720 research outputs found
Thermal gradient-induced forces on geodetic reference masses for LISA
The low frequency sensitivity of space-borne gravitational wave observatories
will depend critically on the geodetic purity of the trajectories of orbiting
test masses. Fluctuations in the temperature difference across the enclosure
surrounding the free-falling test mass can produce noisy forces through several
processes, including the radiometric effect, radiation pressure, and
outgassing. We present here a detailed experimental investigation of thermal
gradient-induced forces for the LISA gravitational wave mission and the LISA
Pathfinder, employing high resolution torsion pendulum measurements of the
torque on a LISA-like test mass suspended inside a prototype of the LISA
gravitational reference sensor that will surround the test mass in orbit. The
measurement campaign, accompanied by numerical simulations of the radiometric
and radiation pressure effects, allows a more accurate and representative
characterization of thermal-gradient forces in the specific geometry and
environment relevant to LISA free-fall. The pressure dependence of the measured
torques allows clear identification of the radiometric effect, in quantitative
agreement with the model developed. In the limit of zero gas pressure, the
measurements are most likely dominated by outgassing, but at a low level that
does not threaten the LISA sensitivity goals.Comment: 21 pages, 16 figures, submitted to Physical Review
Rapid production of Rb BECs in a combined magnetic and optical potential
We describe an apparatus for quickly and simply producing \Rb87
Bose-Einstein condensates. It is based on a magnetic quadrupole trap and a red
detuned optical dipole trap. We collect atoms in a magneto-optical trap (MOT)
and then capture the atom in a magnetic quadrupole trap and force rf
evaporation. We then transfer the resulting cold, dense cloud into a spatially
mode-matched optical dipole trap by lowering the quadrupole field gradient to
below gravity. This technique combines the efficient capture of atoms from a
MOT into a magnetic trap with the rapid evaporation of optical dipole traps;
the approach is insensitive to the peak quadrupole gradient and the precise
trapping beam waist. Our system reliably produces a condensate with
atoms every 16\second
Effective decrease of photoelectric emission threshold from gold plated surfaces
Many applications require charge neutralisation of isolated test bodies and
this has been successfully done using photoelectric emission from surfaces
which are electrically benign(gold) or superconducting (niobium). Gold surfaces
nominally have a high work function (\,eV)which should require deep
UV photons for photoemission. In practice it has been found that it can be
achieved with somewhat lower energy photons with indicative work functions of
(\,eV). A detailed working understanding of the process is lacking
and this work reports on a study of the photoelectric emission properties of
4.6x4.6 cm^2 gold plated surfaces, representative of those used in typical
satellite applications with a film thickness of 800 nm, and measured surface
roughnesses between 7 and 340 nm. Various UV sources with photon energies from
4.8 to 6.2 eV and power outputs from 1 nW to 1000 nW, illuminated a ~0.3 cm^2
of the central surface region at angles of incidence from 0 to 60 degrees.
Final extrinsic quantum yields in the range 10 ppm to 44 ppm were reliably
obtained during 8 campaigns, covering a ~3 year period, but with intermediate
long-term variations lasting several weeks and, in some cases, bake-out
procedures at up to 200 C. Experimental results were obtained in a vacuum
system with a baseline pressure of ~10^{-7} mbar at room temperature. A working
model, designed to allow accurate simulation of any experimental configuration,
is proposed.Comment: 35 pages, 12 figure
Classical Euclidean wormhole solutions in Palatini cosmology
We study the classical Euclidean wormholes in the context of extended
theories of gravity. With no loss of generality, we use the dynamical
equivalence between gravity and scalar-tensor theories to
construct a point-like Lagrangian in the flat FRW space time. We first show the
dynamical equivalence between Palatini gravity and the
Brans-Dicke theory with self-interacting potential, and then show the dynamical
equivalence between the Brans-Dicke theory with self-interacting potential and
the minimally coupled O'Hanlon theory. We show the existence of new Euclidean
wormhole solutions for this O'Hanlon theory and, for an special case, find out
the corresponding form of having wormhole solution. For small
values of the Ricci scalar, this is in agreement with the
wormhole solution obtained for higher order gravity theory .Comment: 11 page
The Behaviour Of Cosmological Models With Varying-G
We provide a detailed analysis of Friedmann-Robertson-Walker universes in a
wide range of scalar-tensor theories of gravity. We apply solution-generating
methods to three parametrised classes of scalar-tensor theory which lead
naturally to general relativity in the weak-field limit. We restrict the
parameters which specify these theories by the requirements imposed by the
weak-field tests of gravitation theories in the solar system and by the
requirement that viable cosmological solutions be obtained. We construct a
range of exact solutions for open, closed, and flat isotropic universes
containing matter with equation of state and in vacuum.
