Super Gaussian enhancers in the Schwinger mechanism

We study the Schwinger mechanism in the presence of an additional uniformly oriented, weak super Gaussian of integer order $4N+2$. Using the worldline approach, we determine the relevant critical points to compute the leading order exponential factor analytically. We show that increasing the parameter $N$ gives rise to a strong dynamical enhancement. For $N=2$, this effect turns out to be larger compared to a weak contribution of Sauter type. For higher orders, specifically, for the rectangular barrier limit, i.e. $N \rightarrow \infty$, we approach the Lorentzian case as an upper bound. Although the mentioned backgrounds significantly differ in Minkowski spacetime, we show that the found coincidence applies due to identical reflection points in the Euclidean instanton plane. In addition, we also treat the background in perturbation theory following recent ideas. By doing so, we show that the parameter $N$ determines whether the weak contribution behaves perturbatively or nonperturbatively with respect to the field strength ratio and, hence, reveals an interesting dependence on the background shape. In particular, we show that for backgrounds, for which higher orders in the field strength ratio turn out to be relevant, a proposed integral condition is not fulfilled. In view of these findings, the latter may serve as an indicator for the necessity of higher order contributions.Comment: 8 pages, 5 figures, v3: text revisions, references updated, matches journal versio

Nonlinear lepton-photon interactions in external background fields

Nonlinear phenomena of lepton-photon interactions in external backgrounds with a generalised periodic plane-wave geometry are studied. We discuss nonlinear Compton scattering in head-on lepton-photon collisions extended properly to beyond the soft-photon regime. In addition, our results are applied to stimulated lepton-antilepton pair production in photon collisions with unrestricted energies. Derivations are considered semi-classically based on unperturbed fermionic Volkov representations encoding the full interaction with the background field. Closed expressions for total probabilities considering S-matrix elements have been derived. The general formula is applied to Compton scattering by an electron propagating in an external laser-like background. We obtain additive contributions in the extended unconstrained result which turns out to be stringently required in the highly nonlinear regime. A detailed comparison of contributing harmonics is discussed for various field parameters.Comment: 39 pages, 8 figures, minor changes in introduction, references added, a few misprints foun

Euclidean mirrors: enhanced vacuum decay from reflected instantons

We study the tunneling of virtual matter-antimatter pairs from the quantum vacuum in the presence of a spatially uniform, time-dependent electric background composed of a strong, slow field superimposed with a weak, rapid field. After analytic continuation to Euclidean spacetime, we obtain from the instanton equations two critical points. While one of them is the closing point of the instanton path, the other serves as an Euclidean mirror which reflects and squeezes the instanton. It is this reflection and shrinking which is responsible for an enormous enhancement of the vacuum pair production rate. We discuss how important features of two different mechanisms can be analysed and understood via such a rotation in the complex plane. a) Consistent with previous studies, we first discuss the standard assisted mechanism with a static strong field and certain weak fields with a distinct pole structure in order to show that the reflection takes place exactly at the poles. We also discuss the effect of possible sub-cycle structures. We extend this reflection picture then to weak fields which have no poles present and illustrate the effective reflections with explicit examples. An additional field strength dependence for the rate occurs in such cases. We analytically compute the characteristic threshold for the assisted mechanism given by the critical combined Keldysh parameter. We discuss significant differences between these two types of fields. For various backgrounds, we present the contributing instantons and perform analytical computations for the corresponding rates treating both fields nonperturbatively. b) In addition, we also study the case with a nonstatic strong field which gives rise to the assisted dynamical mechanism. For different strong field profiles we investigate the impact on the critical combined Keldysh parameter. [...]Comment: 54 pages, 23 figures, revised, restructured to improve readability, matches journal versio

Elli bin sayılık ilk baskı ile:Nazım Hikmet İspanyolca'ya çevrildi

Taha Toros Arşivi, Dosya Adı: Nazım Hikmetİstanbul Kalkınma Ajansı (TR10/14/YEN/0033) İstanbul Development Agency (TR10/14/YEN/0033

Explaining Investment and Intermediate Goods Imports and Estimating Elasticities in Turkey

Turkey has been highly dependable on foreign production goods for Its domestic production since 1980. This necessitates estimation of income, relative import price, exchange rate and term of trade elasticities of investment and intermediate goods import demand to manage trade deficit. Assuming theoretical causalities, this study shows that investment and intermediate goods imports can be explained by gross national product, relative import price, foreign exchange rate, terms of trade, relative import price adjusted and foreign terms of trade adjusted real foreign exchange rate for the period of 1982-2004. The income, relative import price and foreign exchange rate elasticities of investment good import are found less elastic than the related elasticities of intermediate good import. Investment and intermediate good imports increase as export increases. The positively estimated import price relative to domestic wholesale price effect implies the Veblen effect on the real import and complementary goods relationship between domestic and imported production goods

Explaining and Testing Structural Break of Relative Trade of Turkey with European Union Countries

This study explains and tests structural break of Turkey’s relative trade with the European Union countries for the period of 1982-2007. As a result, the data indicated the evidence of structural break and alternating elasticities’ signs in estimations based upon error component model. After (Before) Turkey being the member of the ECU a percentage increase in all the relative foreign income, Turkish TOT, real exchange simultaneously and a unit increase in trend is found improving (decreasing) the relative trade about 0.135 (0.78) percentage in total. However, the dynamic fixed effect model did not validate the evidence of structural break. This implies individual differences matter in relative trade not to face structural break

TV'de bu hafta

Taha Toros Arşivi, Dosya Adı: Ahmet Cevat Paşa-Cevat Şakir Kabaağaçl