1,828 research outputs found
Periodic and discrete Zak bases
Weyl's displacement operators for position and momentum commute if the
product of the elementary displacements equals Planck's constant. Then, their
common eigenstates constitute the Zak basis, each state specified by two phase
parameters. Upon enforcing a periodic dependence on the phases, one gets a
one-to-one mapping of the Hilbert space on the line onto the Hilbert space on
the torus. The Fourier coefficients of the periodic Zak bases make up the
discrete Zak bases. The two bases are mutually unbiased. We study these bases
in detail, including a brief discussion of their relation to Aharonov's modular
operators, and mention how they can be used to associate with the single degree
of freedom of the line a pair of genuine qubits.Comment: 15 pages, 3 figures; displayed abstract is shortened, see the paper
for the complete abstrac
Evidence for the intense exchange of MazG in marine cyanophages by horizontal gene transfer
Background: S-PM2 is a phage capable of infecting strains of unicellular cyanobacteria belonging to the genus Synechococcus. S-PM2, like other myoviruses infecting marine cyanobacteria, encodes a number of bacterial-like genes. Amongst these genes is one encoding a MazG homologue that is hypothesized to be involved in the adaption of the infected host for production of progeny phage.
Methodology/Principal Findings: This study focuses on establishing the occurrence of mazG homologues in other cyanophages isolated from different oceanic locations. Degenerate PCR primers were designed using the mazG gene of S-PM2. The mazG gene was found to be widely distributed and highly conserved among Synechococcus myoviruses and podoviruses from diverse oceanic provinces.
Conclusions/Significance: This study provides evidence of a globally connected cyanophage gene pool, the cyanophage mazG gene having a small effective population size indicative of rapid lateral gene transfer despite being present in a substantial fraction of cyanophage. The Prochlorococcus and Synechococcus phage mazG genes do not cluster with the host mazG gene, suggesting that their primary hosts are not the source of the mazG gene
An 11.6 Micron Keck Search For Exozodiacal Dust
We have begun an observational program to search nearby stars for dust disks
that are analogous to the disk of zodiacal dust that fills the interior of our
solar system. We imaged six nearby main-sequence stars with the Keck telescope
at 11.6 microns, correcting for atmosphere-induced wavefront aberrations and
deconvolving the point spread function via classical speckle analysis. We
compare our data to a simple model of the zodiacal dust in our own system based
on COBE/DIRBE observations and place upper limits on the density of exozodiacal
dust in these systems.Comment: 10 pages, figure1, figure2, figure3, and figures 4a-
Subsonic longitudinal and lateral-directional characteristics of a forward-swept-wing fighter configuration at angles of attack up to 47 deg
Subsonic lateral-direction and longitudinal characteristics of a forward-swept-wing fighter configuration were examined in wind-tunnel tests at Mach numbers of 0.2 and 0.5 for angles of attack from -7 to 47 deg. and over a sidelslip range of +/- 15 deg. The effects of a canard, strakes, vertical tail, and leading- and trailing-edge flaps are examined. The canard and strakes both reduce asymmetric moments and side forces at zero sideslip for angles of attack up to about 30 deg. The canard has a small influence on lateral-directional stability; however, strakes produce a substantial reduction in lateral stability for angles of attack greater than about 20 deg. The vertical tail improves directional stability for angles of attack up to 30 deg. Deflection of the leading-edge flap to 20 deg. at high angles of attack on the strake and canard configurations degrades lateral and directional stability. Deflection of the trailing-edge flap to 20 deg. on the canard configuration generally increases lateral and directional stability at high angles of attack. Leading- and trailing-edge flaps on the wing-body and canard configurations are effective for increased lift only for angles of attack up to about 40 deg. The leading-edge flap remains effective on the strake configuration over the entire angle-of-attack range tested
Lepton Mixing and Cancellation of the Dirac Mass Hierarchy in SO(10) GUTs with Flavor Symmetries T7 and Sigma(81)
In SO(10) grand unified theories (GUTs) the hierarchy which is present in the
Dirac mass term of the neutrinos is generically as strong as the one in the
up-type quark mass term. We propose a mechanism to partially or completely
cancel this hierarchy in the light neutrino mass matrix in the seesaw context.
The two main ingredients of the cancellation mechanism are the existence of
three fermionic gauge singlets and of a discrete flavor symmetry G_f which is
broken at a higher scale than SO(10). Two realizations of the cancellation
mechanism are presented. The realization based on the Frobenius group T7 = Z7 x
Z3 leads to a partial cancellation of the hierarchy and relates maximal 2-3
lepton mixing with the geometric hierarchy of the up-quark masses. In the
realization with the group Sigma(81) the cancellation is complete and
tri-bimaximal lepton mixing is reproduced at the lowest order. In both cases,
to fully accommodate the leptonic data we take into account additional effects
such as effects of higher-dimensional operators involving more than one flavon.
