The screening of the particle-hole interaction to all orders

Starting from Brueckner's G-matrix the effects of the induced interactions on the particle-hole interaction in nuclear matter are investigated to all orders. Although an iterative solution of the non-linear integral equations, which have to be solved, leads to divergent contributions, it can be shown that the renormalization effects summed to all orders are finite. The complete momentum dependence of the particle-hole interaction is treated which in the Landau limit results in less attractive F₀ and more repulsive G′₀ parameters. For finite ph momenta these features are maintained implying that the collectivity in the pion spin-isospin channel is screened also when the renormalization is considered to all orders.     

Variational calculations of asymmetric nuclear matter

We report on variational calculations of the energy E(ρ, β) of asymmetric nuclear matter having ? = ?_n + ?_p = 0.05 to 0.35 fm^?3, and β = (?_n ? ?_p/g9 = 0 to 1. The nuclear h used in this work consists of a realistic two-nucleon interaction, called v₁₄, that fits the available nucleon-nucleon scattering data up to 425 MeV, and a phenomenological three nucleon interaction adjusted to reproduce the empirical properties of symmetric nuclear matter. The variational many-body theory of symmetric nuclear matter is extended to treat matter with neutron excess. Numerical and analytic studies of the β-dependence of various contributions to the nuclear matter energy show that at ? < 0.35 fm^?3 the β⁴ terms are very small, and that the interaction energy EI(ρ, β) defined as E(ρ, β) ? T_F(ρ, β), where T_F is the Fermi-gas energy, is well approximated by EI₀(?) + β²EI₂(ρ). The calculated symmetry energy at equilibrium density is 30 MeV and it increases from 15 to 38 MeV as ? increases from 0.05 to 0.35 fm^?3.     

Measurement of the quadratic Zeeman shift of Rb hyperfine sublevels using stimulated Raman transitions

We demonstrate a technique for directly measuring the quadratic Zeeman shift using stimulated Raman transitions. The quadratic Zeeman shift has been measured yielding Δν=1296.8±3.3 Hz/G² for magnetically insensitive sublevels (5S_1/2,F=2,m_F=0→5S_1/2,F=3,m_F=0) of ⁸⁵Rb by compensating the magnetic field and cancelling the ac Stark shift. We also measured the cancellation ratio of the differential ac Stark shift due to the imbalanced Raman beams by using two pairs of Raman beams (σ⁺, σ⁺) and it is 1:3.67 when the one-photon detuning is 1.5 GHz in the experiment.     

A new limit on the arion interaction constant

The experimental results of the search for a long-range quasimagnetic electron-electron interaction associated with the exchange of a massless pseudoscalar Goldstone boson (arion) are presented. The limit obtained on the arion interaction constant is G_α < 10⁻³G_F (G_F is the Fermi constant).     

Luminescence self-quenching in Tm3+: YLF crystals: II. The luminescence decay and macrorates of energy transfer

Microparameters of migration and self-quenching of luminescence from the (³ P ₀, ¹ I ₆) levels of Tm ³⁺ ions are calculated and the luminescence decay from the ³ H ₄, ¹ G ₄, and ¹ _{D 2} levels are analyzed in the concentration series of LiY_1?x Tm_xF₄(x=0.5–10 at. %) crystals upon weak selective excitation. It is shown that the luminescence decay of a thulium ion concentration higher than 0.5 at. % cannot be described by the energy transfer that takes into account only the dipole-dipole interaction. The comparison of experimental kinetic curves with theoretical curves calculated within the framework of the known energy transfer models showed that multipole-multipole interaction is responsible for the self-quenching of the thulium levels. Contributions of higher multipole interactions to the self-quenching are estimated. The decay of luminescence from the ³ H ₄, ¹ G ₄, and ¹ D ₂ levels of Tm³⁺:YLF crystals with different concentrations of thulium ions is described analytically.     

