• Volume 51, Issue 3-4

      September 1998,   pages  317-546

    • Charges in gauge theories

      Robin Horan Martin Lavelle David Mcmullan

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      In this article we investigate charged particles in gauge theories. After reviewing the physical and theoretical problems, a method to construct charged particles is presented. Explicit solutions are found in the abelian theory and a physical interpretation is given. These solutions and our interpretation of these variables as the true degrees of freedom for charged particles, are then tested in the perturbative domain and are demonstrated to yield infra-red finite, on-shell Green’s functions at all orders of perturbation theory. The extension to collinear divergences is studied and it is shown that this method applies to the case of massless charged particles. The application of these constructions to the charged sectors of the standard model is reviewed and we conclude with a discussion of the successes achieved so far in this programme and a list of open questions.

    • Gravitationally induced particle creation in aq-scalar field

      G Vinod K Babu Joseph

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      Tom and Goodison [5] have shown that for generic values ofq, gravitationally induced particle creation is impossible in the ordinary vacuum state. Here we consider the evolution of aq-deformed scalar field in a curved spacetime and observe that if the field is either represented by a coherent state or a squeezed state, there is a change in the energy density of the field indicating the possibility of particle creation.

    • Correct bound states of the non- linear oscillator through non-variational method: Failure of the variational method

      Biswanath Rath

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      A method for obtaining correct bound states of the non-linear oscillatorH = P2+X2 +X2(X2) is described. It is shown that the variational method fails to yield correct ground state energy.

    • String cosmology in inhomogeneous cylindrically symmetric spacetime

      Subenoy Chakraborty Anusua Roy

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      String-dust cosmology in an inhomogeneous cylindrically symmetric model is considered. Solutions are obtained only for geometric string with the separability assumption for metric coefficients.

    • Existence of multiple attractors and the nature of bifurcations in a discontinuous logistic map

      P R Krishnan Nair V M Nandakumaran

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      We present the analytical investigations on a logistic map with a discontinuity at the centre. An explanation for the bifurcation phenomenon in discontinuous systems is presented. We establish that whenever the elements of ann-cycle (n > 1) approach the discontinuities of thenth iterate of the map, a bifurcation other than the usual period-doubling one takes place. The periods of the cycles decrease in an arithmetic progression, as the control parameter is varied. The system also shows the presence of multiple attractors. Our results are verified by numerical experiments as well.

    • CP-violating dipole form factors of the top quark and tau lepton in scalar leptoquark models

      P Poulose Saurabh D Rindani

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      We calculate the CP-violating electric and weak dipole form factors of the top quark and the tau lepton in models with scalar leptoquarks coupling only to the third generation of quarks and leptons. We obtain numerical values of the real and imaginary parts of these form factors at various energies for different values of leptoquark masses and couplings. The existing limits on the tau electric and weak dipole form factors allow us to put a limit on the masses and couplings of such leptoquarks and therefore on the top electric and weak dipole form factors. We also discuss constraints on the form factors coming from indirect limits on leptoquark masses and couplings deduced from LEP results on Z properties.

    • Nonlinear effects on Nielsen-Olesen instability

      Vishnu M Bannur

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      We have obtained an analytical solution of the nonlinear Nielsen-Olesen mode, which is known to be linearly unstable. We believe that our solution may be a nonlinear saturated state of the instability. It is different from the flux tube structure of chromomagnetic field as proposed by the Copenhagen group [1-3]. We have reanalysed their calculations, including a term, which they have neglected, and find it consistent with our result.

    • Aspects of Planckian scattering beyond the eikonal

      Saurya Das Parthasarathi Majumdar

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      We discuss an approach to compute two-particle scattering amplitudes for spinless light particles colliding at Planckian centre-of-mass energies, with increasing momentum transfer away from the eikonal limit. The leading corrections to the eikonal amplitude, in our ‘external metric’ approach, are shown to be vanishingly small in the limit of the source particle mass going to zero. For massless charged particles, the electromagnetic and gravitational interactions decouple in the eikonal limit, but mix non-trivially for the leading order corrections.

