9,223 research outputs found
Characterization of the Vacuum Birefringence Polarimeter at BMV: Dynamical Cavity Mirror Birefringence
We present the current status and outlook of the optical characterization of
the polarimeter at the Bir\'{e}fringence Magn\'etique du Vide (BMV) experiment.
BMV is a polarimetric search for the QED predicted anisotropy of vacuum in the
presence of external electromagnetic fields. The main challenge faced in this
fundamental test is the measurement of polarization ellipticity on the order of
induced in linearly polarized laser field per pass through a
magnetic field having an amplitude and length
. This challenge is addressed by
understanding the noise sources in precision cavity-enhanced polarimetry. In
this paper we discuss the first investigation of dynamical birefringence in the
signal-enhancing cavity as a result of cavity mirror motion.Comment: To appear in the 2019 CPEM special issue of IEEE Transactions on
Instrumentation and Measuremen
Noise characterization for resonantly-enhanced polarimetric vacuum magnetic-birefringence experiments
In this work we present data characterizing the sensitivity of the
Bir\'{e}fringence Magnetique du Vide (BMV) instrument. BMV is an experiment
attempting to measure vacuum magnetic birefringence (VMB) via the measurement
of an ellipticity induced in a linearly polarized laser field propagating
through a birefringent region of vacuum in the presence of an external magnetic
field. Correlated measurements of laser noise alongside the measurement in the
main detection channel allow us to separate measured sensing noise from the
inherent birefringence noise of the apparatus. To this end we model different
sources of sensing noise for cavity-enhanced polarimetry experiments, such as
BMV. Our goal is to determine the main sources of noise, clarifying the
limiting factors of such an apparatus. We find our noise models are compatible
with the measured sensitivity of BMV. In this context we compare the phase
sensitivity of separate-arm interferometers to that of a polarimetry apparatus
for the discussion of current and future VMB measurements
[TiII] and [NiII] emission from the strontium filament of eta Carinae
We study the nature of the [TiII] and [NiII] emission from the so-called
strontium filament found in the ejecta of eta Carinae. To this purpose we
employ multilevel models of the TiII and NiII systems which are used to
investigate the physical condition of the filament and the excitation
mechanisms of the observed lines. For the TiII ion, for which no atomic data
was previously available, we carry out ab initio calculations of radiative
transition rates and electron impact excitation rate coefficients. It is found
that the observed spectrum is consistent with the lines being excited in a
mostly neutral region with an electron density of the order of cm
and a temperature around 6000 K. In analyzing three observations with different
slit orientations recorded between March~2000 and November~2001 we find line
ratios that change among various observations, in a way consistent with changes
of up to an order of magnitude in the strength of the continuum radiation
field. These changes result from different samplings of the extended filament,
due to the different slit orientations used for each observation, and yield
clues on the spatial extent and optical depth of the filament. The observed
emission indicates a large Ti/Ni abundance ratio relative to solar abundances.
It is suggested that the observed high Ti/Ni ratio in gas is caused by dust-gas
fractionation processes and does not reflect the absolute Ti/Ni ratio in the
ejecta of \etacar. We study the condensation chemistry of Ti, Ni and Fe within
the filament and suggest that the observed gas phase overabundance of TiComment: 14 paginas, 12 figure
Genetic and physical mapping of DNA replication origins in Haloferax volcanii
The halophilic archaeon Haloferax volcanii has a multireplicon genome, consisting of a main chromosome, three secondary chromosomes, and a plasmid. Genes for the initiator protein Cdc6/Orc1, which are commonly located adjacent to archaeal origins of DNA replication, are found on all replicons except plasmid pHV2. However, prediction of DNA replication origins in H. volcanii is complicated by the fact that this species has no less than 14 cdc6/orc1 genes. We have used a combination of genetic, biochemical, and bioinformatic approaches to map DNA replication origins in H. volcanii. Five autonomously replicating sequences were found adjacent to cdc6/orc1 genes and replication initiation point mapping was used to confirm that these sequences function as bidirectional DNA replication origins in vivo. Pulsed field gel analyses revealed that cdc6/orc1-associated replication origins are distributed not only on the main chromosome (2.9 Mb) but also on pHV1 (86 kb), pHV3 (442 kb), and pHV4 (690 kb) replicons. Gene inactivation studies indicate that linkage of the initiator gene to the origin is not required for replication initiation, and genetic tests with autonomously replicating plasmids suggest that the origin located on pHV1 and pHV4 may be dominant to the principal chromosomal origin. The replication origins we have identified appear to show a functional hierarchy or differential usage, which might reflect the different replication requirements of their respective chromosomes. We propose that duplication of H. volcanii replication origins was a prerequisite for the multireplicon structure of this genome, and that this might provide a means for chromosome-specific replication control under certain growth conditions. Our observations also suggest that H. volcanii is an ideal organism for studying how replication of four replicons is regulated in the context of the archaeal cell cycle. © 2007 Norais et al
Dependence of inner-shell vacancy production upon distance in hard Li-Al collisions
We match the predictions of molecular-dynamics simulations of 1.2 keV and 2.0 keV 7Li+ scattered from Al(100) to observed total Li atom spectra measured by time-of-flight spectroscopy. In doing so we determine the relevant parameters in a simple distance of closest approach model for the probability of production of single and double vacancies in the Li 1s shell during hard Li-Al collisions. In the standard Fano-Lichten model of vacancy production, vacancies are produced with unit probability if the collision is hard enough to force the collision partners past some critical distance of closest approach. We find that such an assumption is insufficient to fit our simulations to experimental observations, and that we must allow for a gradual turning on of the vacancy production probability as the distance of closest approach decreases. The resulting model may be useful in modeling atomic excitation effects in simulations of other ion-impact processes
On the CFT duals for near-extremal black holes
We consider Kerr-Newman-AdS-dS black holes near extremality and work out the
near-horizon geometry of these near-extremal black holes. We identify the exact
U(1)_L x U(1)_R isometries of the near-horizon geometry and provide boundary
conditions enhancing them to a pair of commuting Virasoro algebras. The
conserved charges of the corresponding asymptotic symmetries are found to be
well defined and non-vanishing and to yield central charges c_L\neq0 and c_R=0.
The Cardy formula subsequently reproduces the Bekenstein-Hawking entropy of the
black hole. This suggests that the near-extremal Kerr-Newman-AdS-dS black hole
is holographically dual to a non-chiral two-dimensional conformal field theory.Comment: 11 page
A near-NHEK/CFT correspondence
We consider excitations around the recently introduced near-NHEK metric
describing the near-horizon geometry of the near-extremal four-dimensional Kerr
black hole. This geometry has a U(1)_L x U(1)_R isometry group which can be
enhanced to a pair of commuting Virasoro algebras. We present boundary
conditions for which the conserved charges of the corresponding asymptotic
symmetries are well defined and non-vanishing and find the central charges
c_L=12J/hbar and c_R=0 where J is the angular momentum of the black hole.
Applying the Cardy formula reproduces the Bekenstein-Hawking entropy of the
black hole. This suggests that the near-extremal Kerr black hole is
holographically dual to a non-chiral two-dimensional conformal field theory.Comment: 11 pages, v2: references updated, adde
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