Effect of composition on the spontaneous emission probabilities, stimulated emission cross-sections and local environment of Tm in TeO2-WO3 glass

Effect of composition on the structure, spontaneous and stimulated emission probabilities of various 1.0 mol% Tm₂O₃ doped (1−x)TeO₂+(x)WO₃ glasses were investigated using Raman spectroscopy, ultraviolet-visible-near-infrared (UV/VIS/NIR) absorption and luminescence measurements.Absorption measurements in the UV/VIS/NIR region were used to determine spontaneous emission probabilities for the 4f-4f transitions of Tm³⁺ ions. Six absorption bands corresponding to the absorption of the ¹G₄, ³F₂, ³F₃ and ³F₄, ³H₅ and ³H₄ levels from the ³H₆ ground level were observed. Integrated absorption cross-section of each band except that of ³H₅ level was found to vary with the glass composition. Luminescence spectra of the samples were measured upon 457.9 nm excitation. Three emission bands centered at 476 nm (¹G₄→³H₆ transition), 651 nm (¹G₄→³H₄ transition) and 800 nm (¹G₄→³H₅ transition) were observed. Spontaneous emission cross-sections together with the luminescence spectra measured upon 457.9 nm excitation were used to determine the stimulated emission cross-sections of these emissions.The effect of glass composition on the Judd-Ofelt parameters and therefore on the spontaneous and the stimulated emission cross-sections for the metastable levels of Tm³⁺ ions were discussed in detail. The effect of temperature on the stimulated emission cross-sections for the emissions observed upon 457.9 nm excitation was also discussed.     

Role of Yb and Tm ions in upconversion emission of Tb under 798 and 980 nm laser excitations in Tb-Tm-Yb doped tellurite glass

Upconversion emission and energy transfer processes in singly, doubly and triply doped tellurite glasses have been studied under 798 and 980 nm laser excitations. Emissions have been observed at 482, 544, 584, 655 nm and at 477, 655, 698, 800 nm corresponding to Tb³⁺: ⁵D₄ → ⁷F₆, ⁷F₅, ⁷F₄, ⁷F_J (J = 0, 1, 2, 3) and Tm³⁺: ¹G₄ → ³H₆, ¹G₄ → ³F₄, ³F₃ → ³H₆, ³H₄ → ³H₆ transitions, respectively. Among Tm³⁺, Yb³⁺and Tb³⁺ ions only Tm³⁺ has a ground state absorption at 798 nm excitation due to ³H₄ ← ³H₆ transition. For 980 nm excitation only Yb³⁺ can absorb the incident radiation. However, for both types of excitations, emission from all the three ions Tb, Yb and Tm has been observed. Possible mechanisms are proposed as follows: under 798 nm excitation Tm³⁺ ions are excited which excite Yb³⁺ ions through energy transfer. Finally “cooperative energy transfer” from a pair of Yb³⁺ ions to Tm³⁺ and Tb³⁺ ions takes place. Under 980 nm excitation Yb³⁺ ions absorb the incident energy and excite Tm³⁺ and Tb³⁺ ions via cooperative energy transfer. Variation of emission intensity with the ion concentrations of Yb³⁺, Tm³⁺ and Tb³⁺ has been studied. The lifetime of the ¹G₄ level has also been measured.     

