Effective action for Einstein-Maxwell theory at order RF**4

We use a recently derived integral representation of the one-loop effective action in Einstein-Maxwell theory for an explicit calculation of the part of the effective action containing the information on the low energy limit of the five-point amplitudes involving one graviton, four photons and either a scalar or spinor loop. All available identities are used to get the result into a relatively compact form.Comment: 13 pages, no figure

Semiconducting quantum confined silicon-tin alloyed nanocrystals prepared by ns pulsed laser ablation in water

In this letter we demonstrate the material synthetic feasibility for semiconducting alloyed silicon-tin nanocrystals (SiSn-ncs) with quantum confinement effects. An environmentally friendly synthesis is achieved by ns laser ablation of SiSn amorphous target in water at ambient conditions. Plasmas generated in liquid by laser pulses are characterized by spatial confinement with very high pressure (GPa), which allowed the growth of SiSn-ncs via kinetic pathways. We further illustrate that surface engineering by a direct-current atmospheric-pressure microplasma is capable of tailoring SiSn-ncs surface properties without the need of lengthy surfactants and resulting in room temperature photoluminescence (PL); the PL peak wavelength is red-shifted by more than 250 nm with respect to the PL peak wavelengths observed for comparable elemental silicon nanocrystals

Geodetic precession and strong gravitational lensing in the dynamical Chern-Simons modified gravity

We have investigated the geodetic precession and the strong gravitational lensing in the slowly-rotating black hole in the dynamical Chern-Simons modified gravity theory. We present the formulas of the orbital period $T$ and the geodetic precession angle $\Delta\Theta$ for the timelike particles in the circular orbits around the black hole, which shows that the change of the geodetic precession angle with the Chern-Simons coupling parameter $\xi$ is converse to the change of the orbital period with $\xi$ for fixed $a$. We also discuss the effects of the Chern-Simons coupling parameter on the strong gravitational lensing when the light rays pass close to the black hole and obtain that for the stronger Chern-Simons coupling the prograde photons may be captured more easily, and conversely, the retrograde photons is harder to be captured in the slowly-rotating black hole in the dynamical Chern-Simons modified gravity. Supposing that the gravitational field of the supermassive central object of the Galaxy can be described by this metric, we estimated the numerical values of the main observables for gravitational lensing in the strong field limit.Comment: 19 pages, 5 figures, more clarifications and references added, accepted for publication in Classical and Quantum Gravit

Axions In String Theory

In the context of string theory, axions appear to provide the most plausible solution of the strong CP problem. However, as has been known for a long time, in many string-based models, the axion coupling parameter F_a is several orders of magnitude higher than the standard cosmological bounds. We re-examine this problem in a variety of models, showing that F_a is close to the GUT scale or above in many models that have GUT-like phenomenology, as well as some that do not. On the other hand, in some models with Standard Model gauge fields supported on vanishing cycles, it is possible for F_a to be well below the GUT scale.Comment: 62 pages, v2; references, acknowledgements and minor corrections adde

Bounds on masses of bulk fields in string compactifications

In string compactification on a manifold X, in addition to the string scale and the normal scales of low-energy particle physics, there is a Kaluza-Klein scale 1/R associated with the size of X. We present an argument that generic string models with low-energy supersymmetry have, after moduli stabilization, bulk fields with masses which are parametrically lighter than 1/R. We discuss the implications of these light states for anomaly mediation and gaugino mediation scenarios.Comment: 15 page