Formation of ions by high energy photons

We calculate the electron energy spectrum of ionization by a high energy photon, accompanied by creation of electron-positron pair. The total cross section of the process is also obtained. The asymptotics of the cross section does not depend on the photon energies. At the photon energies exceeding a certain value $\omega_0$ this appeares to to be the dominant mechanism of formation of the ions. The dependence of $\omega_0$ on the value of nuclear charge is obtained. Our results are consistent with experimental data.Comment: 16 pages, 6 figure

Photon storage in Lambda-type optically dense atomic media. III. Effects of inhomogeneous broadening

In a recent paper [Gorshkov et al., Phys. Rev. Lett. 98, 123601 (2007)] and in the two preceding papers [Gorshkov et al., Phys. Rev. A 76, 033804 (2007); 76, 033805 (2007)], we used a universal physical picture to optimize and demonstrate equivalence between a wide range of techniques for storage and retrieval of photon wave packets in homogeneously broadened Lambda-type atomic media, including the adiabatic reduction of the photon group velocity, pulse-propagation control via off-resonant Raman techniques, and photon-echo-based techniques. In the present paper, we generalize this treatment to include inhomogeneous broadening. In particular, we consider the case of Doppler-broadened atoms and assume that there is a negligible difference between the Doppler shifts of the two optical transitions. In this situation, we show that, at high enough optical depth, all atoms contribute coherently to the storage process as if the medium were homogeneously broadened. We also discuss the effects of inhomogeneous broadening in solid state samples. In this context, we discuss the advantages and limitations of reversing the inhomogeneous broadening during the storage time, as well as suggest a way for achieving high efficiencies with a nonreversible inhomogeneous profile.Comment: 15 pages, 8 figures. V2: minor changes in presentation, new references, higher resolution of figure

Photon storage in Lambda-type optically dense atomic media. I. Cavity model

In a recent paper [Gorshkov et al., Phys. Rev. Lett. 98, 123601 (2007)], we used a universal physical picture to optimize and demonstrate equivalence between a wide range of techniques for storage and retrieval of photon wave packets in Lambda-type atomic media in free space, including the adiabatic reduction of the photon group velocity, pulse-propagation control via off-resonant Raman techniques, and photon-echo-based techniques. In the present paper, we perform the same analysis for the cavity model. In particular, we show that the retrieval efficiency is equal to C/(1+C) independent of the retrieval technique, where C is the cooperativity parameter. We also derive the optimal strategy for storage and, in particular, demonstrate that at any detuning one can store, with the optimal efficiency of C/(1+C), any smooth input mode satisfying T C gamma >> 1 and a certain class of resonant input modes satisfying T C gamma ~ 1, where T is the duration of the input mode and 2 gamma is the transition linewidth. In the two subsequent papers of the series, we present the full analysis of the free-space model and discuss the effects of inhomogeneous broadening on photon storage.Comment: 16 pages, 2 figures. V2: significant changes in presentation, new references, higher resolution of figure

Photon storage in Lambda-type optically dense atomic media. II. Free-space model

In a recent paper [Gorshkov et al., Phys. Rev. Lett. 98, 123601 (2007)], we presented a universal physical picture for describing a wide range of techniques for storage and retrieval of photon wave packets in Lambda-type atomic media in free space, including the adiabatic reduction of the photon group velocity, pulse-propagation control via off-resonant Raman techniques, and photon-echo based techniques. This universal picture produced an optimal control strategy for photon storage and retrieval applicable to all approaches and yielded identical maximum efficiencies for all of them. In the present paper, we present the full details of this analysis as well some of its extensions, including the discussion of the effects of non-degeneracy of the two lower levels of the Lambda system. The analysis in the present paper is based on the intuition obtained from the study of photon storage in the cavity model in the preceding paper [Gorshkov et al., Phys. Rev. A 76, 033804 (2007)].Comment: 26 pages, 8 figures. V2: significant changes in presentation, new references, higher resolution of figure

Four point function of R-currents in N=4 SYM in the Regge limit at weak coupling

We compute, in N=4 super Yang-Mills theory, the four point correlation function of R-currents in the Regge limit in the leading logarithmic approximation at weak coupling. Such a correlator is the closest analog to photon-photon scattering within QCD, and there is a well-defined procedure to perform the analogous computation at strong coupling via the AdS/CFT correspondence. The main result of this paper is, on the gauge theory side, the proof of Regge factorization and the explicit computation of the R-current impact factors.Comment: 21 pages, 10 figures, typos correcte

Parity nonconservation in electron recombination of multiply charged ions

We discuss a parity nonconserving asymmetry in the cross section of KLL dielectronic recombination of polarized electrons on the hydrogen-like ions with $Z \lesssim 60$. This effect is strongly enhanced because of the near-degeneracy of doubly-excited $2l2l'$ states of opposite parity in He-like ions. For ions with $Z \sim 30$ the asymmetry is of the order of $10^{-9}$. For $Z \approx 48$ a level crossing takes place, leading to the PNC asymmetry of $\pm 5\times 10^{-9}$, which is $10^8$ times greater than the basic strength of the weak interaction in atoms.Comment: 11 pages, 5 figures; v.2: sign errors in Eqs.(29-32,38) corrected, figs.4,5 and related discussion change

On recognition of direct powers of finite simple linear groups by spectrum

The spectrum of a finite group is the set of its element orders. We give an affirmative answer to Problem 20.58(a) from the Kourovka Notebook proving that for every positive integer $k$, the $k$-th direct power of the simple linear group $L_{n}(2)$ is uniquely determined by its spectrum in the class of finite groups provided $n$ is a power of $2$ greater than or equal to $56k^2$.Comment: 17 page