24,805 research outputs found
Theory of pixel lensing towards M31 I: the density contribution and mass of MACHOs
POINT-AGAPE is an Anglo-French collaboration which is employing the Isaac
Newton Telescope (INT) to conduct a pixel-lensing survey towards M31. In this
paper we investigate what we can learn from pixel-lensing observables about the
MACHO mass and fractional contribution in M31 and the Galaxy for the case of
spherically-symmetric near-isothermal haloes. We employ detailed pixel-lensing
simulations which include many of the factors which affect the observables. For
a maximum MACHO halo we predict an event rate in V of up to 100 per season for
M31 and 40 per season for the Galaxy. However, the Einstein radius crossing
time is generally not measurable and the observed full-width half-maximum
duration provides only a weak tracer of lens mass. Nonetheless, we find that
the near-far asymmetry in the spatial distribution of M31 MACHOs provides
significant information on their mass and density contribution. We present a
likelihood estimator for measuring the fractional contribution and mass of both
M31 and Galaxy MACHOs which permits an unbiased determination to be made of
MACHO parameters, even from data-sets strongly contaminated by variable stars.
If M31 does not have a significant population of MACHOs in the mass range
0.001-1 Solar masses strong limits will result from the first season of INT
observations. Simulations based on currently favoured density and mass values
indicate that, after three seasons, the M31 MACHO parameters should be
constrained to within a factor four uncertainty in halo fraction and an order
of magnitude uncertainty in mass (90% confidence). Interesting constraints on
Galaxy MACHOs may also be possible. For a campaign lasting ten years,
comparable to the lifetime of current LMC surveys, reliable estimates of MACHO
parameters in both galaxies should be possible. (Abridged)Comment: 21 pages, 14 figures. Submitted to MNRA
Effective Mass of the Four Flux Composite Fermion at
We have measured the effective mass () of the four flux composite
fermion at Landau level filling factor (CF), using the
activation energy gaps at the fractional quantum Hall effect (FQHE) states
= 2/7, 3/11, and 4/15 and the temperature dependence of the Shubnikov-de
Haas (SdH) oscillations around . We find that the energy gaps show a
linear dependence on the effective magnetic field (), and from this linear dependence we obtain and
a disorder broadening 1 K for a sample of density /cm. The deduced from the temperature dependence of
the SdH effect shows large differences for and . For
, . It scales as with the mass
derived from the data around and shows an increase in as , resembling the findings around . For ,
increases rapidly with increasing and can be described by . This anomalous dependence on is
precursory to the formation of the insulating phase at still lower filling.Comment: 5 pages, 3 figure
Genetic Studies of Sulfadiazine-resistant and Methionine-requiring \u3cem\u3eNeisseria\u3c/em\u3e Isolated From Clinical Material
Deoxyribonucleate (DNA) preparations were extracted from Neisseria meningitidis (four isolates from spinal fluid and blood) and N. gonorrhoeae strains, all of which were resistant to sulfadiazine upon primary isolation. These DNA preparations, together with others from in vitro mutants of N. meningitidis and N. perflava, were examined in transformation tests by using as recipient a drug-susceptible strain of N. meningitidis (Ne 15 Sul-s Met+) which was able to grow in a methionine-free defined medium. The sulfadiazine resistance typical of each donor was introduced into the uniform constitution of this recipient. Production of p-aminobenzoic acid was not significantly altered thereby. Transformants elicited by DNA from the N. meningitidis clinical isolates were resistant to at least 200 μg of sulfadiazine/ml, and did not show a requirement for methionine (Sul-r Met+). DNA from six strains of N. gonorrhoeae, which were isolated during the period of therapeutic use of sulfonamides, conveyed lower degrees of resistance and, invariably, a concurrent methionine requirement (Sul-r/Met−). The requirement of these transformants, and that of in vitro mutants selected on sulfadiazine-agar, was satisfied by methionine, but not by vitamin B12, homocysteine, cystathionine, homoserine, or cysteine. Sul-r Met+ and Sul-r/Met− loci could coexist in the same genome, but were segregated during transformation. On the other hand, the dual Sul-r/Met− properties were not separated by recombination, but were eliminated together. DNA from various Sul-r/Met− clones tested against recipients having nonidentical Sul-r/Met− mutant sites yielded Sul-s Met+ transformants. The met locus involved is genetically complex, and will be a valuable tool for studies of genetic fine structure of members of Neisseria, and of genetic homology between species
Phenomenological Understanding of a Transport Regime with Reflection Symmetry in the Quantum Hall System in a Composite Fermion Picture
In this paper, we present a phenomenological picture based on the composite
fermion theory, in responding to the recent discovery by Shahar et al. of a new
transport regime near the transition from a quantum Hall liquid to a
Hall insulator(ref[8]). In this picture, the seemingly unexpected reflection
symmetry in the longitudinal resistivity can be understood clearly
as due to the symmetry of the gapful excitations which dominate
across the transition, and the abrupt change in at the
transition. The parameter in the linear fit of in ref[8] is
also given a simple physical meaning and the effective mass can be calculated
from , which gives a reasonable value of several electron band mass.
When taking into account the result of network model, the almost invariant Hall
resistivity across the transition is also well-understood.Comment: 4 pages, RevTex, final version to appear in Phys.Rev.
