204 research outputs found
Pure electrical, highly-efficient and sidelobe free coherent Raman spectroscopy using acousto-optics tunable filter (AOTF)
Fast and sensitive Raman spectroscopy measurements are imperative for a large number of applications in biomedical imaging, remote sensing and material characterization. Stimulated Raman spectroscopy offers a substantial improvement in the signal-to-noise ratio but is often limited to a discrete number of wavelengths. In this report, by introducing an electronically-tunable acousto-optical filter as a wavelength selector, a novel approach to a broadband stimulated Raman spectroscopy is demonstrated. The corresponding Raman shift covers the spectral range from 600 cm(−1) to 4500 cm(−1), sufficient for probing most vibrational Raman transitions. We validated the use of the new instrumentation to both coherent anti-Stokes scattering (CARS) and stimulated Raman scattering (SRS) spectroscopies
Measurements of Lifetimes and a Limit on the Lifetime Difference in the Neutral D-Meson System
Using the large hadroproduced charm sample collected in experiment E791 at
Fermilab, we report the first directly measured constraint on the decay-width
difference Delta Gamma for the mass eigenstates of the D0-D0bar system. We
obtain our result from lifetime measurements of the decays D0 --> K-pi+ and D0
--> K-K+, under the assumption of CP invariance, which implies that the CP
eigenstates and the mass eigenstates are the same. The lifetime of D0 --> K-K+
(the CP-even final state is \tau_KK = 0.410 +/- 0.011 +/- 0.006 ps, and the
lifetime of D0 --> K-pi+ (an equal mixture of CP-odd and CP-even final states
is tau_Kpi = 0.413 +/- 0.003 +/- 0.004 ps. The decay-width difference is Delta
Gamma = 2(Gamma_KK - Gamma_Kpi) = 0.04 +/- 0.14 +/- 0.05 ps^-1. We relate these
measurements to measurements of mixing in the neutral D-meson system.Comment: 8 pages + 3 figures + 2 table
DVCS amplitude at tree level: Transversality, twist-3, and factorization
We study the virtual Compton amplitude in the generalized Bjorken region (q^2
-> Infinity, t small) in QCD by means of a light-cone expansion of the product
of e.m. currents in string operators in coordinate space. Electromagnetic gauge
invariance (transversality) is maintained by including in addition to the
twist-2 operators 'kinematical' twist-3 operators which appear as total
derivatives of twist-2 operators. The non-forward matrix elements of the
elementary twist-2 operators are parametrized in terms of two-variable spectral
functions (double distributions), from which twist-2 and 3 skewed distributions
are obtained through reduction formulas. Our approach is equivalent to a
Wandzura-Wilczek type approximation for the twist-3 skewed distributions. The
resulting Compton amplitude is manifestly transverse up to terms of order
t/q^2. We find that in this approximation the tensor amplitude for longitudinal
polarization of the virtual photon is finite, while the one for transverse
polarization contains a divergence already at tree level. However, this
divergence has zero projection on the polarization vector of the final photon,
so that the physical helicity amplitudes are finite.Comment: 34 pages, revtex, 1 eps figure included using epsf. Misprints
corrected, one reference adde
Off-forward parton distributions and Shuvaev's transformations
We review Shuvaev's transformations, that relate off-forward parton
distributions (OFPDs) to so-called effective forward parton distributions
(EFPDs). The latter evolve like conventional forward partons. We express
nonforward amplitudes, depending on OFPDs, directly in terms of EFPDs and
construct a model for the EFPDs, which allows to consistently express them in
terms of the conventional forward parton distributions and nucleon form
factors. Our model is self-consistent for arbitrary x, xi, mu, and t.Comment: 13 pages, 7 eps-figures, LaTeX2e, added references, corrected typo
SU(3) Breaking and D0-D0bar Mixing
The main challenge in the Standard Model calculation of the mass and width
difference in the D0-D0bar system is to estimate the size of SU(3) breaking
effects. We prove that D meson mixing occurs in the Standard Model only at
second order in SU(3) violation. We consider the possibility that phase space
effects may be the dominant source of SU(3) breaking. We find that y=(Delta
Gamma)/(2Gamma) of the order of one percent is natural in the Standard Model,
potentially reducing the sensitivity to new physics of measurements of D meson
mixing.Comment: 18 pages; minor corrections, version to appear in Phys. Rev.
The Potential for Neutrino Physics at Muon Colliders and Dedicated High Current Muon Storage Rings
Conceptual design studies are underway for muon colliders and other
high-current muon storage rings that have the potential to become the first
true ``neutrino factories''. Muon decays in long straight sections of the
storage rings would produce precisely characterized beams of electron and muon
type neutrinos of unprecedented intensity. This article reviews the prospects
for these facilities to greatly extend our capabilities for neutrino
experiments, largely emphasizing the physics of neutrino interactions.Comment: 107 pages, 16 figures, to be published in Physics Report
Bright emission from a random Raman laser
Random lasers are a developing class of light sources that utilize a highly disordered gain medium as opposed to a conventional optical cavity. Although traditional random lasers often have a relatively broad emission spectrum, a random laser that utilizes vibration transitions via Raman scattering allows for an extremely narrow bandwidth, on the order of 10 cm(−1). Here we demonstrate the first experimental evidence of lasing via a Raman interaction in a bulk three-dimensional random medium, with conversion efficiencies on the order of a few percent. Furthermore, Monte Carlo simulations are used to study the complex spatial and temporal dynamics of nonlinear processes in turbid media. In addition to providing a large signal, characteristic of the Raman medium, the random Raman laser offers us an entirely new tool for studying the dynamics of gain in a turbid medium
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