762 research outputs found
Femtosecond energy transfer between chromophores in allophycocyanin trimers
Ultrafast energy-transfer processes in allophycocyanin (APC) trimers from Mastigocladus laminosus have been examined by a femtosecond absorption technique. Isotropic absorption recovery kinetics with τ=440±30 fs were observed in APC trimers at 615 nm. In APC monomers such a fast process was not observed. The anisotropy in both samples was constant and close to 0.4 during the first few picoseconds. The results are consistent with a model of the APC trimer in which the two APC chromophores have different absorption spectra with maxima about 600 and 650 nm. The transfer of energy from the 600 nm chromophore to the 650 nm chromophore occurs in 440 fs and is dominated by the Förster dipole—dipole energy-transfer mechanism
Leptoquark Pair Production in Hadronic Interactions
The scalar and vector leptoquark pair production cross sections in hadronic
collisions are calculated. In a model independent analysis we consider the most
general C and P conserving couplings of gluons to both scalar and vector
leptoquarks described by an effective low--energy Lagangian which obeys SU(3)_c
invariance. Analytrical expressions are derived for the differential and
integral scattering cross sections including the case of anomalous vector
leptoquark couplings, and , to the gluon field. Numerical
predictions are given for the kinematic range of the TEVATRON and LHC. The pair
production cross sections are also calculated for the resolved photon
contributions to at HERA and LEP
LHC, and for the process at possible future linear colliders and
colliders. Estimates of the search potential for scalar and vector leptoquarks
at present and future high energy colliders are given.Comment: 44 pages Latex, including 2 ps-figures and 19 eps-figure
Systems to Control Molten Metal Transfer in Arc Welding
The paper analyzes the systems used for controlling molten wire metal droplets during the arc welding process in shielding gases. The variations for implementing the relevant systems are given, with the positive and negative aspects of such implementation taken into account. Electrical systems are currently investigated to the fullest extent possible and implemented in different power sources for pulsed welding arc. Mechanical systems are represented by different types of feeders that provide the pulsed wire feeding process. The feed mechanisms driven by electric motors and electromagnets are analyzed. In addition to the mechanical and electrical systems, the examples of combined control systems are given
On observation of position in quantum theory
Newtonian and Scroedinger dynamics can be formulated in a physically meaningful way within the same Hilbert space framework. This fact was recently used to discover an unexpected relation between classical and quantum motions that goes beyond the results provided by the Ehrenfest theorem. A formula relating the normal probability distribution and the Born rule was also found. Here the dynamical mechanism responsible for the latter formula is proposed and applied to measurements of macroscopic and microscopic systems. A relationship between the classical Brownian motion and the diffusion of state on the space of states is discovered. The role of measuring devices in quantum theory is investigated in the new framework. It is shown that the so-called collapse of the wave function is not measurement specific and does not require a ``concentration" near the eigenstates of the measured observable. Instead, it is explained by the common diffusion of a state over the space of states under interaction with the apparatus and the environment.
This in turn provides us with a basic reason for the definite position of macroscopic bodies in space
CompHEP-PYTHIA interface: integrated package for the collision events generation based on exact matrix elements
CompHEP, as a partonic event generator, and PYTHIA, as a generator of final
states of detectable objects, are interfaced. Thus, integrated tool is proposed
for simulation of (almost) arbitrary collision processes at the level of
detectable particles. Exact (multiparticle) matrix elements, convolution with
structure functions, decays, partons hadronization and (optionally) parton
shower evolution are basic stages of calculations. The PEVLIB library of event
generators for LHC processes is described.Comment: Standard LaTeX, 4 pages. To appear in the proceedings of the Seventh
International Workshop on Advanced Computing and Analysis Technics in Physics
Research (ACAT2000, Fermilab, October 16-20, 2000
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