MRI edge enhancement as a diffusive discord of spin phase structure

The enhancement of magnetic resonance image intensity near impermeable boundaries can be nicely described by a new approach where the diffusional spin echo attenuation is linked to the correlation function of molecular motion. In this method the spin phase structure created by the applied gradient is considered to be a composition of plane waves with the wave vectors representing feasible momentum states of a particle in confinement. The enhancement of edges on the magnetic resonance images (MRI) comes out as a discord of plane waves due to particle motion. It results from the average of the wave phase by using the cumulant expansion in the Gaussian approximation. The acquired analytical expression describes the MRI signal space distribution where the enhancement of edges depends on the intensity and the duration of gradient sequence as well as on the length of the mean squared particle displacement in restricted geometry. This new method works well with gradients of general waveform and is, therefore, suitable for imaging sequences where finite or even modulated gradients are usually used.     

Bremsstrahlung of a neutral fermi particle in the field of a plane electromagnetic wave

The bremsstrahlung is considered from a neutral Fermi particle with anomalous magnetic and electric moments in the field of a screened Coulomb center and in the presence of a plane electromagnetic wave. The effect of the wave polarization on the scattering cross section and the behavior of the particle spin during the scattering process are considered. Cross sections are given for scattering of a particle at a Coulomb center in the presence of constant, crossed electric and magnetic fields which are equal in magnitude and also for a free particle. It is shown that the effect of the anomalous electric moment is often decisive.     

On the propagation of solutions for the wave equations describing the particles with arbitrary spin

The casual propagation of Hurley wave equations for a massive particle with an arbitrary spin s, interacting minimally with an external electromagnetic field, is found to be due to the nature of the β-matrices satisfying Harish Chandra's condition analogous to the spin-one case.     

Spinning Particle in Interaction with Plane Wave Field Via Path Integral

An exact solution for a spinning particle in interaction with an electromagnetic plane wave field is obtained using classical equations of motion. The Green's function is found and the result agrees with those of the literature. Besides, the Polyakov spin factor has been explicitly deduced.     

Space-time in ordered electromagnetic radiation outside a massive plane

The Einstein-Maxwell equations are solved with an energy-momentum tensor corresponding to the field of a standing electromagnetic wave outside a massive plane, in the limit where the test particle is much larger than the wavelength of the electromagnetic field. Physical properties of the solution are discussed.     

Twistors and linearized Einstein theory on plane-fronted impulsive wave backgrounds

The spinor form of linearized gravitation on a curved vacuum background is obtained from the Bianchi identities. There are no Buchdahl constraints to be satisfied, and for a flat background the approach reduces to the usual massless spin 2 theory. The equations are specialized to the plane-fronted impulsive gravitational wave backgrounds which include the ultrarelativistic limit of a moving black hole. Given a solution of the resulting system, it is possible to construct the metric perturbations up to gauge terms. It is shown that this linearized scheme is equivalent to the proposed twistor Hamiltonian formalism. As an example, colliding plane impulsive waves are considered, and the model compares well with the known exact solution. A discussion is given of the application of these methods to ultrarelativistic black-hole encounters.     

Wave functions in geometric quantization

A geometrical way is described to associate quantum states in the sense of geometric quantization to wave functions in the quantum mechanical sense for each relativistic elementary particle. Explicit computations are made in a number of cases: Klein-Gordon and Dirac equations, neutrino and antineutrino Weyl equations, and very general cases of massive and massless particles of arbitrary spin. In this later case one is led in a canonical way to Penrose wave equations.     

Description of anomalous magnetic moment of massive particle with spin 2 in the theory of relativistic wave equations

Within the framework of the theory of first-order relativistic wave equations in matrix form, the possibility of describing a massive particle with spin 2 having a static anomalous magnetic moment is revealed, on introducing minimal interaction with an external field.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 11–16, March, 1988.     

Reflection of a plane impulsive acoustic pressure wave by a rigid sphere

The reflection of a plane impulsive acoustic pressure wave by a rigid sphere is studied in terms of distributions. The result is a formulation ideally suited for practical applications involving convolutions with arbitrary wave shapes.     

Path integral for a Dirac particle in a quantized plane wave field

The problem of a relativistic spinning particle interacting with a quantized electromagnetic plane wave is treated by employing path integral methods and introducing the notion of a coherent state action. The dynamics of the particle spin is described using the supersymmetric action proposed recently by Alexandrou et al. in the so-called global representation. It is shown that to obtain the relative causal Green function, two fermionic identities, related directly to the classical equations of motion, have to be incorporated. The Green function, as constructed, allows us to extract in a natural way the expressions of the corresponding energy spectrum and wave functions. Received: 26 July 2002 / Revised version: 11 September 2002 / Published online: 25 October 2002     

Projection Operator and Feynman Propagator for a Free MassiVe Particle of Arbitrary Spin

Based on the solution to the Bargmann-Wigner equations, a direct derivation of the projection operator and Feynman propagator for a free massive particle of arbitrary spin is worked out. The projection operator constructed by Behrends and Fronsdal is re-deduced and confirmed, and simplified in the case of half-integral spin, the general commutation rules and Feynman propagator with additional non-covariant terms for a free massive particle with any spin are derived, and explicit expressions for the propagators for spins 3/2, 2, 5/2, 3, 7/2, and 4 are provided.     

