The complex coordinate method for three-particle scattering above the break-up threshold

The complex coordinate method is used to evaluate the elastic scattering above the break-up threshold in a model three-body system. The results are compared with previous estimates.     

Electron-hydrogen scattering just above the ionization threshold

The scattering of electrons by hydrogen atoms is studied for an incident energy of 15 eV, by means of a close coupling calculation using a pseudostate basis. Results are presented for elastic scattering and n = 2 excitation, including angular correlation parameters. The ionization cross section is extracted by an improved projection technique.     

Up-down unification just above the supersymmetric threshold

Large corrections to the quark mass matrices at the supersymmetric threshold allow the theory to have identical Yukawa matrices in the superpotential. We demonstrate that up–down unification can take place in a moderate quark–squark alignment scenario with an average squark mass of the order 1 TeV and with . Received: 22 April 1998 / Revised version: 17 September 1998 / Published online: 25 January 1999     

Behavior of the scattering amplitude at the three-particle threshold for zero-range pair potentials

Differential equations in the momentum space for the 3 → 2 recombination processes of three bosons with pair forces of zero-range radius have been derived.     

Dynamics of a random laser above threshold

We have performed enhanced backscattering experiments in a high gain random laser, under circumstances where a stationary theory predicts the laser intensity to diverge. Above the laser threshold the observed backscatter cone changes only gradually, not showing any signs of the divergence. We present measurements and theory-generalized laser equations with a diffusive transport term for pump and laser light-to explain the observed behavior. The population dynamics prevent an indefinite growth of the intensity. Time dependence is essential for a theory of random lasers above the laser threshold.     

Band structure effects in above threshold photoemission

We have performed an angle-resolved two-photon and three-photon photoemission study of the Ag(111) surface employing ultrashort laser pulses as the excitation source. We show the presence of multi-photon resonances between electronic states at selected points of the Brillouin zone which appear in the high-order photoemission spectral region. We observe clear signatures of electronic band structure effects of the Ag crystal in above-threshold photoemission (ATP) processes, identifying two types of transitions, which either proceed via non-resonant multi-photon excitation from an occupied initial state, or involve a real intermediate state located above the vacuum level of the solid directly influencing the ATP process. For this latter class of phenomena, we suggest that electron populations created by incoherent processes give a contribution to the multi-photon transition, possibly allowing us to trace the transmission of photoexcited electrons through the crystal.     

Nonmonotonic flux flow in inhomogeneous superconductors above the percolation threshold

Flux flow resistivity above critical temperature is analyzed based on the increased evidences that high temperature superconductors are intrinsically inhomogeneous, and local nonpercolating superconducting domains persist high above the critical temperature. It was found that above a certain field, the resistivity originated in the flow of flux motion starts to decrease with increasing field, due to the predominance of the suppression of the superconducting droplets over the increase of vortex core density.     

The role of meson retardation in the NN interaction above pion threshold

A model is developed for the hadronic interaction in the two-nucleon system above pion threshold which is based on meson, nucleon and Δ degrees of freedom and which includes full meson retardation in the exchange operators. For technical reasons, the model allows maximal one meson to be present explicitly. Thus the Hilbert space contains besides NN and NΔ also configurations consisting of two nucleons and one meson. For this reason, only two- and three-body unitarity is obeyed, and the model is suited for reactions in the two-nucleon sector, where only one pion is produced or absorbed. Starting from a realistic pure nucleonic retarded potential, which had to be renormalized because of the additional π and Δ degrees of freedom, a reasonable fit to experimental NN-scattering data could be achieved.     

Evaluation of the LEP centre-of-mass energy above the W-pair production threshold

Knowledge of the centre-of-mass energy at LEP2 is of primary importance to set the absolute energy scale for the measurement of the W-boson mass. The beam energy above 80 GeV is derived from continuous measurements of the magnetic bending field by 16 NMR probes situated in a number of the LEP dipoles. The relationship between the fields measured by the probes and the beam energy is calibrated against precise measurements of the average beam energy between 41 and 55 GeV made using the resonant depolarisation technique. The linearity of the relationship is tested by comparing the fields measured by the probes with the total bending field measured by a flux loop. This test results in the largest contribution to the systematic uncertainty. Several further corrections are applied to derive the centre-of-mass energies at each interaction point. In addition, the centre-of-mass energy spread is evaluated. The beam energy has been determined with a precision of 25 MeV for the data taken in 1997, corresponding to a relative precision of . This is small in comparison to the present uncertainty on the W mass measurement at LEP. However, the ultimate statistical precision on the W mass with the full LEP2 data sample should be around 25 MeV, and a smaller uncertainty on the beam energy is desirable. Prospects for improvements are outlined. Received: 14 December 1998 / Published online: 3 November 1999     

Harmonic mixing in the classical charge density wave model above threshold

The third order response properties of the classical model for charge transport by charge density waves above the conductivity threshold are investigated analytically. The model is isomorphic to that of a resistively shunted Josephson circuit (RSJ model) and thus exhibits an oscillating solution above a threshold electric driving field with a characteristic internal frequencyω _ph . By a systematic perturbation expansion in powers of the perturbing field strengths thedc response of the time varying state to twoac fields of frequencies ω and 2ω is evaluated. This determines the harmonic mixing signal (HMS) as a third order response. The general formula is explicitly evaluated far above threshold giving a result similar to the Kanter-Vernon formula for the rectified signal in second order. In addition to the resonance atω _ph =ω a second resonance is found atω _ph =2ω. A characteristic feature for the HMS above threshold is the absence of an out of phase component, i.e. the internal phase shift is zero.     

Role of elastic scattering in high-order above threshold ionization

We investigate the target and intensity dependence of plateau in high-order above threshold ionization(HATI) by simulating the two-dimensional(2D) momentum distributions and the energy spectra of photoelectrons in HATI of rare gas atoms through using the quantitative rescattering model. The simulated results are compared with the existing experimental measurements. It is found that the slope of the plateau in the HATI photoelectron energy spectrum highly depends on the structure of elastic scattering differential cross section(DCS) of laser-induced returning electron with its parent ion. The investigations of the long- and short-range potential effects in the DCSs reveal that the short-range potential, which reflects the structure of the target, plays an essential role in generating the HATI photoelectron spectra.