Muon pair creation from positronium in a circularly polarized laser field

We study elementary particle reactions that result from the interaction of an atomic system with a very intense laser wave of circular polarization. As a specific example, we calculate the rate for the laser-driven reaction $e^+e^- \to \mu^+\mu^-$, where the electron and positron originate from a positronium atom or, alternatively, from a nonrelativistic $e^+e^-$ plasma. We distinguish accordingly between the coherent and incoherent channels of the process. Apart from numerical calculations, we derive by analytical means compact formulas for the corresponding reaction rates. The rate for the coherent channel in a laser field of circular polarization is shown to be damped because of the destructive interference of the partial waves that constitute the positronium ground-state wave packet. Conditions for the observation of the process via the dominant incoherent channel in a circularly polarized field are pointed out

Breit-Wheeler Process in Intense Short Laser Pulses

Energy-angular distributions of electron-positron pair creation in collisions of a laser beam and a nonlaser photon are calculated using the $S$-matrix formalism. The laser field is modeled as a finite pulse, similar to the formulation introduced in our recent paper in the context of Compton scattering [Phys. Rev. A {\bf 85}, 062102 (2012)]. The nonperturbative regime of pair creation is considered here. The energy spectra of created particles are compared with the corresponding spectra obtained using the modulated plane wave approximation for the driving laser field. A very good agreement in these two cases is observed, provided that the laser pulse is sufficiently long. For short pulse durations, this agreement breaks down. The sensitivity of pair production to the polarization of a driving pulse is also investigated. We show that in the nonperturbative regime, the pair creation yields depend on the polarization of the pulse, reaching their maximal values for the linear polarization. Therefore, we focus on this case. Specifically, we analyze the dependence of pair creation on the relative configuration of linear polarizations of the laser pulse and the nonlaser photon. Lastly, we investigate the carrier-envelope phase effect on angular distributions of created particles, suggesting the possibility of phase control in relation to the pair creation processes.Comment: 13 pages, 8 figure

Friedmann Universes and Exact Solutions in String Cosmology

We show that the classical null strings generate the Hilbert-Einstein gravity corresponding to D-dimensional Friedmann universes.Comment: 8 pages, LATE

Null Strings in Schwarzschild Spacetime

The null string equations of motion and constraints in the Schwarzschild spacetime are given. The solutions are those of the null geodesics of General Relativity appended by a null string constraint in which the "constants of motion" depend on the world-sheet spatial coordinate. Because of the extended nature of a string, the physical interpretation of the solutions is completely different from the point particle case. In particular, a null string is generally not propagating in a plane through the origin, although each of its individual points is. Some special solutions are obtained and their physical interpretation is given. Especially, the solution for a null string with a constant radial coordinate $r$ moving vertically from the south pole to the north pole around the photon sphere, is presented. A general discussion of classical null/tensile strings as compared to massless/massive particles is given. For instance, tensile circular solutions with a constant radial coordinate $r$ do not exist at all. The results are discussed in relation to the previous literature on the subject.Comment: 16 pages, REVTEX, no figure

Tailoring surface properties, biocompatibility and corrosion behavior of stainless steel by laser induced periodic surface treatment towards developing biomimetic stents

Laser-Induced Periodic Surface Structures (LIPSS) holds great potential for regenerative biomedicine. Creating highly precise LIPSS enables to generate biomimetic implant surfaces with improved properties. The present study focuses on the fabrication and investigation of laser-treated stainless steel samples with applied linear LIPSS patterns with grooves made by means of a picosecond laser system using wavelengths of 1064 nm and 532 nm. To investigate properties of the laser-treated surfaces and to understand the basics of cell-surface interactions between the LIPSS and human Umbilical Cord Mesenchymal Stem Cells (UCMSC), flat stainless steel samples with various applied nanopatterns were used. Such LIPSSs demonstrated higher surface roughness, good biocompatibility, lower wettability and higher corrosion resistance compared to the untreated (polished) spec-imens. The surface roughness of laser-treated samples was in microscale that enabled adhesion and migration of endothelial cells, thus increasing the likelihood for endothelialisation. This thereby could reduce the chances for the development of Late Stent Thrombosis (LST) and In-Stent Restenosis (ISR). Furthermore, laser textured surfaces demonstrated an environment supportive for cell attachment, proliferation and alignment with the nanogroves. Therefore, application of the biomimetic nanopatterns could help to overcome frequent post-surgery complications after the stent implantation

String Propagation in Bianchi Type I models: Dynamical anisotropy Damping and Consequences

A generic ansatz is introduced which provides families of exact solutions to the equations of motion and constraints for null-strings in Bianchi type I cosmological models. This is achieved irrespective of the form of the metric. Within classes of dilaton cosmologies a backreaction mapping relation is established where the null string leads to more or less anisotropic members of the family. The equations of motion and constraints for the generic model are casted in their first order form and integrated both analytically and numerically.Comment: 37 pages, 11 figure

Topically applied vitamin C increases the density of dermal papillae in aged human skin

BACKGROUND: The influence of ageing on the density of the functional entities of the papillae containing nutritive capillaries, here in terms as the papillary index, and the effect of topically applied vitamin C were investigated by confocal laser scanning microscopy (CLSM) in vivo. METHODS: The age dependency of the papillary index was determined by CLSM on 3 different age groups. Additionally, we determined the effect of a topical cream containing 3% vitamin C against the vehicle alone using daily applications for four months on the volar forearm of 33 women. RESULTS: There were significant decreases in the papillary index showing a clear dependency on age. Topical vitamin C resulted in a significant increase of the density of dermal papillae from 4 weeks onward compared to its vehicle. Reproducibility was determined in repeated studies. CONCLUSIONS: Vitamin C has the potential to enhance the density of dermal papillae, perhaps through the mechanism of angiogenesis. Topical vitamin C may have therapeutical effects for partial corrections of the regressive structural changes associated with the aging process

Tensionless String in the Notoph Background

We study the interaction between a tensionless (null) string and an antisymmetric background field B_{ab} using a 2-component spinor formalism. A geometric condition for the absence of such an interaction is formulated. We show that only one gauge-invariant degree of freedom of the field B_{ab} does not satisfy this condition. Identification of this degree of freedom with the notoph field \phi of Ogievetskii-Polubarinov-Kalb-Ramond is suggested. Application of a two-component spinor formalism allows us a reduction of the complete system of non-linear partial differential equations and constraints governing the interacting null string dynamics to a system of linear differential equations for the basis spinors of the spin-frame. We find that total effect of the interaction is contained in a single derivation coefficient which is identified with the notoph field.Comment: 15 pages, no figures, RevTeX 3.