On the derivation of effective interactions by matrix diagonalization methods

A technique is developed which in certain cases allows the derivation of a completely linked two body effective interaction for two particles beyond a closed shell using matrix diagonalization methods. In particular this applies to a space consisting of 0 and $\text{2}\text{h}\text{?}\text{ω}$ excitations.     

Conduction mechanism in polycrystalline Na1−xSrxNbO3 niobium bronzes

Electrical conductivity and thermoelectric power of polycrystalline Na_1?xSr_xNbO₃ have been measured between 5 and 300 K for various compositions. The results suggest that the density of states is finite at the Fermi level, whilst the carrier mobility is activated. The observed behavior is attributed to an Anderson localization of states at the Fermi level due to disorder resulting from either a random distribution of Sr²⁺ and Na⁺ ions or from the polycrystalline character of the samples or from both effects.     

Strangeness in stellar matter

A protoneutron star is formed immediately after the gravitational collapse of the core of a massive star. At birth, the hot and high density matter in such a star contains a large number of neutrinos trapped during collapse. Trapped neutrinos generally inhibit the presence of exotic matter — hyperons, a kaon condensate, or quarks. However, as the neutrinos diffuse out in about 10–15 s, the threshold for the appearance of strangeness is reduced; hence, the composition and the structure of the star can change significantly. The effect of exotic, negatively-charged, strangeness-bearing components is always to soften the equation of state, and the possibility exists that the star collapses to a black hole at this time. This could explain why no neutron star has yet been seen in the remnant of supernova SN1987A, even though one certainly existed when neutrinos were detected on Feb. 23, 1987. With new generation neutrino detectors it is feasible to test different theoretical scenarios observationally.     

Self-consistent cluster calculation of the potential for positive muons in Al and Cu

The potential profile for positive muons on the body diagonal in Al and Cu, are calculated using a molecular cluster model within the framework of the Hartree — Fock — Slater theory. The effects of substitutional Ag and Mn impurities are investigated. The extension of the muon wavefunction in a non-spherical potential is compared to the result obtained in a spherically averaged one.     

Challenges for extended technicolour theories

We develop the dialectic between extended technicolour theories and phenomenological constraints on rare processes. The natural suppression of flavour-changing neutral currents and CP-violating processes which can occur in models with elementary scalars is not a general feature of naïve extended technicolour (ETC) models. We study the extent to which naïve ETC estimates must in fact be suppressed in order to be compatible with the phenomenology of such rare processes. We emphasize the potential significance of the exchanges of neutral flavour-changing bosons and find that the strongest constraints arise from considering the combination of the D₁⁰-D₂⁰ and K₁⁰-K₂⁰ systems. CP-violating effects in ETC models must be severely suppressed if they are to be compatible with the observational facts. We point to several rare processes whose further experimental study is of particular concern to ETC theories.     

Electronic structure and bonding in ThO2 and UO2

Electron energy levels and charge densities for ThO₂ and UO₂ are calculated in a molecular cluster approximation, using spin unrestricted Hartree-Fock-Slater and relativistic Dirac-Slater models. Results compare favorably with X-ray photoelectron spectra and reveal similarities in chemical bonding with rare earth oxides.     

Cosmological constraints on supergravity models

We delineate the domain of supersymmetry breaking parameters in minimal supergravity models for which the cosmological relic photino density is no larger than the closure density. Demanding that the relic density equal the closure density as suggested by inflationary cosmology suggests $\text{m}{}_{\mathrm{<mtext>q</mtext><mtext>?</mtext>}}\text{～ 42}\text{GeV}\text{+ 0.89 m}{}_{\mathrm{<mtext>g</mtext><mtext>?</mtext>}}$. We point out that several supergravity scenarios for the monojet events at the CERN $\text{p}$p Collider would yield a relic density considerably greater than the closure density.     

The ΔI = 12 rule with heavy quarks and right-handed currents

The effect of strong interactions on a two-fermion operator is calculated in an asymptotically free theory, thereby completing the calculation of the logarithmic exponents for the operators contributing to non-leptonic amplitudes. The case of models with both left-and right-handed weak currents is discussed and the implications for the $\text{ΔI=}\text{1}\text{2}$ rule and other non-leptonic decays are deduced.     

Why the neutron isn't all neutral

Experimental measurements of the electric form factor of the neutron indicate a spatially inhomogeneous distribution of charge. The quark model, with a spatially dependent spin-spin interaction, can accurately describe this inhomogeneity. We relate the neutron's charge radius to the nucleon-delta mass difference and discuss other experimental consequences of the inhomogeneity.