Possibility of T-violating P-conserving magnetism and its contribution to the T-odd P-even neutron–nucleus forward elastic scattering amplitude

T-violating P-even magnetism is considered. The magnetism arises from the T-violating P-conserving vertex of a spin 1/2 particle interaction with the electromagnetic field. The vertex vanishes for a particle on the mass shell. Considering the particle interaction with a point electric charge we have obtained the T-violating P-even spin dependent potential, which is inversely proportional to the cubed distance from the charge. The matrix element of this potential is zero for particle states on the mass shell; nevertheless, the potential contributes to the T-odd P-even neutron forward elastic scattering amplitude by a deformed nucleus with spin . The contribution arises if we take into account incident neutron plane wave distortion by the strong neutron interaction with the nucleus. Received: 29 August 2000 / Revised version: 22 January 2001 / Published online: 23 March 2001     

相对论粒子的自旋算符

发展了关于相对论态自旋算符的系统理论.考虑了具有非零静质量的粒子情况.对带自旋的相对论粒子,通常的自旋算符需换为相对论的自旋算符.在Poincar啨群不可约表示的框架里,构造了适用于粒子任意正则态的自旋算符,称为运动自旋.本文的讨论限于量子力学.随后将在量子场论中对此作进一步深入研究.     

Worldline path integrals for a Dirac particle in a weak gravitational plane wave

The problem of a relativistic spinning particle interacting with a weak gravitational plane wave in (3+1) dimensions is formulated in the frame work of covariant supersymmetric path integrals. The relative Green function is expressed through a functional integral over bosonic trajectories that describe the external motion and fermionic variables that describe the spin degrees of freedom. The (3+1) dimensional problem is reduced to the (1+1) dimensional one by using an identity. Next, the relative propagator is exactly calculated and the wave functions are extracted. PACS 04.30.-w; 03.65.Ca; 03.65.Db; 03.65.Pm     

Emission and absorption of plane impulsive gravitational waves

Emission and absorption of plane impulsive waves of radiation by a thin shell of dust are studied. At the moment of interaction local conservation laws apply. An impulsive gravitational wave does not participate in the energy balance. Rather, it is associated with a change in isotropy of the source (absorber). Changes in the kinetic energy of the source exactly match the changes in its rest mass.     

De Sitter spacetime as a momentum measuring apparatus

We discuss the evolution of a quantum wave packet in the expanding de Sitter spacetime using the plane wave solutions of the Dirac equation. We concentrate on the case of large negative times when the packet approaches the event horizon and confirm that the evolution accords with that expected from the classical trajectories. We point out that in certain conditions the packet can split into two components that become localized at different parts of the horizon and that this effect can be seen, in an idealized sense, as a measuring process for the momentum of the particle, in direct analogy with the measurement of spin in a Stern-Gerlach experiment.     

Reflection of an Obliquely Incident Impulsive Plane Wave from Isotropic and Uniaxial Cold Lossy Plasma Half-Spaces

An exact solution for the transient reflected wave from a lossy plasma half-space is derived for the case of an obliquely incident impulsive plane electromagnetic wave. The plasma half-space may be isotropic or uniaxially anisotropic.     

Motion of a classical charged particle with spin in the external field of a plane electromagnetic wave

We obtain the solution to the equations of motion for a classical particle with spin, electric and magnetic charges, and electric and magnetic normal and anomalous moments, in the field of a plane electromagnetic wave. An explicit form of the solution is given for linearly polarized waves and monochromatic circularly polarized waves. The possibility is noted that due to the presence of the moments, the particle can be accelerated to speeds arbitrarily close to the speed of light.Translated from Izvestiya Vysshykh Uchebnykh Zavedenii, Fizika, No. 11, pp. 71–74, November, 1981.     

Triplet superconductivity in a 1D itinerant electron system with transverse spin anisotropy

In this paper we study the ground state phase diagram of a one-dimensional t-J-U model away from half-filling. In the large-bandwidth limit and for ferromagnetic exchange with easy-plane anisotropy a phase with gapless charge and massive spin excitations, characterized by the coexistence of triplet superconducting and spin density wave instabilities is realized in the ground state. With increasing ferromagnetic exchange transitions into a ferrometallic and then a spin gapped triplet superconducting phase take place.     

Representation of the propagator of a massive spinning particle as a BFV-BRST path integral

In the index-spinor approach, the transition amplitude for a free massive particle of arbitrary spin is obtained by calculating the relevant path integral in the BFV-BRST formalism. The calculation is performed without any renormalization of the measure in the path integral. The result coincides with the Weinberg propagator in the index-free representation. It is shown that the type of representation for the particle spin—a holomorphic or an antiholomorphic one—is determined by the choice of boundary conditions for the index spinor.