714 research outputs found
Fluctuaciones de las poblaciones de Diuraphis noxia y sus enemigos naturales en cultivos de trigo en la zona de Bahia Blanca, Argentina
The Russian wheat aphid Diuraphis noxia Mordvilko
(Homoptera:Aphididae) is a dangerous pest of economic importance to the
Argentina's wheat growing region, dating from its appearance in 1991.
Fluctuations in D. noxia population and its natural enemies were
evaluated in the area of Bah\ueda Blanca, Buenos Aires , Argentina,
during 1994 and 1995. The information was obtained through direct plant
sampling, water and soil traps and net catches. The first aphids were
found in September during the phenological reed stage and the highest
density was found in November during the phenological tassel stage. The
age structure of the population was similar and was represented by
young juvenile and adult forms. The population fitted an added type of
pattern, constituted mainly of small pupa. The winged forms were found
at the end of November, starting at that time their migration to
natural grass. The alternative hosts were, Hordeum leporinum Link,
Lolium multiflorum Lam., L. perenne L., Bromus unioloides Kunth,
and Bromus brevis Nees. The most outstanding predators during both
years of study were the Coccinelidae, Eriopis connexa (Mulsant),
Hippodamia convergens (Guer.) and Coccinella ancoralis (Germ.)
which presented together with Hymenoptera: Aphidiidae the peak of the
maximum occurrence simultaneous to the aphid's highest maximum
population density. The presence of Diptera: Syrphidae and Nabis spp.
was scarce.En Argentina el pulg\uf3n ruso del trigo, Diuraphis noxia Mordvilko
(Homoptera: Aphididae), es una plaga de importancia econ\uf3mica en
la regi\uf3n triguera a partir de su aparici\uf3n en el pa\ueds
en 1991. En la zona de Bah\ueda Blanca, Buenos Aires, Argentina,
durante los a\uf1os 1994 y 1995 se evaluaron las fluctuaciones
poblacionales D. noxia y sus enemigos naturales. La informaci\uf3n
fue obtenida mediante muestreo directo de plantas, trampas de agua y
suelo, y golpes de red. Los primeros \ue1fidos se registraron en el
mes de septiembre en el estado fenol\uf3gico de enca\uf1ado y la
mayor densidad se produjo en el mes de noviembre durante el estado
fenol\uf3gico de espigado. La estructura por edad de la
poblaci\uf3n fue similar y estuvo representada por las formas
juveniles y adultas. La poblaci\uf3n se ajust\uf3 a un patr\uf3n
de distribuci\uf3n de tipo agregada, constituida en su mayor\ueda
por ninfas peque\uf1as. Las formas aladas se registraron a fines de
noviembre, iniciando en esta \ue9poca la migraci\uf3n hacia las
gram\uedneas naturales. Los hospederos alternativos fueron cola de
zorro, Hordeum leporinum Link; rye grass perenne, Lolium perenne
L.;rye grass criollo, Lolium multiflorum Lam.; cebadilla australiana,
Bromus unioloides Kunth, y cebadilla criolla, Bromus brevis Nees.
Los depredadores m\ue1s destacados para ambos a\uf1os de estudio
fueron los coccin\ue9lidos Eriopis connexa (Mulsant), Hippodamia
convergens (Guer.) y Coccinella ancoralis (Germ.) quepresentaron
junto a Hymenoptera: Aphidiidae el pico de m\ue1xima ocurrencia en
forma coincidente con la m\ue1xima densidad poblacional de los
pulgones. La presencia de Diptera: Syrphidae y de Nabis spp. fue
escasa
The front-end electronics for the COMPASS MWPCs
In the COMPASS experiment 34 planes of MWPCs for about 26,000 readout channels of MWPCs are used. The very high rate of the muon and hadron beams, and the consequently high trigger rate, requires the use of a front-end electronics with new conceptual design, to have a fast DAQ with a minimum dead-time. Its scheme will be described, together with some results of the performances achieved
Non-minimally coupled dark matter: effective pressure and structure formation
We propose a phenomenological model in which a non-minimal coupling between
gravity and dark matter is present in order to address some of the apparent
small scales issues of \lcdm model. When described in a frame in which gravity
dynamics is given by the standard Einstein-Hilbert action, the non-minimal
coupling translates into an effective pressure for the dark matter component.
