parton scatterings (no
re-interactions) are considered, we recover such a linear behavior. It remains
to be studied whether these results on the parton level can be experimentally
seen in final-state observables, such as the charged particle multiplicity, the
magnitude of produced transverse energy, or the number of produced strange
hadrons.Comment: 15 pages including 9 postscript figure
Thermodynamic versus statistical nonequivalence of ensembles for the mean-field Blume-Emery-Griffiths model
We illustrate a novel characterization of nonequivalent statistical
mechanical ensembles using the mean-field Blume-Emery-Griffiths (BEG) model as
a test model. The novel characterization takes effect at the level of the
microcanonical and canonical equilibrium distributions of states. For this
reason it may be viewed as a statistical characterization of nonequivalent
ensembles which extends and complements the common thermodynamic
characterization of nonequivalent ensembles based on nonconcave anomalies of
the microcanonical entropy. By computing numerically both the microcanonical
and canonical sets of equilibrium distributions of states of the BEG model, we
show that for values of the mean energy where the microcanonical entropy is
nonconcave, the microcanonical distributions of states are nowhere realized in
the canonical ensemble. Moreover, we show that for values of the mean energy
where the microcanonical entropy is strictly concave, the equilibrium
microcanonical distributions of states can be put in one-to-one correspondence
with equivalent canonical equilibrium distributions of states. Our numerical
computations illustrate general results relating thermodynamic and statistical
equivalence and nonequivalence of ensembles proved by Ellis, Haven, and
Turkington [J. Stat. Phys. 101, 999 (2000)].Comment: 13 pages, 4 figures, minor typos corrected and one reference adde
Kinematics of deformable media
We investigate the kinematics of deformations in two and three dimensional
media by explicitly solving (analytically) the evolution equations
(Raychaudhuri equations) for the expansion, shear and rotation associated with
the deformations. The analytical solutions allow us to study the dependence of
the kinematical quantities on initial conditions. In particular, we are able to
identify regions of the space of initial conditions that lead to a singularity
in finite time. Some generic features of the deformations are also discussed in
detail. We conclude by indicating the feasibility and utility of a similar
exercise for fluid and geodesic flows in flat and curved spacetimes.Comment: 28 pages, 12 figure
Vacuum-Induced Quantum Decoherence and the Entropy Puzzle
Or: ``How to generate an ensemble in a single event?'' Following recent work
on entropy in strong interactions, I explain the concept of environment-induced
quantum decoherence in elementary quantum mechanics. The classically chaotic
inverted oscillator becomes partially decoherent already in the environment of
a single other oscillator performing only vacuum fluctuations. One finds
exponential entropy growth in the subsystem with a Lyapunov exponent, which
approaches the classical one for weak coupling.
Presented at the Workshops on ``Quantum Infrared Physics'', Paris,
6 - 10.6.94, and ``QCD 94'', Montpellier, 7 - 13.7.94.
Invited talk at the NATO Adv. Res. Workshop on ``Hot Hadronic Matter'',
Divonne, 27.6. - 1.7.94. To appear in the Proceedings.Comment: 12 pages, CERN-TH.7372/9
Can Cosmic Ray Catalysed Vacuum Decay Dominate Over Tunnelling?
We consider the question of whether cosmic ray catalysed false vacuum decay
can be phenomenologically more important than spontaneous decay via quantum
tunnelling. We extend the zero bubble wall width Landau-WKB analysis of
catalysed false vacuum decay to include the leading order effects of finite
wall width and derive an expression for the thin-wall bubble action. Using this
we calculate the exponential suppression factor for the catalysed decay rate at
the critical bubble energy, corresponding to the largest probability of
catalysed decay. We show that, in general, cosmic ray catalysed decay is likely
to be more important than spontaneous decay for sufficiently thin-walled
bubbles (wall thickness less than about 30 % of the initial bubble radius), but
that spontaneous decay will dominate for the case of thick-walled bubbles.
