3,278 research outputs found
Two-Loop Quark and Gluon Form Factors in Dimensional Regularisation
We compute the two-loop corrections to the massless quark form factor
and gluon form factor to all orders in the
dimensional regularisation parameter . The two-loop
contributions to the form factors are reduced to linear combinations of master
integrals, which are computed in a closed form, expressed as -functions
and generalised hypergeometric functions of unit argument. Using the newly
developed HypExp-package, these can be expanded to any desired order, yielding
Laurent expansions in . We provide expansions of the form factors to
order , as required for ultraviolet renormalisation and infrared
factorisation of the three-loop form factors.Comment: 9 pages, latex, references update
Isolated Photons in Deep Inelastic Scattering
Photon radiation at large transverse momenta at colliders is a detailed probe
of hard interaction dynamics. The isolated photon production cross section in
deep inelastic scattering was measured recently by the ZEUS experiment, and
found to be considerably larger than theoretical predictions obtained with
widely used event generators. To investigate this discrepancy, we perform a
dedicated parton-level calculation of this observable, including contributions
from fragmentation and large-angle radiation. Our results are in good agreement
with all aspects of the experimental measurement.Comment: 4 pages, 3 figure
The diurnal evolution of the urban heat island of Paris: a model-based case study during Summer 2006
The urban heat island (UHI) over Paris during summer 2006 was simulated using the Advanced Regional Prediction System (ARPS) updated with a simple urban parametrization at a horizontal resolution of 1 km. Two integrations were performed, one with the urban land cover of Paris and another in which Paris was replaced by cropland. The focus is on a five-day clear-sky period, for which the UHI intensity reaches its maximum. The diurnal evolution of the UHI intensity was found to be adequately simulated for this five day period. The maximum difference at night in 2 m temperature between urban and rural areas stemming from the urban heating is reproduced with a relative error of less than 10%. The UHI has an ellipsoidal shape and stretches along the prevailing wind direction. The maximum UHI intensity of 6.1 K occurs at 23:00 UTC located 6 km downstream of the city centre and this largely remains during the whole night. An idealized one-column model study demonstrates that the nocturnal differential sensible heat flux, even though much smaller than its daytime value, is mainly responsible for the maximum UHI intensity. The reason for this nighttime maximum is that additional heat is only affecting a shallow layer of 150 m. An air uplift is explained by the synoptic east wind and a ramp upwind of the city centre, which leads to a considerable nocturnal adiabatic cooling over cropland. The idealized study demonstrates that the reduced vertical adiabatic cooling over the city compared to cropland induces an additional UHI build-up of 25%. The UHI and its vertical extent is affected by the boundary-layer stability, nocturnal low-level jet as well as radiative cooling. Therefore, improvements of representing these boundary-layer features in atmospheric models are important for UHI studies
NNLO QCD corrections to event shape variables in electron positron annihilation
Precision studies of QCD at electron-positron colliders are based on
measurements of event shapes and jet rates. To match the high experimental
accuracy, theoretical predictions to next-to-next-to-leading order (NNLO) in
QCD are needed for a reliable interpretation of the data. We report the first
calculation of NNLO corrections O(alpha_s^3) to three-jet production and
related event shapes, and discuss their phenomenological impact.Comment: Contributed to 2007 Europhysics Conference on High Energy Physics,
Manchester, England 19-25 July 200
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When is the Fiscal Multiplier High? A Comparison of Four Business Cycle Phases
This paper compares the effect of fiscal spending on economic activity across four phases of the business cycle. We show that the fiscal multiplier is higher when unemployment is increasing than when it is decreasing. Conversely, fiscal multipliers do not depend on whether the unemployment rate is above or below its long-term trend. This result emerges both in the analysis of long time-series at the U.S. national level as well as for a post-Vietnam War panel of U.S. states. Our findings synthesize previous, at times conflicting, evidence on the state-dependence of fiscal multipliers and imply that fiscal intervention early on in economic downturns is most effective at stabilizing output
The most storage economical Runge-Kutta methods for the solution of large systems of coupled first-order differential equations
AbstractIt is shown how the attainable minimum for the memory requirements of Runge-Kutta methods can be realised for methods of the third order. These economisable third order methods belong to a one parameter sub-family from which two particular members with low error bound are selected
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