6,409 research outputs found
Cross Sections for Charm Production in Collisions: Massive versus Massless Scheme
The next--to--leading order inclusive cross section for large-
photoproduction of charm quarks at HERA is calculated in two different
approaches. In the first approach the charm quarks are treated as massive
objects which are strictly external to the proton and the photon while in the
second approach the charm mass is neglected and the quark is assumed to be
one of the active flavours in the proton and photon structure functions. We
present single-inclusive distributions in transverse momentum and rapidity
including direct and resolved photons. The cross section in the massless
approach is found to be significantly larger than in the massive scheme. The
deviation originates from several contributions which are disentangled. We
argue that large- photoproduction of charm quarks at HERA will be
sensitive to the charm content of the photon structure function.Comment: 11 pages, Latex, epsfig, 6 figures appended as uuencoded file
(hardcopy can be obtained upon request from [email protected]
Higgs radiation off quarks in supersymmetric theories at e^+e^- colliders
Yukawa couplings between Higgs bosons and quarks in supersymmetric theories
can be measured in the processes e^+e^- -> Q Qbar + Higgs. We have determined
the cross sections of these processes in the minimal supersymmetric model
including the complete set of next-to-leading order QCD corrections for all
channels.Comment: 12 pages, latex, 3 figure
Hemodynamic and inotropic effects of endothelin-1 in vivo
Endothelin-1 (ET-1) is known to have strong vasoactive properties. Contradictory results have been reported with regard to its inotropic effects. This study examined the dose-dependent (500, 1000, 2500, 5000 and 10,000 ng ET-1/kg vs. NaCl controls) hemodynamic and inotropic effects of ET-1 in 53 open-chest rats during and after a 7-min infusion. Besides measurements in the intact circulation the myocardial function was examined by isovolumic registrations independent of peripheral vascular effects. A transient ET-1 induced (500, 1000, 2500, 5000 ng ET-1/kg) decrease of the left ventricular systolic pressure (LVSP) and the mean aortic pressure (AoPmean) was followed by a dose-related rise of these pressures (LVSP: -1%, -1%, +8%, +16% vs. preinfusion values; AoPmean: -11%, +9%, +39%, +52%). Heart rate (HR) was not influenced by ET-1. Due to the dose-dependent decrease of the stroke volume (SV) the cardiac output (CO) was reduced (CO: -8%, -23%, -40%, -50%). After an initial vasodilatation ET-1 elevates the total peripheral resistance (TPR: -1%, +49%, +139%, +215%) dose-dependently. 10,000 ng ET-1/kg was a lethal dose resulting in cardiac failure within minutes (low output). Since the maximum of the isovolumic LVSP (peak LVSP) and the corresponding dP/dtmax (peak dP/dtmax) were unchanged under ET-1, the isovolumic measurements do not indicate a positive inotropic effect of ET-1 in vivo in contrast to published results of in vitro experiments. It may be possible that a direct positive inotropic effect of ET-1 observed in in vitro studies is counterbalanced in vivo by an indirect negative inotropic effect due to the coronary-constrictive effect of ET-1
Parametric excitation of a Bose-Einstein condensate in a 1D optical lattice
We study the response of a Bose-Einstein condensate to a periodic modulation
of the depth of an optical lattice. Using Gross-Pitaevskii theory, we show that
a modulation at frequency Omega drives the parametric excitation of Bogoliubov
modes with frequency Omega/2. The ensuing nonlinear dynamics leads to a rapid
broadening of the momentum distribution and a consequent large increase of the
condensate size after free expansion. We show that this process does not
require the presence of a large condensate depletion. Our results reproduce the
main features of the spectrum measured in the superfluid phase by Stoeferle et
al., Phys. Rev. Lett. 92, 130403 (2004).Comment: 4 pages, 4 figures, more results added, to appear in PRA Rapid
Communication
Precision Calculations for Associated WH and ZH Production at Hadron Colliders
Recently the next-to-next-to-leading order QCD corrections and the
electroweak O(alpha) corrections to the Higgs-strahlung processes ppbar/pp ->
WH/ZH + X have been calculated. Both types of corrections are of the order of
5-10%. In this article the various corrections are briefly discussed and
combined into state-of-the-art predictions for the cross sections. The
theoretical uncertainties from renormalization/factorization scales and from
the parton distribution functions are discussed.Comment: 8 pages, Contributed to 3rd Les Houches Workshop: Physics at TeV
Collider
Exciting Collective Oscillations in a Trapped 1D Gas
We report on the realization of a trapped one dimensional Bose gas and its
characterization by means of measuring its lowest lying collective excitations.
The quantum degenerate Bose gas is prepared in a 2D optical lattice and we find
the ratio of the frequencies of the lowest compressional (breathing) mode and
the dipole mode to be , in accordance with the
Lieb-Liniger and mean-field theory. For a thermal gas we measure
. By heating the quantum degenerate gas we have
studied the transition between the two regimes. For the lowest number of
particles attainable in the experiment the kinetic energy of the system is
similar to the interaction energy and we enter the strongly interacting regime.Comment: 4 pages, 4 figure
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