1,283 research outputs found
Maximal Accuracy and Minimal Disturbance in the Arthurs-Kelly Simultaneous Measurement Process
The accuracy of the Arthurs-Kelly model of a simultaneous measurement of
position and momentum is analysed using concepts developed by Braginsky and
Khalili in the context of measurements of a single quantum observable. A
distinction is made between the errors of retrodiction and prediction. It is
shown that the distribution of measured values coincides with the initial state
Husimi function when the retrodictive accuracy is maximised, and that it is
related to the final state anti-Husimi function (the P representation of
quantum optics) when the predictive accuracy is maximised. The disturbance of
the system by the measurement is also discussed. A class of minimally
disturbing measurements is characterised. It is shown that the distribution of
measured values then coincides with one of the smoothed Wigner functions
described by Cartwright.Comment: 12 pages, 0 figures. AMS-Latex. Earlier version replaced with final
published versio
Time-of-arrival distributions from position-momentum and energy-time joint measurements
The position-momentum quasi-distribution obtained from an Arthurs and Kelly
joint measurement model is used to obtain indirectly an ``operational''
time-of-arrival (TOA) distribution following a quantization procedure proposed
by Kocha\'nski and W\'odkiewicz [Phys. Rev. A 60, 2689 (1999)]. This TOA
distribution is not time covariant. The procedure is generalized by using other
phase-space quasi-distributions, and sufficient conditions are provided for
time covariance that limit the possible phase-space quasi-distributions
essentially to the Wigner function, which, however, provides a non-positive TOA
quasi-distribution. These problems are remedied with a different quantization
procedure which, on the other hand, does not guarantee normalization. Finally
an Arthurs and Kelly measurement model for TOA and energy (valid also for
arbitrary conjugate variables when one of the variables is bounded from below)
is worked out. The marginal TOA distribution so obtained, a distorted version
of Kijowski's distribution, is time covariant, positive, and normalized
Low Q^2 Jet Production at HERA and Virtual Photon Structure
The transition between photoproduction and deep-inelastic scattering is
investigated in jet production at the HERA ep collider, using data collected by
the H1 experiment. Measurements of the differential inclusive jet
cross-sections dsigep/dEt* and dsigmep/deta*, where Et* and eta* are the
transverse energy and the pseudorapidity of the jets in the virtual
photon-proton centre of mass frame, are presented for 0 < Q2 < 49 GeV2 and 0.3
< y < 0.6. The interpretation of the results in terms of the structure of the
virtual photon is discussed. The data are best described by QCD calculations
which include a partonic structure of the virtual photon that evolves with Q2.Comment: 20 pages, 5 Figure
Physics of Solar Prominences: II - Magnetic Structure and Dynamics
Observations and models of solar prominences are reviewed. We focus on
non-eruptive prominences, and describe recent progress in four areas of
prominence research: (1) magnetic structure deduced from observations and
models, (2) the dynamics of prominence plasmas (formation and flows), (3)
Magneto-hydrodynamic (MHD) waves in prominences and (4) the formation and
large-scale patterns of the filament channels in which prominences are located.
Finally, several outstanding issues in prominence research are discussed, along
with observations and models required to resolve them.Comment: 75 pages, 31 pictures, review pape
Signaling of angiotensin II-induced vascular protein synthesis in conduit and resistance arteries in vivo
BACKGROUND: From in vitro studies, it has become clear that several signaling cascades are involved in angiotensin II-induced cellular hypertrophy. The aim of the present study was to determine some of the signaling pathways mediating angiotensin II (Ang II)-induced protein synthesis in vivo in large and small arteries. METHODS: Newly synthesized proteins were labeled during 4 hours with tritiated leucine in conscious control animals, or animals infused for 24 hours with angiotensin II (400 ng/kg/min). Hemodynamic parameters were measure simultaneously. Pharmacological agents affecting signaling cascades were injected 5 hours before the end of Ang II infusion. RESULTS: Angiotensin II nearly doubled the protein synthesis rate in the aorta and small mesenteric arteries, without affecting arterial pressure. The AT(1 )receptor antagonist Irbesartan antagonized the actions of Ang II. The Ang II-induced protein synthesis was associated with increased extracellular signal-regulated kinases (ERK)1/2 phosphorylation in aortic, but not in mesenteric vessels. Systemic administration of PD98059, an inhibitor of the ERK-1/2 pathway, produced a significant reduction of protein synthesis rate in the aorta, and only a modest decrease in mesenteric arteries. Rapamycin, which influences protein synthesis by alternative signaling, had a significant effect in both vessel types. Rapamycin and PD98059 did not alter basal protein synthesis and had minimal effects on arterial pressure. CONCLUSION: ERK1/2 and rapamycin-sensitive pathways are involved in pressure-independent angiotensin II-induced vascular protein synthesis in vivo. However, their relative contribution may vary depending on the nature of the artery under investigation
A Search for Selectrons and Squarks at HERA
Data from electron-proton collisions at a center-of-mass energy of 300 GeV
are used for a search for selectrons and squarks within the framework of the
minimal supersymmetric model. The decays of selectrons and squarks into the
lightest supersymmetric particle lead to final states with an electron and
hadrons accompanied by large missing energy and transverse momentum. No signal
is found and new bounds on the existence of these particles are derived. At 95%
confidence level the excluded region extends to 65 GeV for selectron and squark
masses, and to 40 GeV for the mass of the lightest supersymmetric particle.Comment: 13 pages, latex, 6 Figure
Energy Flow in the Hadronic Final State of Diffractive and Non-Diffractive Deep-Inelastic Scattering at HERA
An investigation of the hadronic final state in diffractive and
non--diffractive deep--inelastic electron--proton scattering at HERA is
presented, where diffractive data are selected experimentally by demanding a
large gap in pseudo --rapidity around the proton remnant direction. The
transverse energy flow in the hadronic final state is evaluated using a set of
estimators which quantify topological properties. Using available Monte Carlo
QCD calculations, it is demonstrated that the final state in diffractive DIS
exhibits the features expected if the interaction is interpreted as the
scattering of an electron off a current quark with associated effects of
perturbative QCD. A model in which deep--inelastic diffraction is taken to be
the exchange of a pomeron with partonic structure is found to reproduce the
measurements well. Models for deep--inelastic scattering, in which a
sizeable diffractive contribution is present because of non--perturbative
effects in the production of the hadronic final state, reproduce the general
tendencies of the data but in all give a worse description.Comment: 22 pages, latex, 6 Figures appended as uuencoded fil
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