146 research outputs found
Parton energy loss limits and shadowing in Drell-Yan dimuon production
A precise measurement of the ratios of the Drell-Yan cross section per
nucleon for an 800 GeV/c proton beam incident on Be, Fe and W targets is
reported. The behavior of the Drell-Yan ratios at small target parton momentum
fraction is well described by an existing fit to the shadowing observed in
deep-inelastic scattering. The cross section ratios as a function of the
incident parton momentum fraction set tight limits on the energy loss of quarks
passing through a cold nucleus
Measurement of Angular Distributions of Drell-Yan Dimuons in Interactions at 800 GeV/c
We report a measurement of the angular distributions of Drell-Yan dimuons
produced using an 800 GeV/c proton beam on a hydrogen target. The polar and
azimuthal angular distribution parameters have been extracted over the
kinematic range GeV/c (excluding the
resonance region), GeV/c, and . The angular
distributions are similar to those of , and both data sets are compared
with models which attribute the distribution either to the
presence of the transverse-momentum-dependent Boer-Mulders structure function
or to QCD effects. The data indicate the presence of both
mechanisms. The validity of the Lam-Tung relation in Drell-Yan is also
tested.Comment: 4 pages, 3 figure
Measurement of Angular Distributions of Drell-Yan Dimuons in p + d Interaction at 800 GeV/c
We report a measurement of the angular distributions of Drell-Yan dimuons
produced using an 800 GeV/c proton beam on a deuterium target. The muon angular
distributions in polar angle and azimuthal angle have been
measured over the kinematic range GeV/c, GeV/c, and . No significant cos dependence is found
in these proton-induced Drell-Yan data, in contrast to the situation for
pion-induced Drell-Yan. The data are compared with expectations from models
which attribute the cos distribution to a QCD vacuum effect or to the
presence of the transverse-momentum-dependent Boer-Mulders structure function
. Constraints on the magnitude of the sea-quark
structure functions are obtained.Comment: 4 pages, 3 figure
The effects of CO2, climate and land-use on terrestrial carbon balance, 1920-1992: An analysis with four process-based ecosystem models
The concurrent effects of increasing atmospheric CO2 concentration, climate variability, and cropland establishment and abandonment on terrestrial carbon storage between 1920 and 1992 were assessed using a standard simulation protocol with four process-based terrestrial biosphere models. Over the long-term(1920–1992), the simulations yielded a time history of terrestrial uptake that is consistent (within the uncertainty) with a long-term analysis based on ice core and atmospheric CO2 data. Up to 1958, three of four analyses indicated a net release of carbon from terrestrial ecosystems to the atmosphere caused by cropland establishment. After 1958, all analyses indicate a net uptake of carbon by terrestrial ecosystems, primarily because of the physiological effects of rapidly rising atmospheric CO2. During the 1980s the simulations indicate that terrestrial ecosystems stored between 0.3 and 1.5 Pg C yr−1, which is within the uncertainty of analysis based on CO2 and O2 budgets. Three of the four models indicated (in accordance with O2 evidence) that the tropics were approximately neutral while a net sink existed in ecosystems north of the tropics. Although all of the models agree that the long-term effect of climate on carbon storage has been small relative to the effects of increasing atmospheric CO2 and land use, the models disagree as to whether climate variability and change in the twentieth century has promoted carbon storage or release. Simulated interannual variability from 1958 generally reproduced the El Niño/Southern Oscillation (ENSO)-scale variability in the atmospheric CO2 increase, but there were substantial differences in the magnitude of interannual variability simulated by the models. The analysis of the ability of the models to simulate the changing amplitude of the seasonal cycle of atmospheric CO2 suggested that the observed trend may be a consequence of CO2 effects, climate variability, land use changes, or a combination of these effects. The next steps for improving the process-based simulation of historical terrestrial carbon include (1) the transfer of insight gained from stand-level process studies to improve the sensitivity of simulated carbon storage responses to changes in CO2 and climate, (2) improvements in the data sets used to drive the models so that they incorporate the timing, extent, and types of major disturbances, (3) the enhancement of the models so that they consider major crop types and management schemes, (4) development of data sets that identify the spatial extent of major crop types and management schemes through time, and (5) the consideration of the effects of anthropogenic nitrogen deposition. The evaluation of the performance of the models in the context of a more complete consideration of the factors influencing historical terrestrial carbon dynamics is important for reducing uncertainties in representing the role of terrestrial ecosystems in future projections of the Earth system
Measurement of the Light Antiquark Flavor Asymmetry in the Nucleon Sea
A precise measurement of the ratio of Drell-Yan yields from an 800 GeV/c
proton beam incident on hydrogen and deuterium targets is reported. Over
140,000 Drell-Yan muon pairs with dimuon mass M_{mu+ mu-} >= 4.5 GeV/c^2 were
recorded. From these data, the ratio of anti-down (dbar) to anti-up (ubar)
quark distributions in the proton sea is determined over a wide range in
Bjorken-x. A strong x dependence is observed in the ratio dbar/ubar, showing
substantial enhancement of dbar with respect to ubar for x<0.2. This result is
in fair agreement with recent parton distribution parameterizations of the sea.
For x>0.2, the observed dbar/ubar ratio is much nearer unity than given by the
parameterizations.Comment: REVTeX, to be published in Phys. Rev. Let
dbar/ubar Asymmetry and the Origin of the Nucleon Sea
The Drell-Yan cross section ratios, , measured in
Fermilab E866, have led to the first determination of ,
, and the integral of for the
proton over the range . The E866 results are compared
with predictions based on parton distribution functions and various theoretical
models. The relationship between the E866 results and the NMC measurement of
the Gottfried integral is discussed. The agreement between the E866 results and
models employing virtual mesons indicates these non-perturbative processes play
an important role in the origin of the , asymmetry in the
nucleon sea.Comment: 5 pages, 3 figures, ReVTe
Tracking the Atlantic Multidecadal Oscillation through the last 8,000 years
Understanding the internal ocean variability and its influence on climate is imperative for society. A key aspect concerns the enigmatic Atlantic Multidecadal Oscillation (AMO), a feature defined by a 60- to 90-year variability in North Atlantic sea-surface temperatures. The nature and origin of the AMO is uncertain, and it remains unknown whether it represents a persistent periodic driver in the climate system, or merely a transient feature. Here, we show that distinct, ∼55- to 70-year oscillations characterized the North Atlantic ocean-atmosphere variability over the past 8,000 years. We test and reject the hypothesis that this climate oscillation was directly forced by periodic changes in solar activity. We therefore conjecture that a quasi-persistent ∼55- to 70-year AMO, linked to internal ocean-atmosphere variability, existed during large parts of the Holocene. Our analyses further suggest that the coupling from the AMO to regional climate conditions was modulated by orbitally induced shifts in large-scale ocean-atmosphere circulation
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