Negative power spectra in quantum field theory

We consider the spatial power spectra associated with fluctuations of quadratic operators in field theory, such as quantum stress tensor components. We show that the power spectrum can be negative, in contrast to most fluctuation phenomena where the Wiener-Khinchin theorem requires a positive power spectrum. We show why the usual argument for positivity fails in this case, and discuss the physical interpretation of negative power spectra. Possible applications to cosmology are discussed.     

Proper bases and internal energy in electromagnetic field theory

Orthonormal proper bases are determined for an electromagnetic field. An expression is proposed for the internal electromagnetic field energy that is a four-dimensional scalar. The Maxwell equations are written in the proper bases.Translated form Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 8–12, October, 1989.     

Reconstruction of spatial spectra of the isotropic field of background internal waves

Model reconstruction of the two-dimensional spatial spectra of isotropic background internal waves is considered in the framework of computer modeling. The solution of the inverse problem is based on information about the frequency shifts of interference maxima of an acoustic field. The results of reconstruction of spectra with and without focusing of the inverse wave field are presented. The possibilities of monitoring are illustrated in relation to the interference pattern formed by different modal composition.     

Temperature dependence of bound state spectra in field theory

We analyze the behaviour of kinks and semiclassical bound states at finite temperatures by applying quantum statistics to the fluctuations which determine the quantum dynamics of these states. We consider two theories in one space dimension — the ?⁴ theory with a dynamical symmetry breaking and the Gross-Neveu model. For the ?⁴ theory, the one-loop temperature corrections are obtained by using temperature-dependent Green function techniques. We show that the same result can be obtained by applying quantum statistics to the fluctuations around the kink. For the Gross-Neveu model, the temperature dependence of the bound states, which correspond to time-independent field configurations, is obtained. We show that for every bound state there exists a critical temperature at which this state breaks up into its constituents. This critical temperature increases with the number of constituents of the bound state.     

Exact wave functions in a noncommutative field theory

We consider the nonrelativistic field theory with a quartic interaction on a noncommutative plane and compute the 2-->2 scattering amplitude within perturbative analysis to all orders. We regain the results of the perturbative analysis by finding the scattering and the bound state wave functions of the two particle Schrodinger equation. These wave functions unusually have two center positions in the relative coordinates, whose separation is transverse to the total momentum and scales linearly with its magnitude, exhibiting the stringy nature of the noncommutative field theory.     

Self-consistent field theory of a helix traveling wave tubeamplifier

A self-consistent relativistic field theory of a helix traveling wave tube (TWT) is presented for a configuration in which a thin annular beam propagates through a sheath helix enclosed within a loss-free wall. A linear analysis of the interaction is carried out, subject to the boundary conditions imposed by the beam, helix, and wall. A detrimental dispersion equation is obtained which implicitly includes beam space-charge effects without recourse to a heuristic model of the space-charge field. The equation is valid for arbitrary azimuthal mode number and is solved numerically for the azimuthally symmetric case. The coupled-wave Pierce theory is recovered in the near-resonant limit. Numerical comparisons between the complete dispersion equation and the Pierce model are described. A discrepancy is found between the Pierce and the field theory even for low currents in the nominally ballistic regime, owing to the dielectric effect of the beam on the helix modes     

Energy and momentum as observables in quantum field theory

The spectrum condition implies that energy and momentum are limits of local observables.     

On the spectrum of internal symmetries in the algebraic quantum field theory

It is shown that the point spectrum of internal symmetries is always symmetric. It is a group provided the intersection of all local subalgebras is trivial.     

Horizontal array beamforming in an azimuthally anisotropic internal wave field

A numerical study of beamforming on a horizontal array is performed in a shallow water waveguide where a summer thermocline is perturbed by a time evolving realization of an internal wave field. The components of the internal wave field consist of a horizontally (azimuthally) isotropic, spatially homogeneous contribution, and a horizontally anisotropic, spatially inhomogeneous component. These terms represent a diffuse ("background") internal wave field and a localized solitary wave packet, respectively. Conventional beamforming is performed as a function of time while the internal wave field evolves throughout a computational volume containing the source-receiver paths. Source-receiver orientation with respect to the azimuthally anisotropic component has a significant effect on the beamformed output. When the source-receiver configuration is oriented approximately parallel to the solitary wave crests, beam wander, fading, beam splitting and coherence length degradation occurs in a time-dependent manner as the solitary wave packet passes through the environment. Both horizontal refraction of energy and a time-dependent modal source excitation distribution are responsible for these beamforming effects. In cases where source-receiver orientation is not approximately parallel to the wave crests, these effects are substantially reduced or eliminated, indicating that an azimuthally selective perturbation of the acoustic field can be attributed to the wave packet. Modal decomposition of the acoustic field and single mode starting fields are used to infer that, for the source-receiver orientation along the wave crests and troughs, acoustic propagation is predominantly adiabatic. A modal phase speed analysis explains several features associated with the beamformed power.     