    • ΛN Space-exchange effects in thes-shell hypernuclei andΛ9Be

      Mohammad Shoeb Q N Usmani A R Bodmer

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      Variational Monte Carlo calculations of the ground state separation energiesBΛ of thes-shell hypernuclei and also ofΛ9Be have been made for an Urbana-type central space-exchange ΛN potential consistent with Λp scattering, and also including three-body ΛNN forces. Thes-shell hypernuclei are treated asA-body systems (A = baryon number), andΛ9Be is analysed as a partially nine-body problem in the Λ — 2α model. The reduction ofBΛ due to the space-exchange ΛN potential has been calculated for thes-shell hypernuclei for a range of interactions: both ΛN and ΛN + ΛNN forces. ForA = 3,4,5 the exchange energy is approximately, 0.04, 0.15 and 0.50 MeV, respectively. ForΛ9Be a much more limited study gives ≅ 1.3 MeV. These values are much larger than that for ‘soft’ ΛN +NN potentials when the correlations are weak.

    • Investigation of two- and three-nucleon transfer reactions in12C +56Fe

      Hari S Patel B Srinivasan B J Roy M G Betigeri

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      Multi-nucleon transfer reactions56Fe(12C, X) have been studied at an incident12C energy of 60 MeV. Angular distributions of10Be and9Be corresponding to 2p and 2p 1n transfer reactions in transition to low-lying states in the residual nuclei have been measured. The angular distribution data for 2p transfer have been analysed in terms of finite range DWBA calculations assuming a one-step transfer of two protons. The spectroscopic factors for three low-lying transitions observed in56Fe(12C,10Be)58Ni have been extracted. Transfer probabilities for the ground state transition in two- and three-nucleon stripping channels have been obtained and compared with the corresponding sequential transfer probabilities in order to emphasise the role of direct transfer of nucleons vis-a-vis sequential transfer.

    • On the Wigner-Witmer correlation rules for a homonuclear diatomic molecule with the like atoms in identical atomic states

      V P Bellary T K Balasubramanian B J Shetty

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      The problem of building the manifold of electronic states of a homonuclear diatomic molecule formed from two like atoms in identical atomic states is readdressed. A conceptually simple approach is presented by invoking a model and all the standard results originally obtained by Wigner and Witmer are reproduced.

    • Total electron scattering cross-sections for carbon monoxide at low electron energies

      P Rawat K P Subramanian Vijay Kumar

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      Absolute total electron scattering cross-sections for carbon monoxide have been measured at low electron energies using a photoelectron source. The measurements have been carried out at 22 electron energies varying from 0.73 to 9.14 eV with an accuracy of ±3%. The cross-sections obtained in the present experiment have been compared with other measurements and theoretical computations.

    • Design considerations and scaling laws for high power convective cooled CW CO2 lasers

      A K Nath V S Golubev

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      Various criteria for designing high power convective cooled CO2 lasers have been discussed. Considering the saturation intensity, optical damage threshold of the optical resonator components and the small-signal gain, the scaling laws for designing high power CW CO2 lasers have been established. In transverse flow CO2 lasers having discharge of square cross-section, the discharge lengthL and its widthW for a specific laser powerP (Watt) and gas flow velocityV (cm/s) can be given byL = 1.4 x 104p1/2V-1 cms andW = 0.04P1/2 cms.

      The optimum transmitivity of the output coupler is found to be almost constant (about 60%), independent of the small signal gain and laser power. In fast axial flow CO2 lasers the gas flow should be divided into several discharge tubes to maintain the flow velocity within sonic limit. The discharge length in this type of laser does not depend explicitly on the laser power, instead it depends on the input power density in the discharge and the gas flow velocity. Various considerations for ensuring better laser beam quality are also discussed.

    • Amplification of extra-ordinary mode wave in the presence of kinetic Alfvén wave turbulence

      B K Saikia B J Saikia K S Goswami S Bujarbarua

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      The amplification mechanism of extra-ordinary mode radiation in the presence of kinetic Alfvén wave turbulence driven by an electron beam is studied. It is shown that plasma maser process may be responsible for the amplification of the extra-ordinary mode through up-conversion of turbulent energy via nonlinear wave particle interaction. The relevance of this investigation to space plasmas is discussed.

