8,894 research outputs found
Diquark and light four-quark states
Four-quark states with different internal clusters are discussed within the
constituent quark model. It is pointed out that the diquark concept is not
meaningful in the construction of a tetraquark interpolating current in the QCD
sum rule approach, and hence existing sum-rule studies of four-quark states are
incomplete. An updated QCD sum-rule determination of the properties of diquark
clusters is then used as input for the constituent quark model to obtain the
masses of light tetraquark states ({\it i.e.\} a bound state of two
diquark clusters). The results support the identification of ,
and as the light tetraquark states, and seem to
be inconsistent with the tetraquark state interpretation of the new BES
observations of the near-threshold enhancements, X(1835) and X(1812),
with the possible exception that X(1576) may be an "exotic" first orbital
excitation of or .Comment: 7 pages, 4 eps figures, RevTex, two figures and some references
added, published version in PR
The impact of population-based faecal occult blood test screening on colorectal cancer mortality:a matched cohort study
BACKGROUND: Randomised trials show reduced colorectal cancer (CRC) mortality with faecal occult blood testing (FOBT). This outcome is now examined in a routine, population-based, screening programme. METHODS: Three biennial rounds of the UK CRC screening pilot were completed in Scotland (2000–2007) before the roll out of a national programme. All residents (50–69 years) in the three pilot Health Boards were invited for screening. They received a FOBT test by post to complete at home and return for analysis. Positive tests were followed up with colonoscopy. Controls, selected from non-pilot Health Boards, were matched by age, gender, and deprivation and assigned the invitation date of matched invitee. Follow-up was from invitation date to 31 December 2009 or date of death if earlier. RESULTS: There were 379 655 people in each group (median age 55.6 years, 51.6% male). Participation was 60.6%. There were 961 (0.25%) CRC deaths in invitees, 1056 (0.28%) in controls, rate ratio (RR) 0.90 (95% confidence interval (CI) 0.83–0.99) overall and 0.73 (95% CI 0.65–0.82) for participants. Non-participants had increased CRC mortality compared with controls, RR 1.21 (95% CI 1.06–1.38). CONCLUSION: There was a 10% relative reduction in CRC mortality in a routine screening programme, rising to 27% in participants
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Minor industrial crops in the Southern Africa region: an appraisal of developmental potential with special reference to interregional trade. (June-July 1995). Vol.1: The main report
This report presents the findings and conclusions arising from a team study carried out in June-August 1995 of the potential for development of 'minor industrial crops' in the Southern Africa region. The study focussed on spices, essential oils, gums, resins, natural colourants and castor oil. Information was gathered on markets and production by means of field studies in South Africa, Malawi, Zimbabwe and Zambia and was supplemented by examination of trade statistics and discussions with overseas traders. Time constraints on the study precluded a thorough investigation of all topics and, therefore, the conclusions reached must be regarded as a first phase overview of the regional market and a preliminary identification of the more promising crops for future development
Quantum transport in carbon nanotubes
Carbon nanotubes are a versatile material in which many aspects of condensed
matter physics come together. Recent discoveries, enabled by sophisticated
fabrication, have uncovered new phenomena that completely change our
understanding of transport in these devices, especially the role of the spin
and valley degrees of freedom. This review describes the modern understanding
of transport through nanotube devices.
Unlike conventional semiconductors, electrons in nanotubes have two angular
momentum quantum numbers, arising from spin and from valley freedom. We focus
on the interplay between the two. In single quantum dots defined in short
lengths of nanotube, the energy levels associated with each degree of freedom,
and the spin-orbit coupling between them, are revealed by Coulomb blockade
spectroscopy. In double quantum dots, the combination of quantum numbers
modifies the selection rules of Pauli blockade. This can be exploited to read
out spin and valley qubits, and to measure the decay of these states through
coupling to nuclear spins and phonons. A second unique property of carbon
nanotubes is that the combination of valley freedom and electron-electron
interactions in one dimension strongly modifies their transport behaviour.
Interaction between electrons inside and outside a quantum dot is manifested in
SU(4) Kondo behavior and level renormalization. Interaction within a dot leads
to Wigner molecules and more complex correlated states.
This review takes an experimental perspective informed by recent advances in
theory. As well as the well-understood overall picture, we also state clearly
open questions for the field. These advances position nanotubes as a leading
system for the study of spin and valley physics in one dimension where
electronic disorder and hyperfine interaction can both be reduced to a very low
level.Comment: In press at Reviews of Modern Physics. 68 pages, 55 figure
Dilepton and Photon Emission Rates from a Hadronic Gas
We analyze the dilepton and photon emission rates from a hadronic gas using
chiral reduction formulas and a virial expansion. The emission rates are
reduced to pertinent vacuum correlation functions, most of which can be
assessed from experiment. Our results indicate that in the low mass region, the
dilepton and photon rates are enhanced compared to most of the calculations
using chiral Lagrangians. The enhancement is further increased through a finite
pion chemical potential. An estimate of the emission rates is also made using
Haag's expansion for the electromagnetic current. The relevance of these
results to dilepton and photon emission rates in heavy-ion collisions is
discussed.Comment: 7 pages, LaTeX using revTeX, 6 figures imbedded in text. Figures
slightly changed, text left unchange
Isospin Fluctuations in QCD and Relativistic Heavy-Ion Collisions
We address the role of fluctuations in strongly interacting matter during the
dense stages of a heavy-ion collision through its electromagnetic emission.