Modeling of nanoshells spectra in evanescent wave field via discrete sources method

Simple, fast and flexible computer model to analyze optical spectra of individual nanoshells deposited on a prism surface has been developed. It is based on rigorous theory of discrete sources method and enables to account for complete interaction between scatterer and prism surface analytically. Model is implemented to examine local biosensor operation in a field of evanescent waves.     

Conformal field theory and the symmetries of string field theory

The two-dimensional conformal field theory representation of Witten's bosonic string field theory is discussed. The basic overlap equations, K_n symmetry and BRST invariance are proved directly, without the usual expansion in oscillators. The conformal field theory approach naturally provides local overlap identities which (when integrated over half the string) can be used to verify properties of the cubic action. In particular, a recently proposed diffeomorphism invariance is shown to be free of anomalies. Finally, a new class of symmetries, including generalizations of the K_n symmetries which are local in spacetime, are presented.     

Electromagnetic wave scattering on a cylinder. High-frequency asymptotic behavior of the internal field

Electromagnetic wave scattering on a dielectric multilayer cylinder is studied. It is shown that the asymptotic behavior of the field in the inner layer of the cylinder is a superposition of two flat waves running towards each other. A simple way is found of computing the coefficients of this superposition.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 100–103, August, 1991.     

Relativistic wave packets in the quantum field approach to the theory of neutrino oscillations

A covariant theory of wave packets and its application to the quantum field method of calculation of the probabilities of neutrino oscillations in vacuum that is based on the technique of macroscopic Feynman diagrams, which describe the processes of emission and absorption of virtual massive neutrinos ν _i (i = 1, 2, 3) at macroscopicly separated space-time points, is considered. The effect of flavor oscillations is reduced to an interference of amplitudes with different vi in an intermediate state. A macroscopic amplitude is calculated that describes a class of processes which go with nonconservation of leptonic numbers, and statistical averaging of the squared modulus of this amplitude is performed. The averaged probability of a process with ultrarelativistic neutrino exchange is representable in the form of an integral of the product of three factors: the flux of massless neutrinos from the source, the differential cross-section for the interaction of a neutrino with the detector, and a dimensionless factor that describes the flavor transition. The conditions under which the last factor can be interpreted as the probability of the flavor transition in the conventional quantummechanical sense are analyzed.     

Energy spectra and quantum crystallization in two-electron quantum dots in a magnetic field

Quantum crystallization of electrons in a quantum dot (QD) subjected to an external magnetic field is considered. Two-electron QDs with two-dimensional (2D) parabolic confining potential in an external transverse magnetic field are calculated. The Hamiltonian is numerically diagonalized in the basis of one-particle functions to find the energy spectra and wave functions for the relative motion of electrons with inclusion of electron-electron interaction for a broad range of the confining-potential steepness (α) and external magnetic fields (B). The region of the external parameters (α, B) within which a gradual transition to quantum crystalline order occurs is numerically determined. In contrast to a 2D unbounded system, a magnetic field acts nonmonotonically on “crystallization” in a quantum dot with several electrons because of a competition between two effects taking place with increasing B, namely, decreasing spread of the electron wave functions and increasing effective steepness of the confining potential, which reduces the average separation between electrons. Fiz. Tverd. Tela (St. Petersburg) 40, 1753–1759 (September 1998)     

Central charges and Lorentz and internal symmetries in massive supersymmetric quantum field theory

The properties of central charges in the framework of the massive supersymmetric quantum field theory related to internal symmetries, Lorentz covariance and locality of the fields are investigated. It is shown that in the presence of z central charges the largest semisimple part of the internal symmetry algebra is a direct sum of z compact symplectic group algebras and possibly an additional term representing the unimodular unitary group algebra. Next it is shown that 4j ? N + K, where j is the highest spin value of the underlying fields, N is the number of spinorial charges and K the number of these spinorial charges which are not linked to other spinorial charges by a central charge. It is further demonstrated that, in general, the central charge can not be redefined in such a way that it is at the same time real and preserves the locality principle. The discussion of the obtained results concludes the paper.