We consider some phenomenological examples and describe both background and
linear perturbations. We show that the presence of an effective pressure may
lead these scenarios to differ from \lcdm at the scales where the non-minimal
coupling (and therefore the pressure) is active. In particular two effects are
present: a pressure term for the dark matter component that is able to reduce
the growth of structures at galactic scales, possibly reconciling simulations
and observations; an effective interaction term between dark matter and baryons
that could explain observed correlations between the two components of the
cosmic fluid within Tully-Fisher analysis.Comment: 18 pages, 6 figures, references added. Published in JCA
Gluon polarization in the nucleon from quasi-real photoproduction of high-pT hadron pairs
We present a determination of the gluon polarization Delta G/G in the
nucleon, based on the helicity asymmetry of quasi-real photoproduction events,
Q^2<1(GeV/c)^2, with a pair of large transverse-momentum hadrons in the final
state. The data were obtained by the COMPASS experiment at CERN using a 160 GeV
polarized muon beam scattered on a polarized 6-LiD target. The helicity
asymmetry for the selected events is = 0.002 +- 0.019(stat.) +-
0.003(syst.). From this value, we obtain in a leading-order QCD analysis Delta
G/G=0.024 +- 0.089(stat.) +- 0.057(syst.) at x_g = 0.095 and mu^2 =~ 3
(GeV}/c)^2.Comment: 10 pages, 3 figure
The Deuteron Spin-dependent Structure Function g1d and its First Moment
We present a measurement of the deuteron spin-dependent structure function
g1d based on the data collected by the COMPASS experiment at CERN during the
years 2002-2004. The data provide an accurate evaluation for Gamma_1^d, the
first moment of g1d(x), and for the matrix element of the singlet axial
current, a0. The results of QCD fits in the next to leading order (NLO) on all
g1 deep inelastic scattering data are also presented. They provide two
solutions with the gluon spin distribution function Delta G positive or
negative, which describe the data equally well. In both cases, at Q^2 = 3
(GeV/c)^2 the first moment of Delta G is found to be of the order of 0.2 - 0.3
in absolute value.Comment: fits redone using MRST2004 instead of MRSV1998 for G(x), correlation
matrix adde
Spin asymmetry A_1^d and the spin-dependent structure function g_1^d of the deuteron at low values of x and Q^2
We present a precise measurement of the deuteron longitudinal spin asymmetry
A_1^d and of the deuteron spin-dependent structure function g_1^d at Q^2 < 1
GeV^2 and 4*10^-5 < x < 2.5*10^-2 based on the data collected by the COMPASS
experiment at CERN during the years 2002 and 2003. The statistical precision is
tenfold better than that of the previous measurement in this region. The
measured A_1^d and g_1^d are found to be consistent with zero in the whole
range of x.Comment: 17 pages, 10 figure
A new measurement of the Collins and Sivers asymmetries on a transversely polarised deuteron target
New high precision measurements of the Collins and Sivers asymmetries of
charged hadrons produced in deep-inelastic scattering of muons on a
transversely polarised 6LiD target are presented. The data were taken in 2003
and 2004 with the COMPASS spectrometer using the muon beam of the CERN SPS at
160 GeV/c. Both the Collins and Sivers asymmetries turn out to be compatible
with zero, within the present statistical errors, which are more than a factor
of 2 smaller than those of the published COMPASS results from the 2002 data.
The final results from the 2002, 2003 and 2004 runs are compared with naive
expectations and with existing model calculations.Comment: 40 pages, 28 figure
Measurement of the Spin Structure of the Deuteron in the DIS Region
We present a new measurement of the longitudinal spin asymmetry A_1^d and the
spin-dependent structure function g_1^d of the deuteron in the range 1 GeV^2 <
Q^2 < 100 GeV^2 and 0.004< x <0.7. The data were obtained by the COMPASS
experiment at CERN using a 160 GeV polarised muon beam and a large polarised
6-LiD target. The results are in agreement with those from previous experiments
and improve considerably the statistical accuracy in the region 0.004 < x <
0.03.Comment: 10 pages, 6 figures, subm. to PLB, revised: author list, Fig. 4,
details adde
The COMPASS Experiment at CERN
The COMPASS experiment makes use of the CERN SPS high-intensitymuon and
hadron beams for the investigation of the nucleon spin structure and the
spectroscopy of hadrons. One or more outgoing particles are detected in
coincidence with the incoming muon or hadron. A large polarized target inside a
superconducting solenoid is used for the measurements with the muon beam.
Outgoing particles are detected by a two-stage, large angle and large momentum
range spectrometer. The setup is built using several types of tracking
detectors, according to the expected incident rate, required space resolution
and the solid angle to be covered. Particle identification is achieved using a
RICH counter and both hadron and electromagnetic calorimeters. The setup has
been successfully operated from 2002 onwards using a muon beam. Data with a
hadron beam were also collected in 2004. This article describes the main
features and performances of the spectrometer in 2004; a short summary of the
2006 upgrade is also given.Comment: 84 papes, 74 figure
Transverse spin effects in hadron-pair production from semi-inclusive deep inelastic scattering
First measurements of azimuthal asymmetries in hadron-pair production in
deep-inelastic scattering of muons on transversely polarised ^6LiD (deuteron)
and NH_3 (proton) targets are presented. The data were taken in the years
2002-2004 and 2007 with the COMPASS spectrometer using a muon beam of 160 GeV/c
at the CERN SPS. The asymmetries provide access to the transversity
distribution functions, without involving the Collins effect as in single
hadron production. The sizeable asymmetries measured on the NH_ target indicate
non-vanishing u-quark transversity and two-hadron interference fragmentation
functions. The small asymmetries measured on the ^6LiD target can be
interpreted as indication for a cancellation of u- and d-quark transversities.Comment: 13 pages, 4 figures, updated to the published versio
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