865 research outputs found
The Refractive Index of Silicon at Gamma Ray Energies
The index of refraction n(E_{\gamma})=1+\delta(E_{\gamma})+i\beta(E_{\gamma})
is split into a real part \delta and an absorptive part \beta. The absorptive
part has the three well-known contributions to the cross section \sigma_{abs}:
the photo effect, the Compton effect and the pair creation, but there is also
the inelastic Delbr\"uck scattering. Second-order elastic scattering cross
sections \sigma_{sca} with Rayleigh scattering (virtual photo effect), virtual
Compton effect and Delbr\"uck scattering (virtual pair creation) can be
calculated by integrals of the Kramers-Kronig dispersion relations from the
cross section \sigma_{abs}. The real elastic scattering amplitudes are
proportional to the refractive indices \delta_{photo}, \delta_{Compton} and
\delta_{pair}. While for X-rays the negative \delta_{photo} dominates, we show
for the first time experimentally and theoretically that the positive
\delta_{pair} dominates for \gamma rays, opening a new era of \gamma optics
applications, i.e. of nuclear photonics.Comment: 4 pages, 3 figure
Axions, their Relatives and Prospects for the Future
The observation of a non-vanishing rotation of linear polarized laser light
after passage through a strong magnetic field by the PVLAS collaboration has
renewed the interest in light particles coupled to photons. Axions are a
species of such particles that is theoretically well motivated. However, the
relation between coupling and mass predicted by standard axion models conflicts
with the PVLAS observation. Moreover, light particles with a coupling to
photons of the strength required to explain PVLAS face trouble from
astrophysical bounds. We discuss models that can avoid these bounds. Finally,
we present some ideas to test these possible explanations of PVLAS
experimentally.Comment: 11 pages, 4 figures. Contributed to the ``Third Symposium on Large
TPCs for Low Energy Rare Event Detection'' in Paris, December 200
Optimal Economic Growth under Stochastic Environmental Impact: Sensitivity Analysis
In this work we present an approach toward the sensitivity analysis of optimal economic growth to a negative environmental impact driven by random natural hazards that damage the production output . We use a simplified model of the GDP whose growth leads to the increase of GHG in the atmosphere provided investment in cleaning is insufficient. The hypothesis of the Poisson probability distribution of the natural hazards is used at the first stage of the research. We apply the standard utility function - the discounted integral consumption and construct an optimal investment policy in production and cleaning together with optimal GDP trajectories. We calibrate the model in the global scale and analyze the sensitivity of obtained optimal growth scenarios with respect to uncertain parameters of the Poisson distribution
Can Light Signals Travel Faster than c in Nontrivial Vacuua in Flat space-time? Relativistic Causality II
In this paper we show that the Scharnhorst effect (Vacuum with boundaries or
a Casimir type vacuum) cannot be used to generate signals showing measurable
faster-than-c speeds. Furthermore, we aim to show that the Scharnhorst effect
would violate special relativity, by allowing for a variable speed of light in
vacuum, unless one can specify a small invariant length scale. This invariant
length scale would be agreed upon by all inertial observers. We hypothesize the
approximate scale of the invariant length.Comment: 12 pages no figure
External Fields as a Probe for Fundamental Physics
Quantum vacuum experiments are becoming a flexible tool for investigating
fundamental physics. They are particularly powerful for searching for new light
but weakly interacting degrees of freedom and are thus complementary to
accelerator-driven experiments. I review recent developments in this field,
focusing on optical experiments in strong electromagnetic fields. In order to
characterize potential optical signatures, I discuss various low-energy
effective actions which parameterize the interaction of particle-physics
candidates with optical photons and external electromagnetic fields.
Experiments with an electromagnetized quantum vacuum and optical probes do not
only have the potential to collect evidence for new physics, but
special-purpose setups can also distinguish between different particle-physics
scenarios and extract information about underlying microscopic properties.Comment: 12 pages, plenary talk at QFEXT07, Leipzig, September 200
Dispersive properties of quasi-phase-matched optical parametric amplifiers
The dispersive properties of non-degenerate optical parametric amplification
in quasi-phase-matched (QPM) nonlinear quadratic crystals with an arbitrary
grating profile are theoretically investigated in the no-pump-depletion limit.
The spectral group delay curve of the amplifier is shown to be univocally
determined by its spectral power gain curve through a Hilbert transform. Such a
constraint has important implications on the propagation of spectrally-narrow
optical pulses through the amplifier. In particular, it is shown that anomalous
transit times, corresponding to superluminal or even negative group velocities,
are possible near local minima of the spectral gain curve. A possible
experimental observation of such effects using a QPM Lithium-Niobate crystal is
suggested.Comment: submitted for publicatio
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