We study the range of early and late-time behaviours displayed, examine when
there is a `bounce' at early times, and expansion maxima in closed models.Comment: 58 pages LaTeX, 6 postscript figures, uses eps
Towards a European military culture?
Recent discussion of the Common Security and Foreign Policy has focussed on the international relations between European member states. Such a focus is entirely valid since the project is being driven forward by nation states. However, the success of the Common Security and Foreign Policy and especially the development of a specifically European military capability under the European Security and Defence Policy will depend not merely on the will of the participating nation-states. Above all, it will depend on the development of a common military culture at the level of weapons development and procurement and at the level of doctrine. The problem is that at neither level is the development of a European culture remotely in sight
Measurement of the mass and lifetime of the baryon
A proton-proton collision data sample, corresponding to an integrated
luminosity of 3 fb collected by LHCb at and 8 TeV, is used
to reconstruct , decays. Using the , decay mode for calibration, the lifetime ratio and absolute
lifetime of the baryon are measured to be \begin{align*}
\frac{\tau_{\Omega_b^-}}{\tau_{\Xi_b^-}} &= 1.11\pm0.16\pm0.03, \\
\tau_{\Omega_b^-} &= 1.78\pm0.26\pm0.05\pm0.06~{\rm ps}, \end{align*} where the
uncertainties are statistical, systematic and from the calibration mode (for
only). A measurement is also made of the mass difference,
, and the corresponding mass, which
yields \begin{align*} m_{\Omega_b^-}-m_{\Xi_b^-} &= 247.4\pm3.2\pm0.5~{\rm
MeV}/c^2, \\ m_{\Omega_b^-} &= 6045.1\pm3.2\pm 0.5\pm0.6~{\rm MeV}/c^2.
\end{align*} These results are consistent with previous measurements.Comment: 11 pages, 5 figures, All figures and tables, along with any
supplementary material and additional information, are available at
https://lhcbproject.web.cern.ch/lhcbproject/Publications/LHCbProjectPublic/LHCb-PAPER-2016-008.htm
Bose-Einstein correlations of same-sign charged pions in the forward region in pp collisions at √s=7 TeV
Bose-Einstein correlations of same-sign charged pions, produced in protonproton collisions at a 7 TeV centre-of-mass energy, are studied using a data sample collected
by the LHCb experiment. The signature for Bose-Einstein correlations is observed in the
form of an enhancement of pairs of like-sign charged pions with small four-momentum
difference squared. The charged-particle multiplicity dependence of the Bose-Einstein correlation parameters describing the correlation strength and the size of the emitting source
is investigated, determining both the correlation radius and the chaoticity parameter. The
measured correlation radius is found to increase as a function of increasing charged-particle
multiplicity, while the chaoticity parameter is seen to decreas
Immersion education outcomes and the Gaelic community:Identities and language ideologies among Gaelic-medium educated adults in Scotland
Scholars have consistently theorised that language ideologies can influence the ways in which bilingual speakers in minority language settings identify and engage with the linguistic varieties available to them. Research conducted by the author examined the interplay of language use and ideologies among a purposive sample of adults who started in Gaelic medium education during the first years of its availability. Crucially, the majority of participants’ Gaelic use today is limited, although notable exceptions were found among individuals who were substantially socialised in the language at home during childhood, and a small number of new speakers. In this paper, I draw attention to some of the language ideologies that interviewees conveyed when describing their cultural identifications with Gaelic. I argue that the ideologies that informants express seem to militate against their more frequent use of the language and their association with the wider Gaelic community. In particular, I discuss interviewees’ negative perceptions of the traditionally defined, ethnolinguistic identity category ‘Gael(s)’ in their expression of language ideologies and identities, and the implications of this finding for other contexts of minority language revitalisation
Precision measurement of violation in decays
The time-dependent asymmetry in decays is
measured using collision data, corresponding to an integrated luminosity
of fb, collected with the LHCb detector at centre-of-mass energies
of and TeV. In a sample of 96 000 decays, the
-violating phase is measured, as well as the decay widths
and of the light and heavy mass eigenstates of the
system. The values obtained are rad, ps, andps, where the first uncertainty is
statistical and the second systematic. These are the most precise single
measurements of those quantities to date. A combined analysis with decays gives rad. All
measurements are in agreement with the Standard Model predictions. For the
first time the phase is measured independently for each polarisation
state of the system and shows no evidence for polarisation
dependence.Comment: 6 figure
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