Fluctuations of isospin charge are considered in a thermal system at rest as
well as in a moving hadronic fluid at fixed proper time within a finite bin of
pseudo-rapidity. In the former case, we use general thermodynamic relations to
establish a connection between fluctuations and the space-like screening limit
of the retarded photon self-energy, which directly relates to the emissivities
of dileptons and photons. Effects of hadronic interactions are highlighted
through two illustrative calculations. In the latter case, we show that a
finite time scale inherent in the evolution of a heavy-ion collision
implies that equilibrium fluctuations involve both space-like and time-like
components of the photon self-energy in the system. Our study of non-thermal
effects, explored here through a stochastic treatment, shows that an early and
large fluctuation in isospin survives only if it is accompanied by a large
temperature fluctuation at freeze-out, an unlikely scenario in hadronic phases
with large heat capacity. We point out prospects for the future which include:
(1) A determination of the Debye mass of the system at the dilute freeze-out
stage of a heavy-ion collision, and (2) A delineation of the role of charge
fluctuations during the dense stages of the collision through a study of
electromagnetic emissivities.Comment: 12 pages ReVTeX incl. 4 ps-fig
Consequences of a Killing symmetry in spacetime's local structure
In this paper we discuss the consequences of a Killing symmetry on the local
geometrical structure of four-dimensional spacetimes. We have adopted the point
of view introduced in recent works where the exterior derivative of the Killing
plays a fundamental role. Then, we study some issues related with this approach
and clarify why in many circumstances its use has advantages with respect to
other approaches. We also extend the formalism developed in the case of vacuum
spacetimes to the general case of an arbitrary energy-momentum content.
Finally, we illustrate our framework with the case of spacetimes with a
gravitating electromagnetic field.Comment: 20 pages, LaTeX2e, IOP style. Revised version accepted for
publication in Classical and Quantum Gravit
Medium effect on photon production in ultrarelativistic nuclear collisions
The effect of in-medium vector and axial-vector meson masses on photon
production is studied. We assume that the effective mass of a vector meson in
hot nuclear matter decreases according to a universal scaling law, while that
of an axial-vector meson is given by Weinberg's mass formula. We find that the
thermal production rate of photons increases with reduced masses, and is
enhanced by an order of magnitude at T=160 MeV with MeV. Assuming
a hydrodynamic evolution, we estimate the effect of the reduced masses on
photon production in nucleus-nucleus collisions. The result is compared to
experimental data from the WA80/WA98 collaboration.Comment: 21 pages, REVTEX + 9 figures (ps file
Inferring statistics of planet populations by means of automated microlensing searches
(abridged) The study of other worlds is key to understanding our own, and not
only provides clues to the origin of our civilization, but also looks into its
future. Rather than in identifying nearby systems and learning about their
individual properties, the main value of the technique of gravitational
microlensing is in obtaining the statistics of planetary populations within the
Milky Way and beyond. Only the complementarity of different techniques
currently employed promises to yield a complete picture of planet formation
that has sufficient predictive power to let us understand how habitable worlds
like ours evolve, and how abundant such systems are in the Universe. A
cooperative three-step strategy of survey, follow-up, and anomaly monitoring of
microlensing targets, realized by means of an automated expert system and a
network of ground-based telescopes is ready right now to be used to obtain a
first census of cool planets with masses reaching even below that of Earth
orbiting K and M dwarfs in two distinct stellar populations, namely the
Galactic bulge and disk. The hunt for extra-solar planets acts as a principal
science driver for time-domain astronomy with robotic-telescope networks
adopting fully-automated strategies. Several initiatives, both into facilities
as well as into advanced software and strategies, are supposed to see the
capabilities of gravitational microlensing programmes step-wise increasing over
the next 10 years. New opportunities will show up with high-precision
astrometry becoming available and studying the abundance of planets around
stars in neighbouring galaxies becoming possible. Finally, we should not miss
out on sharing the vision with the general public, and make its realization to
profit not only the scientists but all the wider society.Comment: 10 pages in PDF format. White paper submitted to ESA's Exo-Planet
Roadmap Advisory Team (EPR-AT); typos corrected. The embedded figures are
available from the author on request. See also "Towards A Census of
Earth-mass Exo-planets with Gravitational Microlensing" by J.P. Beaulieu, E.
Kerins, S. Mao et al. (arXiv:0808.0005
The Role of Nucleons in Electromagnetic Emission Rates
Electromagnetic emission rates from a thermalized hadronic gas are important
for the interpretation of dilepton signals from heavy-ion collisions. Although
there is a consensus in the literature about rates for a pure meson gas,
qualitative differences appear with a finite baryon density. We show this to be
essentially due to the way in which the pi-N background is treated in regards
to the nucleon resonances. Using a background constrained by unitarity and
broken chiral symmetry, it is emphasized that the thermalized hadronic gas can
be considered dilute.Comment: 9 pages, 7 figures, minor change
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