    • On a class of three-dimensional corner eddies

      P N Shankar

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      Consider Stokes flow in a viscous fluid filling a corner, of angle 2α, bounded by two infinite plane walls. Assume that the flow is symmetrical about some plane which is normal to the walls bounding the corner. Since superposition is valid we may consider flows that are symmetrical about the plane bisecting the comer and those that are antisymmetrical about this plane. In either case it is shown that for a class of corner eddies, the corner flow is made up of an infinite sequence of eddies asr → 0, wherer is the radial distance from the corner. Moreover, the eigenvalues λ which determine the structure of the corner eddy fields satisfy the same equation, sin λα = ± λ sin 2α, that arises in the corresponding plane case. The three-dimensional velocity fields are, however, quite different from those seen in the plane case. In particular, in the symmetric case the streamlines are not closed and foci, rather than elliptic stagnation points, are the centres of the eddies in the plane of symmetry. These results represent, in this special context, a generalization to three-dimensions of Moffatt’s classical result for planar corner eddies.

    • Magnetic anomalies in Gd2PdSi3

      R Mallik E V Sampathkumaran P L Paulose H Sugawara H Sato

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      The results of ac and dc magnetic susceptibility, thermopower and Hall effect measurements of a compound, Gd2PdSi3, establish that this compound orders magnetically below Tn = 20 K. Though the ordering appears to be of an antiferromagnetic-type, the paramagnetic Curie temperature is positive with the magnitude being nearly the same as that of TN, suggestive of the existence of ferromagnetic correlations. The thermopower at 300 K is large, apparently due to Pd 4d electrons, decreasing monotonically with temperature. There is a change in the sign of Hall constant well below TN Also considering the observation of Kondo-like characteristics above 21K earlier by us, the overall thermal, transport and magnetic behaviour of this compound is interesting.

    • Dielectric properties of electrolytic solutions

      V V Navarkhele R S Agrawal M L Kurtadikar

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      The variation in dielectric properties of water with the addition of ionic salts have been measured using automated frequency domain experimental microwave C-band. The dielectric properties, that is dielectric constant (ε′) and dielectric loss (ε″) of two electrolyte solutions for various concentrations have been measured at 5-GHz frequency at room temperature. It has been observed that for concentration between 0.2 to 1.0 mole the dielectric constant of water is smaller and some larger than that of pure water and dielectric loss increases with increasing concentration of these salts. It has been also observed that the variation in dielectric loss is different, though the ionic concentration of the two salts are equal.

    • 3-d Model for strain ordering in steel: I Static effects

      S K Ghoshal S Dattagupta

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      We present a spring-defect model in 3-dimensions to describe the connection between elastic distortion and interstitial carbon ordering associated with phase transition from a body centred cubic (BCC) to body centered tetragonal (BCT) structure in BCC metals such as α-iron. The presence or the absence of the carbon is modelled in terms of a pseudo spinŝ=+1or -l.An Ising interaction between carbon atoms is recovered after eliminating the lattice degrees of freedom, which is longranged. The coupling between the spin and lattice degrees of freedom allows for a systematic study of ferroelasticity and the variation of the lattice parameter with carbon concentration. The mean field results for the paraelastic to ferroelastic transition, lattice parameter and static compliance are presented. The significant feature of this calculation is not only a derivation of the defect-defect interaction, but also an explicit calculation of the strain dipole tensor associated with each defect, from a microscopic model.

    • 3-d Model for strain ordering in steel: II Relaxational effects

      S K Ghoshal S Dattagupta

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      The spring-defect model developed by us in the accompanying paper I to discuss ferroelasticity, exhibited in the BCT phase of α-iron (BCC metals), is used to analyse anelastic relaxation across the paraelastic to ferroelastic phase transition. The kinetics of the underlying Hamiltonian representing strain-strain interactions is treated within mean-field theory. The relaxation-response relation of the linear response theory is employed to derive explicit expressions for the anelastic strain, the frequency-dependent compliance and the internal friction in terms of the basic parameters of the spring-defect model.

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