828 research outputs found
Scintillation reduction for combined Gaussian-vortex beam propagating through turbulent atmosphere
We numerically examine the spatial evolution of the structure of coherent and
partially coherent laser beams (PCBs), including the optical vortices,
propagating in turbulent atmospheres. The influence of beam fragmentation and
wandering relative to the axis of propagation (z-axis) on the value of the
scintillation index (SI) of the signal at the detector is analyzed. A method
for significantly reducing the SI, by averaging the signal at the detector over
a set of PCBs, is described. This novel method is to generate the PCBs by
combining two laser beams - Gaussian and vortex beams, with different
frequencies (the difference between these two frequencies being significantly
smaller than the frequencies themselves). In this case, the SI is effectively
suppressed without any high-frequency modulators.Comment: 13 pages, 8 figure
On the forward-backward charge asymmetry in e+e- -annihilation into hadrons at high energies
The forward-backward asymmetry in e+ e- annihilation into a quark-antiquark
pair is considered in the double-logarithmic approximation at energies much
higher than the masses of the weak bosons. It is shown that after accounting to
all orders for the exchange of virtual photons and W, Z -bosons one is lead to
the following effect (asymmetry): quarks with positive electric charge (e.g. u,
\bar{d}) tend to move in the e+ - direction whereas quarks with negative
charges (e.g. d, \bar{u}) tend to move in the e- - direction. The value of the
asymmetry grows with increasing energy when the produced quarks are within a
cone with opening angle, in the cmf, \theta_0\sim 2M_Z/\sqrt{s} around the e+e-
-beam. Outside this cone, at \theta_0 << \theta << 1, the asymmetry is
inversely proportional to \theta .Comment: 17 Pages, 2 Tables, 4 Figures. Hadronization effects to the asymmetry
are considered with more detail
Faddeev-type calculations of few-body nuclear reactions including Coulomb interaction
The method of screening and renormalization is used to include the Coulomb
interaction between the charged particles in the description of few-body
nuclear reactions. Calculations are done in the framework of Faddeev-type
equations in momentum-space. The reliability of the method is demonstrated. The
Coulomb effect on observables is discussed.Comment: Proceedings of the 4th Asia-Pacific Conference on Few-Body Problems
in Physics (APFB08), Depok, Indonesia, August 19 - 23, 2008, to be published
in Mod. Phys. Lett.
Production of a pion in association with a high-Q2 dilepton pair in antiproton-proton annihilation at GSI-FAIR
We evaluate the cross section for anti-p p -> l+ l- pi0 in the forward
direction and for large lepton pair invariant mass. In this kinematical region,
the leading-twist amplitude factorises into a short-distance matrix element,
long-distance dominated antiproton Distribution Amplitudes and proton to pion
Transition Distribution Amplitudes (TDA). Using a modelling inspired from the
chiral limit for these TDAs, we obtain a first estimate of this cross section,
thus demonstrating that this process can be measured at GSI-FAIR.Comment: Latex, 5 pages, 3 figure
Elastic Scattering Amplitude at 1.8 TeV and Determination of Total Cross Section
The data on p elastic scattering at 1.8 and 1.96 TeV are
analysed in terms of real and imaginary amplitudes, in a treatment with high
accuracy, covering the whole t-range and satisfying the expectation of
dispersion relation for amplitudes and for slopes. A method is introduced for
determination of the total cross section and the other forward scattering
parameters and to check compatibility of E-710, CDF and the recent D0 data.
Slopes and of the real and imaginary amplitudes, treated as
independent quantities, influence the amplitudes in the whole t-range and are
important for the determination of the total cross section. The amplitudes are
fully constructed, and a prediction is made of a marked dip in in
the range 3 - 5 GeV due to the universal contribution of the process
of three gluon exchange.Comment: 22 pages, 12 figures, 2 table
Formation of ions by high energy photons
We calculate the electron energy spectrum of ionization by a high energy
photon, accompanied by creation of electron-positron pair. The total cross
section of the process is also obtained. The asymptotics of the cross section
does not depend on the photon energies. At the photon energies exceeding a
certain value this appeares to to be the dominant mechanism of
formation of the ions. The dependence of on the value of nuclear
charge is obtained. Our results are consistent with experimental data.Comment: 16 pages, 6 figure
Photon storage in Lambda-type optically dense atomic media. II. Free-space model
In a recent paper [Gorshkov et al., Phys. Rev. Lett. 98, 123601 (2007)], we
presented a universal physical picture for describing a wide range of
techniques for storage and retrieval of photon wave packets in Lambda-type
atomic media in free space, including the adiabatic reduction of the photon
group velocity, pulse-propagation control via off-resonant Raman techniques,
and photon-echo based techniques. This universal picture produced an optimal
control strategy for photon storage and retrieval applicable to all approaches
and yielded identical maximum efficiencies for all of them. In the present
paper, we present the full details of this analysis as well some of its
extensions, including the discussion of the effects of non-degeneracy of the
two lower levels of the Lambda system. The analysis in the present paper is
based on the intuition obtained from the study of photon storage in the cavity
model in the preceding paper [Gorshkov et al., Phys. Rev. A 76, 033804 (2007)].Comment: 26 pages, 8 figures. V2: significant changes in presentation, new
references, higher resolution of figure
Realization of Coherent Optically Dense Media via Buffer-Gas Cooling
We demonstrate that buffer-gas cooling combined with laser ablation can be
used to create coherent optical media with high optical depth and low Doppler
broadening that offers metastable states with low collisional and motional
decoherence. Demonstration of this generic technique opens pathways to coherent
optics with a large variety of atoms and molecules. We use helium buffer gas to
cool 87Rb atoms to below 7 K and slow atom diffusion to the walls.
Electromagnetically induced transparency (EIT) in this medium allows for 50%
transmission in a medium with initial OD >70 and for slow pulse propagation
with large delay-bandwidth products. In the high-OD regime, we observe
high-contrast spectrum oscillations due to efficient four-wave mixing.Comment: 4 pages, 4 figures. V2: modified title, abstract, introduction,
conclusion; added references; improved theoretical fit in figure 3(b);
shortened slow light theory description; clarified simplicity of apparatus.
Final version as published in Phys. Rev.
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