Influence of Yb concentration on upconversion luminescence of Ho

Upconversion (UC) emissions at 360 ((⁵F, ³F, ⁵G)₂ → ⁵I₈), 392 (³K₇/⁵G₄ → ⁵I₈), 428 (⁵G₅ → ⁵I₈), 554 (⁵S₂/⁵F₄ → ⁵I₈), 667 (⁵F₅ → ⁵I₈) and 754 (⁵S₂/⁵F₄ → ⁵I₇) nm were obtained in 0.1 mol% Ho³⁺/x mol% Yb³⁺:Y₂O₃ (x = 2, 5, 8, 11, 15) bulk ceramics under infrared (IR) excitation at 976 nm. The intensity of the UC luminescence centered at 554 and 754 nm increased with Yb³⁺ concentration from 2 to 5 mol% and decreased from 5 to 15 mol%, while the UC luminescence centered at 392, 428 and 667 nm increased with Yb³⁺ concentration from 2 to 11 mol%, then started to reduce with Yb³⁺ concentration until 15 mol%. This comes from the competition between the energy back transfer (EBT) process [⁵S₂/⁵F₄(Ho) + ²F_7/2(Yb) → ⁵I₆(Ho) + ²F_5/2(Yb) as well as ⁵F₅(Ho) + ²F_7/2(Yb) → ⁵I₇(Ho) + ²F_5/2(Yb)] and spontaneous radiation process. The intensity of the UC luminescence centered at 360 nm always increases with Yb³⁺ concentration from 2 to 15 mol%. We believe that it may come from the cooperation of energy transfer process from Yb³⁺ ions in the ²F_5/2 state and the cross energy transfer process ⁵S₂/⁵F₄ + ⁵I₆ → (⁵F, ³F, ⁵G)₂ + ⁵I₈.     

Local-potential approximation for the Brueckner G matrix and problem of optimally choosing model subspace

The validity of the local-potential approximation, which was proposed previously for the singlet-pairing problem in semi-infinite nuclear matter, is investigated in the Bethe-Goldstone equation for the Brueckner G matrix. For this purpose, use is made of the method developed earlier for solving this equation for a planar slab of nuclear matter in the case of a separable form of NN interaction. The ¹ S ₀ singlet and the ³ S ₁+³ D ₁ triplet channel are considered. The complete two-particle Hilbert space is split into a model and the complementary subspace that are separated by an energy E ₀. The two-particle propagator is calculated precisely in the first subspace, and the local-potential approximation is used in the second subspace. With an eye to subsequently employing the G matrix to calculate the Landau-Migdal amplitude, the total two-particle energy is fixed at E=2μ, where μ is the chemical potential of the system under consideration. Specific numerical calculations are performed at μ=?8 MeV. The applicability of the local-potential approximation is investigated versus the cutoff energy E ₀. It is shown that, in either channel being considered, the accuracy of the local-potential approximation is rather high for E ₀≥10 MeV.     

Search for a parity violation induced by neutral currents in the 6S–7S transition of atomic cesium

Parity violating, time-reversal invariant, weak neutral currents can induce an electric dipole transition moment, E₁^p.v., between atomic states of same parity. We report here on the preliminary results of an experiment designed to measure E₁^p.v. in the 6S–7S transition of atomic cesium, using a polarization effect characteristic of the interference of E₁^p.v. with the electric dipole transition moment E₁^ind. induced by a d.c. electric field. At a 90% confidence level we find the upper limit: |E₁^p.v.| < 2.0 × 10^?9|e|a₀. As a consequence the coupling constant of the electron-nucleon interaction involving the product of an axial electronic neutral current by a vector nucleonic one must be less than 44 G_F.     

Masses of lowest lying heavy mesons in QCD

Using QCD sum rules for the polarization operator ∏^J(Q²) induced by all possible currents with J^PC=0^??, 1^??, 0⁺⁺, 1⁺⁺ and 2⁺⁺ we calculate the S- and P-states of charmonium. Power corrections due to the gluon condensate operator G^α_μνG^α_μν are included and shown to be very important. All known lowest lying charmonium states are reproduced extremely well and a firm prediction is made for the ¹P₁ state.     

Magnetic polaritons in four-sublattice magnetic systems of the GzFx type

Magnetic polaritons in four-sublattice magnetic compounds of the space symmetry D¹⁶_2h with magnetic ordering of the G_zF_x and G_z types, like KMnF₃, RbFeF₃ etc., are considered. Dispersion curves of mixed states of photons and all four magnetic modes are given for some peculiar directions of propagation. The interaction between photons and high-frequency (optic) magnetic modes is shown to be significantly weaker than that between photons and low-frequency spin waves.     

