20,614 research outputs found
Diffractive charged meson pair production
We investigate the possibility to measure the nonforward gluon distribution
function by means of diffractively produced \pi^+\pi^- and K^+K^- pairs in
polarized lepton nucleon scattering. The resulting cross sections are small and
are dominated by the gluonic contribution. We find relatively large spin
asymmetries, both for \pi^+\pi^- and for K^+K^- pairs.Comment: 15 pages, version with changed kinematical cuts, to be pubished in
Phys.Lett.
Algebras of Measurements: the logical structure of Quantum Mechanics
In Quantum Physics, a measurement is represented by a projection on some
closed subspace of a Hilbert space. We study algebras of operators that
abstract from the algebra of projections on closed subspaces of a Hilbert
space. The properties of such operators are justified on epistemological
grounds. Commutation of measurements is a central topic of interest. Classical
logical systems may be viewed as measurement algebras in which all measurements
commute. Keywords: Quantum measurements, Measurement algebras, Quantum Logic.
PACS: 02.10.-v.Comment: Submitted, 30 page
Detection of X-ray emission from the host clusters of 3CR quasars
We report the detection of extended X-ray emission around several powerful
3CR quasars with redshifts out to 0.73. The ROSAT HRI images of the quasars
have been corrected for spacecraft wobble and compared with an empirical
point-spread function. All the quasars examined show excess emission at radii
of 15 arcsec and more; the evidence being strong for the more distant objects
and weak only for the two nearest ones, which are known from other wavelengths
not to lie in strongly clustered environments. The spatial profiles of the
extended component is consistent with thermal emission from the intracluster
medium of moderately rich host clusters to the quasars. The total luminosities
of the clusters are in the range 4x10^44 - 3x10^45 erg/s, assuming a
temperature of 4keV. The inner regions of the intracluster medium are, in all
cases, dense enough to be part of a cooling flow.Comment: 21 pages including 4 figures and 4 tables. To be published in MNRA
A quantitative analysis of measures of quality in science
Condensing the work of any academic scientist into a one-dimensional measure
of scientific quality is a difficult problem. Here, we employ Bayesian
statistics to analyze several different measures of quality. Specifically, we
determine each measure's ability to discriminate between scientific authors.
Using scaling arguments, we demonstrate that the best of these measures require
approximately 50 papers to draw conclusions regarding long term scientific
performance with usefully small statistical uncertainties. Further, the
approach described here permits the value-free (i.e., statistical) comparison
of scientists working in distinct areas of science.Comment: 11 pages, 8 figures, 4 table
The first-mover advantage in scientific publication
Mathematical models of the scientific citation process predict a strong
"first-mover" effect under which the first papers in a field will, essentially
regardless of content, receive citations at a rate enormously higher than
papers published later. Moreover papers are expected to retain this advantage
in perpetuity -- they should receive more citations indefinitely, no matter how
many other papers are published after them. We test this conjecture against
data from a selection of fields and in several cases find a first-mover effect
of a magnitude similar to that predicted by the theory. Were we wearing our
cynical hat today, we might say that the scientist who wants to become famous
is better off -- by a wide margin -- writing a modest paper in next year's
hottest field than an outstanding paper in this year's. On the other hand,
there are some papers, albeit only a small fraction, that buck the trend and
attract significantly more citations than theory predicts despite having
relatively late publication dates. We suggest that papers of this kind, though
they often receive comparatively few citations overall, are probably worthy of
our attention.Comment: 7 pages, 3 figure
Evolutionary conservation of excision repair in Schizosaccharomyces pombe: Evidence for a family of sequences related to the Saccharomyces cerevisiae RAD2 gene
Cells mutated at the rad13 locus in the fission yeast, Schizosaccharomyces pombe are deficient in excision-repair of UV damage. We have cloned the S.pombe rad13 gene by its ability to complement the UV sensitivity of a rad13 mutant. The gene is not essential for cell proliferation. Sequence analysis of the cloned gene revealed an open reading-frame of 1113 amino acids with structural homology to the RAD2 gene of the distantly related Saccharomyces cerevisiae. The sequence similarity is confined to three domains, two close to the N-terminus of the encoded protein, the third being close to the C-terminus. The central region of about 500 amino acids shows little similarity between the two organisms. The first and third domains are also found in a related yet distinct pair of homologous S.pombe/S.cerevisiae DNA repair genes (rad2/YKL510), which have only a very short region between these two conserved domains. Using the polymerase chain reaction with degenerate primers, we have isolated fragments from a gene homologous to rad13/RAD2 from Aspergillus nidulans. These findings define new functional domains involved in excision-repair, as well as identifying a conserved family of genes related to RAD2
Structural and functional conservation of the human homolog of the Schizosaccharomyces pombe rad2 gene, which is required for chromosome segregation and recovery from DNA damage
The rad2 mutant of Schizosaccharomyces pombe is sensitive to UV irradiation and deficient in the repair of UV damage. In addition, it has a very high degree of chromosome loss and/or nondisjunction. We have cloned the rad2 gene and have shown it to be a member of the Saccharomyces cerevisiae RAD2/S. pombe rad13/human XPG family. Using degenerate PCR, we have cloned the human homolog of the rad2 gene. Human cDNA has 55% amino acid sequence identity to the rad2 gene and is able to complement the UV sensitivity of the rad2 null mutant. We have thus isolated a novel human gene which is likely to be involved both in controlling the fidelity of chromosome segregation and in the repair of UV-induced DNA damage. Its involvement in two fundamental processes for maintaining chromosomal integrity suggests that it is likely to be an important component of cancer avoidance mechanisms
KIC 4247791: A SB4 system with two eclipsing binaries (2EBs)
KIC 4247791 is an eclipsing binary observed by the Kepler satellite mission.
We wish to determine the nature of its components and in particular the origin
of a shallow dip in its Kepler light curve that previous investigations have
been unable to explain in a unique way. We analyze newly obtained
high-resolution spectra of the star using synthetic spectra based on atmosphere
models, derive the radial velocities of the stellar components from
cross-correlation with a synthetic template, and calculate the orbital
solution. We use the JKTEBOP program to model the Kepler light curve of KIC
4247791. We find KIC 4247791 to be a SB4 star. The radial velocity variations
of its four components can be explained by two separate eclipsing binaries. In
contradiction to previous photometric findings, we show that the observed
composite spectrum as well as the derived masses of all four of its components
correspond to spectral type F. The observed small dip in the light curve is not
caused by a transit-like phenomenon but by the eclipses of the second binary
system. We find evidence that KIC 4247791 might belong to the very rare
hierarchical SB4 systems with two eclipsing binaries.Comment: 6 pages, 8 figures, 2 table
Spectator detection for the measurement of proton neutron interactions at ANKE
A telescope of three silicon detectors has been installed close to the
internal target position of the ANKE spectrometer, which is situated inside the
ultra-high vacuum of the COSY-Juelich light-ion storage ring. The detection and
identification of slow protons and deuterons emerging from a deuterium
cluster-jet target thus becomes feasible. A good measurement of the energy and
angle of such a spectator proton (p_sp) allows one to identify a reaction as
having taken place on the neutron in the target and then to determine the
kinematical variables of the ion-neutron system on an event-by-event basis over
a range of c.m. energies.
The system has been successfully tested under laboratory conditions. By
measuring the spectator proton in the p d to p_sp d pi^0 reaction in
coincidence with a fast deuteron in the ANKE Forward Detector, values of the p
n to d pi^0 total cross-section have been deduced. Further applications of the
telescope include the determination of the luminosity and beam polarisation
which are required for several experiments.Comment: 16 pages, 9 figure
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