Since any perturbative model with a cosmologically significant false vacuum
decay rate will almost certainly produce thick-walled bubbles, we can conclude
that cosmic ray catalysed false vacuum decay will never dominate over
tunnelling in imposing phenomenological constraints on perturbative particle
physics models.Comment: 22 pages Latex, 3 figures include
Reality and causality in quantum gravity modified electrodynamics
We present a general description of the propagation properties of quantum
gravity modified electrodynamics characterized by constitutive relations up to
second order in the correction parameter. The effective description corresponds
to an electrodynamics in a dispersive and absorptive non-local medium, where
the Green functions and the refraction indices can be explicitly calculated.
The reality of the electromagnetic field together with the requirement of
causal propagation in a given referrence frame leads to restrictions in the
form of such refraction indices. In particular, absorption must be present in
all cases and, contrary to the usual assumption, it is the dominant aspect in
those effective models which exhibit linear effects in the correction parameter
not related to birefringence. In such a situation absorption is linear while
propagation is quadratical in the correction parameter.Comment: 15 pages, LaTex, minor changes to clarify some points, version
accepted for publicatio
Role of longitudinal fluctuations in L10 FePt
L10 FePt is a technologically important material for a range of novel data storage applications. In the ordered FePt structure the normally nonmagnetic Pt ion acquires a magnetic moment, which depends on the local field originating from the neighboring Fe atoms. In this work a model of FePt is constructed in which the induced Pt moment is simulated by using combined longitudinal and rotational spin dynamics. The model is parameterized to include a linear variation of the moment with the exchange field, so that at the Pt site the magnetic moment depends on the Fe ordering. The Curie temperature of FePt is calculated and agrees well with similar models that incorporate the Pt dynamics through an effective Fe-only Hamiltonian. By computing the dynamic correlation function the anisotropy field and the Gilbert damping are extracted over a range of temperatures. The anisotropy exhibits a power-law dependence on the magnetization with exponent n≈2.1. This agrees well with what was observed experimentally, and it is obtained without including a two-ion anisotropy term as in other approaches. Our work shows that incorporating longitudinal fluctuations into spin dynamics calculations is crucial for understanding the properties of materials with induced moments
Scattering and Resonances in QCD_2
Extending previous works on the spectrum of QCD_2, we now investigate the 2D
analogue of meson-baryon scattering. We use semi-classical methods, perturbing
around classical soliton solutions. We start with the abelian case,
corresponding to one flavor,and find that in this case the effective potential
is reflectionless. We obtain an explicit expression for the forward phase
shift. In the non-abelian case of several flavors, the method yields a
potential which depends on the momentum of the incoming particle. In this case
there is both transmission and reflection. In both cases no resonances appear.
As a byproduct, we derive the general conditions for a 2D quantum field
theoretical action to yield a reflectionless effective potential when one
expands in small fluctuations about the classical solution.Comment: Figs. 2 and 3 and the accompanying text amended, following sign
correction in the first term in the potential V in eq. (58). Qualitative
conclusions unchange
Perfect magnetohydrodynamics as a field theory
We propose the generally covariant action for the theory of a self-coupled
complex scalar field and electromagnetism which by virtue of constraints is
equivalent, in the regime of long wavelengths, to perfect magnetohydrodynamics
(MHD). We recover from it the Euler equation with Lorentz force, and the
thermodynamic relations for a prefect fluid. The equation of state of the
latter is related to the scalar field's self potential. We introduce 1+3
notation to elucidate the relation between MHD and field variables. In our
approach the requirement that the scalar field be single valued leads to the
quantization of a certain circulation in steps of ; this feature leads,
in the classical limit, to the conservation of that circulation. The
circulation is identical to that in Oron's generalization of Kelvin's
circulation theorem to perfect MHD; we here characterize the new conserved
helicity associated with it. We also demonstrate the existence for MHD of two
Bernoulli-like theorems for each spacetime symmetry of the flow and geometry;
one of these is pertinent to suitably defined potential flow. We exhibit the
conserved quantities explicitly in the case that two symmetries are
simultaneously present, and give examples. Also in this case we exhibit a new
conserved MHD circulation distinct from Oron's, and provide an example.Comment: RevTeX, 16 pages, no figures; clarifications added and typos
corrected; version to be published in Phys. Rev.
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