3,143 research outputs found
Spin properties of top quark pairs produced at hadron colliders
We discuss the spin properties of top quark pairs produced at hadron
colliders at next-to-leading order in the coupling constant alpha_s of the
strong interaction. Specifically we present, for some decay channels, results
for differential angular distributions that are sensitive to t tbar spin
correlations.Comment: Invited talk given by A. Brandenburg at the Cracow epiphany
conference on heavy flavours, 3 - 6 January 2003, Cracow, Polan
Investigation of Top quark spin correlations at hadron collider
We report on our results about hadronic production at NLO QCD
including spin effects, especially on spin correlations.Comment: talk given at the 32nd International Conference on High Energy
Physics (ICHEP'04), Beijing, China, 16-22 Aug. 200
Dissociation in a polymerization model of homochirality
A fully self-contained model of homochirality is presented that contains the
effects of both polymerization and dissociation. The dissociation fragments are
assumed to replenish the substrate from which new monomers can grow and undergo
new polymerization. The mean length of isotactic polymers is found to grow
slowly with the normalized total number of corresponding building blocks.
Alternatively, if one assumes that the dissociation fragments themselves can
polymerize further, then this corresponds to a strong source of short polymers,
and an unrealistically short average length of only 3. By contrast, without
dissociation, isotactic polymers becomes infinitely long.Comment: 16 pages, 6 figures, submitted to Orig. Life Evol. Biosp
Compact Drawings of 1-Planar Graphs with Right-Angle Crossings and Few Bends
We study the following classes of beyond-planar graphs: 1-planar, IC-planar,
and NIC-planar graphs. These are the graphs that admit a 1-planar, IC-planar,
and NIC-planar drawing, respectively. A drawing of a graph is 1-planar if every
edge is crossed at most once. A 1-planar drawing is IC-planar if no two pairs
of crossing edges share a vertex. A 1-planar drawing is NIC-planar if no two
pairs of crossing edges share two vertices. We study the relations of these
beyond-planar graph classes (beyond-planar graphs is a collective term for the
primary attempts to generalize the planar graphs) to right-angle crossing (RAC)
graphs that admit compact drawings on the grid with few bends. We present four
drawing algorithms that preserve the given embeddings. First, we show that
every -vertex NIC-planar graph admits a NIC-planar RAC drawing with at most
one bend per edge on a grid of size . Then, we show that
every -vertex 1-planar graph admits a 1-planar RAC drawing with at most two
bends per edge on a grid of size . Finally, we make two
known algorithms embedding-preserving; for drawing 1-planar RAC graphs with at
most one bend per edge and for drawing IC-planar RAC graphs straight-line
Axino dark matter in brane world cosmology
We discuss dark matter in the brane world scenario. We work in the
Randall-Sundrum type II brane world and assume that the lightest supersymmetric
particle is the axino. We find that the axinos can play the role of cold dark
matter in the universe, provided that the five-dimensional Planck mass is
bounded both from below and from above. This is possible for higher reheating
temperatures compared to the conventional four-dimensional cosmology due to a
novel expansion law for the universe.Comment: 1+11 pages, version submitted to JCA
Looking for CP Violation in W Production and Decay
We describe CP violating observables in resonant and plus one
jet production at the Tevatron. We present simple examples of CP violating
effective operators, consistent with the symmetries of the Standard Model,
which would give rise to these observables. We find that CP violating effects
coming from new physics at the scale could in principle be observable at
the Tevatron with decays.Comment: 15 pgs with standard LATEX, 7 ps figures embedded with eps
Gravitino Dark Matter and Cosmological Constraints
The gravitino is a promising candidate for cold dark matter. We study
cosmological constraints on scenarios in which the gravitino is the lightest
supersymmetric particle and a charged slepton the next-to-lightest
supersymmetric particle (NLSP). We obtain new results for the hadronic
nucleosynthesis bounds by computing the 4-body decay of the NLSP slepton into
the gravitino, the associated lepton, and a quark-antiquark pair. The bounds
from the observed dark matter density are refined by taking into account
gravitinos from both late NLSP decays and thermal scattering in the early
Universe. We examine the present free-streaming velocity of gravitino dark
matter and the limits from observations and simulations of cosmic structures.