Irreducible kernels and nonperturbative expansions in a theory with purem →m interaction

Recent results on the structure of theS matrix at them-particle threshold (m≧2) in a simplifiedm→m scattering theory with no subchannel interaction are extended to the Green functionF on the basis of off-shell unitarity, through an adequate mathematical extension of some results of Fredholm theory: local two-sheeted or infinite-sheeted structure ofF arounds=(mμ)² depending on the parity of (m?1)(ν?1) (where μ>0 is the mass and ν is the dimension of space-time), off-shell definition of the irreducible kernelU which is the analogue of theK matrix in the two different parity cases (m?1)(ν?1) odd or even, and related local expansion ofF, for (m?1)(ν?1) even, in powers of σ_β ln σ(σ=(mμ)²?s). It is shown that each term in this expansion is the dominant contribution to a Feynman-type integral in which each vertex is a kernelU. The links between the kernelU and Bethe-Salpeter type kernelsG of the theory are exhibited in both parity cases, as also the links between the above expansion ofF and local expansions, in the Bethe-Salpeter type framework, ofF _λ in terms of Feynman-type integrals in which each vertex is a kernelG and which include both dominant and subdominant contributions.     

Discriminative deep inelastic tests of strong interaction field theories

It is demonstrated that recent measurements of ∫¹₀F₂(x, Q²)dx eliminate already all strong interaction field theories except QCD. A detailed study of scaling violations of F₂(x, Q²) in QCD shows their insensitivity to the gluon content of the hadron at presently measured values of Q².     

Investigation of zero-frequency solutions to the pion dispersion equation

The instability of nuclear matter is considered for the case where it is generated by the vanishing of the frequencies of collective excitations belonging to specific types (specifically, excitations that have the pion quantum numbers J ^π = 0^?). The behavior of zero-frequency solutions to the pion dispersion equation is analyzed versus the strength G′ of spin—isospin particle—hole interaction. It is shown that there exists a strength value G′_tr (|G′_tr| ? 1) such that, for G′ < G′_tr, zero-frequency solutions are excitations of the ω _P type, while, for G′ ≥ G′_tr, such solutions are excitations of the ω _c type. Excitations of the ω _P type for G′ < ?1 describe the instability of nuclear matter against small density fluctuations (Pomeranchuk’s instability), while excitations of the ω _c type are responsible for the instability associated with pion condensation at G′ ≈ 2. For stable nuclear matter, the solutions ω _P(κ) and ω _c (κ) lie on unphysical sheets of the complex plane of frequency.     

Axions from chiral family symmetry

We investigate the possibility that family symmetry, G_F, is spontaneously broken chiral global symmetry. We classify the interesting cases when family symmetry can result in an automatic Peccei-Quinn symmetry U(1)_PQ and thus provide a solution to the strong CP problem. The result disfavors having two or four families. For more than four families, U(1)_PQ is in general automatic. In the case of three families, a unique Higgs sector allows U(1)_PQ in the simplest case of G_F = [SU(3)]³. Cosmological consideration also puts strong constraint on the number of families. For G_F = [SU(N)]³ cosmology singles out the three-family (N = 3) case as a unique solution if there are three light neutrinos. Possible implication of decoupling theorem as applied to family symmetry breaking is also discussed.     

Synthesis, characterization and spectroscopic investigation of new tetrakis(acetylacetonato)thulate(III) complexes containing alkaline metals as countercations

In this work a series of tetrakis complexes C[Tm(acac)₄], where C⁺=Li⁺, Na⁺ and K⁺ countercations and acac=acetylacetonate ligand, were synthesized and characterized for photoluminescence investigation. The relevant aspect is that these complexes are water-free in the first coordination sphere. The emission spectra of the tetrakis Tm³⁺-complexes present narrow bands characteristic of the ¹G₄→³H₆ (479 nm), ¹G₄→³F₄ (650 nm) and ¹G₄→³H₅ (779 nm) transitions of the Tm³⁺ ion, with the blue emission color at 479 nm as the most prominent one. The lifetime values (τ) of the emitting ¹G₄ level of the C[Tm(acac)₄] complexes were 344, 360 and 400 ns for the Li⁺, Na⁺ and K⁺ countercations, respectively, showing an increasing linear behavior versus the ionic radius of the alkaline ion. An efficient intramolecular energy transfer process from the triplet state (T) of the ligands to the emitting ¹G₄ state of the Tm³⁺ ion is observed. This fact, together with the absence of water molecules in first coordination sphere, allows these tetrakis Tm³⁺-complexes to act as efficient blue light conversion molecular devices.     