The heavy neutral fermion mass spectra are considered. For both realizations we
analyze the flavon potential at the renormalizable level as well as ways to
generate the Cabibbo angle.Comment: 31 page
Decay of a Yukawa fermion at finite temperature and applications to leptogenesis
We calculate the decay rate of a Yukawa fermion in a thermal bath using
finite temperature cutting rules and effective Green's functions according to
the hard thermal loop resummation technique. We apply this result to the decay
of a heavy Majorana neutrino in leptogenesis. Compared to the usual approach
where thermal masses are inserted into the kinematics of final states, we find
that deviations arise through two different leptonic dispersion relations. The
decay rate differs from the usual approach by more than one order of magnitude
in the temperature range which is interesting for the weak washout regime. We
discuss how to arrive at consistent finite temperature treatments of
leptogenesis.Comment: 16 pages, 5 figure
Ice-age megafauna in Arctic Alaska: extinction, invasion, survival
Radical restructuring of the terrestrial, large mammal fauna living in arctic Alaska occurred between 14,000 and 10,000 years ago at the end of the last ice age. Steppe bison, horse, and woolly mammoth became extinct, moose and humans invaded, while muskox and caribou persisted. The ice age mega fauna was more diverse in species and possibly contained 6x more individual animals than live in the region today. Mega faunal biomass during the last ice age may have been 30x greater than present. Horse was the dominant species in terms of number of individuals. Lions, short-faced bears, wolves, and possibly grizzly bears comprised the predator/scavenger guild. The youngest mammoth so far discovered lived ca 13,800 years ago, while horses and bison persisted on the North Slope until at least 12,500 years ago during the Younger Dry as cold interval. The first people arrived on the North Slope ca 13,500 years ago. Bone-isotope measurements and foot-loading characteristics suggest mega faunal niches were segregated along a moisture gradient, with the surviving species (muskox and caribou) utilizing the warmer and moister portions of the vegetation mosaic. As the ice age ended, the moisture gradient shifted and eliminated habitats utilized by the dry land, grazing species (bison, horse, mammoth). The proximate cause for this change was regional paludification, the spread of organic soil horizons and peat. End-Pleistocene extinctions in arctic Alaska represent local, not global extinctions since the mega faunal species lost there persisted to later times elsewhere. Hunting seems unlikely as the cause of these extinctions, but it cannot be ruled out as the final blow to mega faunal populations that were already functionally extinct by the time humans arrived in the region
Measurement in Quantum Physics
The conceptual problems in quantum mechanics -- related to the collapse of
the wave function, the particle-wave duality, the meaning of measurement --
arise from the need to ascribe particle character to the wave function. As will
be shown, all these problems dissolve when working instead with quantum fields,
which have both wave and particle character. Otherwise the predictions of
quantum physics, including Bell's inequalities, coincide with those of the
standard treatments. The transfer of the results of the quantum measurement to
the classical realm is also discussed.Comment: 34 pages, in Latex, revised and expanded version with an extra
appendix on decoherenc
A Comprehensive Mechanism Reproducing the Mass and Mixing Parameters of Quarks and Leptons
It is shown that if, from the starting point of a universal rank-one mass
matrix long favoured by phenomenologists, one adds the assumption that it
rotates (changes its orientation in generation space) with changing scale, one
can reproduce, in terms of only 6 real parameters, all the 16 mass ratios and
mixing parameters of quarks and leptons. Of these 16 quantities so reproduced,
10 for which data exist for direct comparison (i.e. the CKM elements including
the CP-violating phase, the angles in
-oscillation, and the masses ) agree well with
experiment, mostly to within experimental errors; 4 others (), the experimental values for which can only be inferred, agree
reasonably well; while 2 others ( for leptons), not yet
measured experimentally, remain as predictions. In addition, one gets as
bonuses, estimates for (i) the right-handed neutrino mass and (ii)
the strong CP angle inherent in QCD. One notes in particular that the
output value for from the fit agrees very well with
recent experiments. By inputting the current experimental value with its error,
one obtains further from the fit 2 new testable constraints: (i) that
must depart from its "maximal" value: , (ii) that the CP-violating (Dirac) phase in the PMNS would be
smaller than in the CKM matrix: of order only if
not vanishing altogether.Comment: 37 pages, 1 figur
A Submillimetre Survey of the Hubble Deep Field: Unveiling Dust-Enshrouded Star Formation in the Early Universe
The advent of sensitive sub-mm array cameras now allows a proper census of
dust-enshrouded massive star-formation in very distant galaxies, previously
hidden activity to which even the deepest optical images are insensitive. We
present the deepest sub-mm survey, taken with the SCUBA camera on the James
Clerk Maxwell Telescope (JCMT) and centred on the Hubble Deep Field (HDF). The
high source density on this image implies that the survey is confusion-limited
below a flux density of 2 mJy. However within the central 80 arcsec radius
independent analyses yield 5 reproducible sources with S(850um) > 2 mJy which
simulations indicate can be ascribed to individual galaxies. These data lead to
integral source counts which are completely inconsistent with a no evolution
model, whilst the combined brightness of the 5 most secure sources in our map
is sufficient to account for 30-50% of the previously unresolved sub-mm
background, and statistically the entire background is resolved at about the
0.3 mJy level. Four of the five brightest sources appear to be associated with
galaxies which lie in the redshift range 2 < z < 4. With the caveat that this
is a small sample of sources detected in a small survey area, these submm data
imply a star-formation density over this redshift range that is at least five
times higher than that inferred from the rest-frame ultraviolet output of HDF
galaxies.Comment: to appear in the proceedings of `The Birth of Galaxies', Xth
Rencontres de Blois, 4 pages, 1 postscript figure, uses blois.sty (included
- …