147 research outputs found
A chip-scale atomic beam clock
Atomic beams are a longstanding technology for atom-based sensors and clocks
with widespread use in commercial frequency standards. Here, we report the
demonstration a chip-scale microwave atomic beam clock using coherent
population trapping (CPT) interrogation in a passively pumped atomic beam
device. The beam device consists of a hermetically sealed vacuum cell
fabricated from an anodically bonded stack of glass and Si wafers. Atomic beams
are created using a lithographically defined microcapillary array connected to
a Rb reservoir1 and propagate in a 15 mm long drift cavity. We present a
detailed characterization of the atomic beam performance (total Rb flux
at 363 K device temperature) and of the
vacuum environment in the device (pressure < 1 Pa), which is sustained using
getter materials which pump residual gases and Rb vapor. A chip-scale beam
clock is realized using Ramsey CPT spectroscopy of the 87Rb ground state
hyperfine transition over a 10 mm Ramsey distance in the atomic beam device.
The prototype atomic beam clock demonstrates a fractional frequency stability
of for integration times from 1
s to 250 s, limited by detection noise. Optimized atomic beam clocks based on
this approach may exceed the long-term stability of existing chip-scale clocks,
and leading long-term systematics are predicted to limit the ultimate
fractional frequency stability below .Comment: 22 pages, 4 figure
Evidence for the accelerated expansion of the Universe from weak lensing tomography with COSMOS
We present a tomographic cosmological weak lensing analysis of the HST COSMOS
Survey. Applying our lensing-optimized data reduction, principal component
interpolation for the ACS PSF, and improved modelling of charge-transfer
inefficiency, we measure a lensing signal which is consistent with pure
gravitational modes and no significant shape systematics. We carefully estimate
the statistical uncertainty from simulated COSMOS-like fields obtained from
ray-tracing through the Millennium Simulation. We test our pipeline on
simulated space-based data, recalibrate non-linear power spectrum corrections
using the ray-tracing, employ photometric redshifts to reduce potential
contamination by intrinsic galaxy alignments, and marginalize over systematic
uncertainties. We find that the lensing signal scales with redshift as expected
from General Relativity for a concordance LCDM cosmology, including the full
cross-correlations between different redshift bins. For a flat LCDM cosmology,
we measure sigma_8(Omega_m/0.3)^0.51=0.75+-0.08 from lensing, in perfect
agreement with WMAP-5, yielding joint constraints Omega_m=0.266+0.025-0.023,
sigma_8=0.802+0.028-0.029 (all 68% conf.). Dropping the assumption of flatness
and using HST Key Project and BBN priors only, we find a negative deceleration
parameter q_0 at 94.3% conf. from the tomographic lensing analysis, providing
independent evidence for the accelerated expansion of the Universe. For a flat
wCDM cosmology and prior w in [-2,0], we obtain w<-0.41 (90% conf.). Our dark
energy constraints are still relatively weak solely due to the limited area of
COSMOS. However, they provide an important demonstration for the usefulness of
tomographic weak lensing measurements from space. (abridged)Comment: 26 pages, 25 figures, matches version accepted for publication by
Astronomy and Astrophysic
Measurement of Angular Distributions and R= sigma_L/sigma_T in Diffractive Electroproduction of rho^0 Mesons
Production and decay angular distributions were extracted from measurements
of exclusive electroproduction of the rho^0(770) meson over a range in the
virtual photon negative four-momentum squared 0.5< Q^2 <4 GeV^2 and the
photon-nucleon invariant mass range 3.8< W <6.5 GeV. The experiment was
performed with the HERMES spectrometer, using a longitudinally polarized
positron beam and a ^3He gas target internal to the HERA e^{+-} storage ring.
The event sample combines rho^0 mesons produced incoherently off individual
nucleons and coherently off the nucleus as a whole. The distributions in one
production angle and two angles describing the rho^0 -> pi+ pi- decay yielded
measurements of eight elements of the spin-density matrix, including one that
had not been measured before. The results are consistent with the dominance of
helicity-conserving amplitudes and natural parity exchange. The improved
precision achieved at 47 GeV,
reveals evidence for an energy dependence in the ratio R of the longitudinal to
transverse cross sections at constant Q^2.Comment: 15 pages, 15 embedded figures, LaTeX for SVJour(epj) document class
Revision: Fig. 15 corrected, recent data added to Figs. 10,12,14,15; minor
changes to tex
Determination of the Deep Inelastic Contribution to the Generalised Gerasimov-Drell-Hearn Integral for the Proton and Neutron
The virtual photon absorption cross section differences [sigma_1/2-sigma_3/2]
for the proton and neutron have been determined from measurements of polarised
cross section asymmetries in deep inelastic scattering of 27.5 GeV
longitudinally polarised positrons from polarised 1H and 3He internal gas
targets. The data were collected in the region above the nucleon resonances in
the kinematic range nu < 23.5 GeV and 0.8 GeV**2 < Q**2 < 12 GeV**2. For the
proton the contribution to the generalised Gerasimov-Drell-Hearn integral was
found to be substantial and must be included for an accurate determination of
the full integral. Furthermore the data are consistent with a QCD
next-to-leading order fit based on previous deep inelastic scattering data.
Therefore higher twist effects do not appear significant.Comment: 6 pages, 3 figures, 1 table, revte
Analysis of the impact of the tax system on the cash flow of small businesses: a report for HM Revenue and Customs (HMRC)
Group IVB metalloidal substituent effects in the naphthalene system by fluorine-19 nuclear magnetic resonance
19F Substituent chemical shift (SCS) data for substituents of the type Me3M (where M represents Si, Ge, Sn, and Pb) attached to the naphthalene system provide experimental evidence that all the metalloidal substituents engage in dπ-pπ conjugative electron-withdrawal (-M) in the ground state: and suggested that certain approaches to the analysis of 19F SCS may be invalid
Electrophilic Substitution with Allylic Rearrangement (SE′). Anti Stereoselectivity in Trifluoroacetolysis of Some CycIohex-2-enylsilanes, -germanes, and -stannanes
Trifluoroacetolysis of various cyclohex-2-enylsilanes, -germanes, and -stannanes is demonstrated to be γ regiospecific and highly anti stereoselective, as predicted for a dominating LUMO-HOMO interaction in a concerted S2′ process
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