Spin forbidden transitions in the Ka = 0 subband of NO2 in the λ = 592.5 nm region

In a high resolution laser excitation spectrum of NO₂, lines were recorded which do not follow the selection rule ΔN = ΔJ = ΔF of “spin allowed” transitions. Line positions and intensities of these “spin forbidden” lines were investigated for all rotational lines up to N″ = 12 of the K_a = 0 subband around λ = 592.5 nm. While the observed line intensities of “spin allowed” transitions can be well described by the J-coupling scheme, neither the J- nor the G-coupling scheme sufficiently describes the “spin forbidden” transitions. The observations can be fitted satisfactorily by perturbation theory, in which the Fermi interaction in ²A₁ is treated as the perturber. This looks similar to a superposition of J and G scheme in the ²A₁ ground state.     

Limits on a lepton-number violating process from e+e− collisions

If a new hypothetical particle induces a lepton-number violating process, it could also affect the reactions e⁺e⁻→μ⁺μ⁻ and e⁺e⁻→τ⁺τ⁻ through the t-channel exchange of the particle. A fit of the cross sections and forward-backward asymmetries measured by experiments at PEP and PETRA yields the 90% confidence level limits on the coupling of the partile: G_eμ < 0.119G_F and G_eτ < 0.085G_F, where G_F is the Fermi coupling constant. These limits exclude certain regions of couplings allowed by other lepton-number violating searches.     

The 2ν1, 2ν2 and 2ν3 overtones of FClO3

The infrared spectra of the 2ν₁, 2ν₂ and 2ν₃ overtones of perchloryl fluoride, FClO₃, have been recorded at high resolution using monoisotopic pure samples. Four symmetric top species have been investigated: F³⁵Cl¹⁶O₃, F³⁷Cl¹⁶O₃, F³⁵Cl¹⁸O₃ and F³⁷Cl¹⁸O₃. The v_i = 2, i = 1, 2, 3 vibrationally excited states are totally symmetric, so these overtones correspond to parallel bands of medium/weak intensity, centered from 2010 to 2120 cm⁻¹ (2ν₁), from 1390 to 1430 cm⁻¹ (2ν₂) and from 1070 to 1100 cm⁻¹ (2ν₃). Most of the bands are unperturbed and their analysis was straightforward. The band origins, the rotational and centrifugal molecular constants in the v₁ = 2, v₂ = 2 and v₃ = 2 states have been determined, with standard deviations of the fits from 0.00024 to 0.00067 cm⁻¹. The 2ν₁ overtones of F³⁵Cl¹⁶O₃ and F³⁷Cl¹⁶O₃ are perturbed by an A₁/E Coriolis resonance between the v₁ = 2 state and one E component of the v₄ = 1, v₆ = 2 manifold. The 2ν₂ of F³⁷Cl¹⁸O₃ is perturbed by the same kind of interaction involving the v₁ = v₆ = 1 (E) state, at about 1396 cm⁻¹. In these bands the resonance is localized on rotational levels with specific J and K values. As a consequence, a few transitions of the perpendicular bands involving the interacting levels could be identified in the spectra. A simultaneous fit of the transitions assigned to the dyads has been performed and the parameters of the excited states have been determined, including the high order Coriolis interaction coefficient . The anharmonic constants x₁₁, x₂₂, x₃₃ of all the studied isotopologues of FClO₃, x₄₆ of F³⁵Cl¹⁶O₃, x₄₆ + g₄₆ of F³⁷Cl¹⁶O₃ and x₁₆ of F³⁷Cl¹⁸O₃, have been derived.