7,781 research outputs found
On amplitude zeros at threshold
The occurrence of zeros of 2 to n amplitudes at threshold in scalar theories
is studied. We find a differential equation for the scalar potential, which
incorporates all known cases where the 2 to n amplitudes at threshold vanish
for all sufficiently large , in all space-time dimensions, . This
equation is related to the reflectionless potentials of Quantum Mechanics and
to integrable theories in 1+1 dimensions. As an application, we find that the
sine-Gordon potential and its hyperbolic version, the sinh-Gordon potential,
also have amplitude zeros at threshold, , for and
, independently of the mass and the coupling constant.Comment: 6 pages, Latex, CERN-TH.6762/9
Multiscalar production amplitudes beyond threshold
We present exact tree-order amplitudes for , for final states
containing one or two particles with non-zero three-momentum, for various
interaction potentials. We show that there are potentials leading to tree
amplitudes that satisfy unitarity, not only at threshold but also in the above
kinematical configurations and probably beyond. As a by-product, we also
calculate tree amplitudes at threshold and show that for the unbroken
theory they vanish for , for the Standard Model Higgs they
vanish for and for a model potential, respecting tree-order
unitarity, for even and . Finally, we calculate the imaginary part of
the one-loop amplitude in both symmetric and spontaneously broken
theory.Comment: 11 pages, Latex, 3 figures (available on request
Extended dust emission and atomic hydrogen, a reservoir of diffuse H_2 in NGC 1068
We report on sensitive sub-mm imaging observations of the prototype
Seyfert~2/starburst galaxy NGC 1068 at 850 m and 450 m using the
Submillimetre Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell
Telescope (JCMT). We find clear evidence of dust emission associated with the
extended HI component which together with the very faint CO J=1--0
emission give a gas-to-dust ratio of . This contrasts with the larger ratio estimated within a galactocentric radius of kpc, where the
gas is mostly molecular and starburst activity occurs. The large gas-to-dust
ratio found for the starburst region is attributed to a systematic overestimate
of the molecular gas mass in starburst environments when the luminosity of the
CO J=1--0 line and a standard galactic conversion factor is used. On
the other hand sub-mm imaging proves to be a more powerful tool than
conventional CO imaging for revealing the properties of the diffuse
that coexists with HI. This molecular gas phase is characterized by low
densities ( cm), very faint emission from
sub-thermally excited CO, and contains more mass than HI, namely .Comment: Accepted for publication in the Astrophysical Journal Letter
Zero-dimensional field theory
A study of zero-dimensional theories, based on exact results, is presented.
First, relying on a simple diagrammatic representation of the theory, equations
involving the generating function of all connected Green's functions are
constructed. Second, exact solutions of these equations are obtained for
several theories. Finally, renormalization is carried out. Based on the
anticipated knowledge of the exact solutionsthe full dependence on the
renormalized coupling constant is studied.Comment: 38 pages, LaTe
Recursive actions for scalar theories
We introduce a class of self-interacting scalar theories in which the various
coupling contants obey a recursive relation. These imply a particularly simple
form for the generating function of the Feynman amplitudes with vanishing
external momenta, as well as for the effective potential. In addition we
discuss an interesting duality inherent in these models. Specializing to the
case of zero spacetime dimensions we find intriguing nullification properties
for the amplitudes.Comment: 28 pages, 2 figures Replaced contract numbe
(1,0) superconformal theories in six dimensions and Killing spinor equations
We solve the Killing spinor equations of 6-dimensional (1,0) superconformal
theories in all cases. In particular, we derive the conditions on the fields
imposed by the Killing spinor equations and demonstrate that these depend on
the isotropy group of the Killing spinors. We focus on the models proposed by
Samtleben et al in \cite{ssw} and find that there are solutions preserving 1,2,
4 and 8 supersymmetries. We also explore the solutions which preserve 4
supersymmetries and find that many models admit string and 3-brane solitons as
expected from the M-brane intersection rules. The string solitons are smooth
regulated by the moduli of instanton configurations.Comment: 26 page
Tsunami hazards in the Eastern Mediterranean: strong earthquakes and tsunamis in the East Hellenic Arc and Trench system
International audienceData on tsunami phenomena occurring in the East Hellenic Arc and Trench system (HA-T) from antiquity up to the present have been updated, critically evaluated and compiled in the standard GITEC format developed in the last decade for the New European Tsunami Catalogue. New field observations are presented for the tsunamis of 9 February 1948 and 24 March 2002. From the 18 tsunamis reported eight are rather well-documented while another nine remain doubtful. The mean recurrence of strong tsunamis is likely equal to about 142 years. Most of the tsunamis documented are caused by strong earthquakes occurring in the area offshore Rhodes to the east or northeast of the island. However, there are large earthquakes near Rhodes that do not cause tsunamis, like the 1926 and 1957 ones, which is of particular importance for the tsunami hazard assessment
Dynamic performance of a low voltage microgrid with droop controlled distributed generation
Microgrids are small-scale highly controlled networks designed to supply electrical energy. From the operational point of view, microgrids are active distribution networks, facilitating the integration of distributed generation units. Major technical issues in this concept include system stability and protection coordination which are significantly influenced by the high penetration of inverter-interfaced distributed energy sources. These units often adopt the frequency-active power and voltage-reactive power droop control strategy to participate in the load sharing of an islanded microgrid. The scope of the paper is to investigate the dynamic performance of a low voltage laboratory-scale microgrid system, using experimental results and introduce the concept of Prony analysis for understanding the connected components. Several small disturbance test cases are conducted and the investigations focus on the influence of the droop controlled distributed generation sources
Potential impact and controversy of stem cells in public health
Stem cells are versatile in the bodies which are able to both reproduce themselves and to produce more specialized cells. As such, they are of great potential values in repairing and regenerating damaged cells and tissues. Many different kinds of stem cells have been discovered. The most common are embryonic, foetal and adult stem cells. Stem cell research has the potential to provide an increased understanding of development and differentiation, as well as leading to treatments and cures for many diseases. They are important to the future of medicine and public health because with adequate research, stem cells have the potential to treat degenerative conditions through transplanting human stem cells into patients. With sufficient development of stem cell medicine, chronic diseases such as diabetes, heart disease, and Parkinsonâs disease will be effectively managed. Embryonic stem cell (ESC) research has been a source of ethical, legal, and social controversy which has slowed the pace of stem cell science and shaped many aspects of its subsequent development
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