Assuming that the NLSP sleptons freeze out with a thermal abundance before
their decay, we derive new bounds on the slepton and gravitino masses. The
implications of the constraints for cosmology and collider phenomenology are
discussed and the potential insights from future experiments are outlined. We
propose a set of benchmark scenarios with gravitino dark matter and long-lived
charged NLSP sleptons and describe prospects for the Large Hadron Collider and
the International Linear Collider.Comment: 51 pages, 20 figures, revised version matches published version
(results unchanged, JHEP style used, figures replaced with new high-quality
figures, typos corrected, references added
Top Production in Hadron-Hadron Collisions and Anomalous Top-Gluon Couplings
We discuss the influence of anomalous tbar-t-G couplings on total and
differential tbar-t production cross sections in hadron-hadron collisions. We
study in detail the effects of a chromoelectric and a chromomagnetic dipole
moment, d' and \mu', of the top quark. In the d'-\mu' plane, we find a whole
region where the anomalous couplings give a zero net contribution to the total
top production rate. In differential cross sections, the anomalous moments have
to be quite sizable to give measurable effects. We estimate the values of d'
and \mu' which are allowed by the present Tevatron experimental results on top
production. A chromoelectric dipole moment of the top violates CP invariance.
We discuss a simple CP-odd observable which allows for a direct search for CP
violation in top production.Comment: footnote pg. 4 changed, acknowledgments extende
How much do we really lose?âYield losses in the proximity of natural landscape elements in agricultural landscapes
Natural landscape elements (NLEs) in agricultural landscapes contribute to biodiversity and ecosystem services, but are also regarded as an obstacle for largeâscale agricultural production. However, the effects of NLEs on crop yield have rarely been measured. Here, we investigated how different bordering structures, such as agricultural roads, fieldâtoâfield borders, forests, hedgerows, and kettle holes, influence agricultural yields. We hypothesized that (a) yield values at field borders differ from midâfield yields and that (b) the extent of this change in yields depends on the bordering structure.
We measured winter wheat yields along transects with logâscaled distances from the border into the agricultural field within two intensively managed agricultural landscapes in Germany (2014 near Göttingen, and 2015â2017 in the Uckermark).
We observed a yield loss adjacent to every investigated bordering structure of 11%â38% in comparison with midâfield yields. However, depending on the bordering structure, this yield loss disappeared at different distances. While the proximity of kettle holes did not affect yields more than neighboring agricultural fields, woody landscape elements had strong effects on winter wheat yields. Notably, 95% of midâfield yields could already be reached at a distance of 11.3 m from a kettle hole and at a distance of 17.8 m from hedgerows as well as forest borders.
Our findings suggest that yield losses are especially relevant directly adjacent to woody landscape elements, but not adjacent to inâfield water bodies. This highlights the potential to simultaneously counteract yield losses close to the field border and enhance biodiversity by combining different NLEs in agricultural landscapes such as creating strips of extensive grassland vegetation between woody landscape elements and agricultural fields. In conclusion, our results can be used to quantify ecocompensations to find optimal solutions for the delivery of productive and regulative ecosystem services in heterogeneous agricultural landscapes
The declining representativeness of the British party system, and why it matters
In a recent article, Michael Laver has explained âWhy Vote-Seeking Parties May Make Voters Miserableâ. His model shows that, while ideological convergence may boost congruence between governments and the median voter, it can reduce congruence between the party system and the electorate as a whole. Specifically, convergence can increase the mean distance between voters and their nearest party. In this article we show that this captures the reality of todayâs British party system. Policy scale placements in British Election Studies from 1987 to 2010 confirm that the pronounced convergence during the past decade has left the Conservatives and Labour closer together than would be optimal in terms of minimising the policy distance between the average voter and the nearest major party. We go on to demonstrate that this comes at a cost. Respondents who perceive themselves as further away from one of the major parties in the system tend to score lower on satisfaction with democracy. In short, vote-seeking parties have left the British party system less representative of the ideological diversity in the electorate, and thus made at least some British voters miserable
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