Anisotropic transport for FQH state at intermediate magnetic field
The state is spin-unpolarized at weak magnetic field and fully
polarized at strong field. At intermediate field, a plateau of half the maximal
polarization is observed. We study this phenomenon in the frame of composite
fermion theory. Due to the mixing of the composite fermion Landau levels, the
unidirectional charge/spin density wave state of composite fermions is lower in
energy than the Wigner crystal. It means that transport anisotropy, similar to
those for electrons in higher Landau levels at half fillings, may take place at
this fractional quantum Hall state when the external magnetic field is in an
appropriate range. When the magnetic field is tilted an angle, the easy
transport direction is perpendicular to the direction of the in-plane field.
Varying the partial filling factor of composite fermion Landau level from 0 to
1, we find that the energy minimum occurs in the vicinity of one-half.Comment: 2 figure
Dielectric selective mirror for intracavity wavelength selection in far-infrared p-Ge lasers
A robust metal-free intracavity fixed-wavelength selector for the cryogenically cooled far-infrared p-Ge laser is demonstrated. The device is a back mirror consisting of a thin silicon etalon and dielectric SrTiO3 flat. A laser line width of 0.2 cm-1 is achieved, which corresponds to an active cavity finesse of similar to0.15. The wavelength position and spectral purity are maintained over a wide range of laser operating fields. Use of SrTiO3 lowers the laser resonance line frequencies by similar to1 cm-1 compared with expectations for metal mirrors. The effect is due to phase shift, which is determined from far-infrared reflectivity measurements of SrTiO3. A p-Ge laser with such selector is free from danger of electrical breakdown and mirror oxidation during repeatable thermal cycling, which makes it more reliable than previous selection schemes for practical applications
Noise-Resilient Group Testing: Limitations and Constructions
We study combinatorial group testing schemes for learning -sparse Boolean
vectors using highly unreliable disjunctive measurements. We consider an
adversarial noise model that only limits the number of false observations, and
show that any noise-resilient scheme in this model can only approximately
reconstruct the sparse vector. On the positive side, we take this barrier to
our advantage and show that approximate reconstruction (within a satisfactory
degree of approximation) allows us to break the information theoretic lower
bound of that is known for exact reconstruction of
-sparse vectors of length via non-adaptive measurements, by a
multiplicative factor .
Specifically, we give simple randomized constructions of non-adaptive
measurement schemes, with measurements, that allow efficient
reconstruction of -sparse vectors up to false positives even in the
presence of false positives and false negatives within the
measurement outcomes, for any constant . We show that, information
theoretically, none of these parameters can be substantially improved without
dramatically affecting the others. Furthermore, we obtain several explicit
constructions, in particular one matching the randomized trade-off but using measurements. We also obtain explicit constructions
that allow fast reconstruction in time \poly(m), which would be sublinear in
for sufficiently sparse vectors. The main tool used in our construction is
the list-decoding view of randomness condensers and extractors.Comment: Full version. A preliminary summary of this work appears (under the
same title) in proceedings of the 17th International Symposium on
Fundamentals of Computation Theory (FCT 2009
Charmless two-body B decays: A global analysis with QCD factorization
In this paper, we perform a global analysis of and decays
with the QCD factorization approach. It is encouraging to observe that the
predictions of QCD factorization are in good agreement with experiment. The
best fit is around . The penguin-to-tree ratio of decays is preferred to be larger than 0.3.
We also show the confidence levels for some interesting channels: , and , . For decays, they are expected to have smaller branching ratios with
more precise measurements.Comment: 20 pages, 4 figures, version to appear in Phys. Rev.
The Onset of Anisotropic Transport of Two-Dimensional Electrons in High Landau Levels: An Isotropic-to-Nematic Liquid Crystal Phase Transition?
The recently discovered anisotropy of the longitudinal resistance of
two-dimensional electrons near half filling of high Landau levels is found to
persist to much higher temperatures T when a large in-plane magnetic field B||
is applied. Under these conditions we find that the longitudinal resistivity
scales quasi-linearly with B||/T. These observations support the notion that
the onset of anisotropy at B||=0 does not reflect the spontaneous development
of charge density modulations but may instead signal an isotropic-to-nematic
liquid crystal phase transition.Comment: 5 pages, 4 figure
Characterisation of Pinotage Wine During Maturation on Different Oak Products
The effect of oak contact on the phenolic composition, total antioxidant capacity (TAC) and colour of Pinotage wineswas investigated during maturation. Oak maturation included traditional treatments, such as new, second-fill andthird-fill barrels, as well as alternative treatments (oak chips, staves, extract and dust) applied in old barrels over aperiod of 28 weeks. Oak maturation using traditional and alternative treatments improved the objective colour ofPinotage wine by decreasing the L* value. Losses in TAC caused by decreased concentrations of monomeric phenoliccompounds (most anthocyanins, flavan-3-ols, flavonols and hydroxycinnamic acids) during oak maturation werenegated by increased concentrations of gallic acid and the formation of new oligomeric and polymeric pigments.Wine maturation in stainless steel containers also resulted in a decrease in anthocyanin content. The decreasein phenolic acid content for wines matured in stainless steel was less pronounced, while their flavan-3-ol contentremained stable. The new-barrel treatment had the most pronounced effect on all parameters. Oak maturation canbe used for the production of Pinotage wine when the retention of TAC is a high priority
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