Contribution of the σ meson exchange to pionic double charge exchange reaction

The contribution of the σ meson exchange to the pionic double charge exchange (DCX) reaction is investigated. A concrete calculation on the forward excitation function of the low energy DCX reaction ¹⁴C(π⁺, π⁻)¹⁴O has been performed. It shows that the contribution of the σ meson exchange can reproduce well the resonance like excitation function of low energy DCX reaction. Received: 5 August 1998     

Holographic and photoelectric characterization of a novel photorefractive organic glass

We present a detailed study of the photoelectric as well as the holographic properties of a novel organic photorefractive glass based on triphenylamine. We studied the quantum efficiency Φ of the photogeneration of charges by means of photoinduced discharge measurements. The photoconductivity σ and the charge carrier mobility μ were obtained via dc photoconduction and pulsed time-of-flight experiments, respectively. The holographic characterization was performed by two-wave and degenerate four-wave mixing experiments allowing for the determination of properties such as diffraction efficiency η, modulation of the refractive index Δn, gain coefficient Γ, and phase-shift φ_p of the investigated system. The experimental data for Φ could be successfully described by the Onsager formalism with a thermalization radius of r₀=24 ? and a primary quantum yield of Φ₀=40%. We evaluated the E field and temperature-dependent measurements of μ using the B?ssler formalism yielding a width of the density of states of σ=0.13 eV and a disorder parameter Σ=3.6. On this basis the lifetime and the average drift length of the charge carriers could be estimated from the dc photoconduction experiments. From the photoelectric measurements we also calculated the holographic response time that matched very well to the measured response time and described the E-field dependence satisfactorily. The presented photorefractive system shows outstanding optical properties and stability with respect to degradation. We measured a gain coefficient of Γ=90 cm^-1, and a diffraction efficiency of η=27% at a response time of 30 ms for only 40-μm-thick samples. Orientational enhancement was observed and evaluated quantitatively. To our knowledge, this work presents the first determination of each of the above quantities all in one single organic photorefractive material. Received: 16 November 1998 / Revised version: 4 January 1999 / Published online: 31 March 1999     

Electromagnetic polarizabilities of the nucleon and properties of the σ-meson pole contribution

The t-channel contribution to the difference of electromagnetic polarizabilities of the nucleon, (α - β)^t, can be quantitatively understood in terms of a σ-meson pole in the complex t-plane of the invariant scattering amplitude A ₁(s, t) with properties of the σ-meson as given by the quark-level Nambu-Jona-Lasinio model (NJL). Equivalently, this quantity may be understood in terms of a cut in the complex t-plane where the properties of the σ-meson are taken from the ππ → σ → ππ, γγ → σ → ππ and Nˉ → σ → ππ reactions. This equivalence may be understood as a sum rule where the properties of the σ-meson as predicted by the NJL model are related to the f ₀(600) particle observed in the three reactions. In the following, we describe details of the derivation of (α - β)^t making use of predictions of the quark-level NJL model for the σ-meson mass. An erratum to this article is available at .     

Molecular-orbital structure in neutron-rich Be and C isotopes

The structure of Be and C isotopes are investigated based on the molecular-orbit (MO) model. The low-lying states are characterized by several configurations of valence neutrons, which are constructed as combinations of basic orbits. In ¹⁰Be, all of the observed positive-parity bands and the negative-parity bands are described within the model. The second 0⁺ state of ¹⁰Be has a large α-α cluster structure, and this is characterized by a (1/2⁺ _σ)² configuration. An enlargement of the α-α distance due to two-valence neutrons along the α-α axis makes their wave function smooth and reduces the kinetic energy drastically. Furthermore, the contribution of the spin-orbit interaction due to coupling between the S _z = 0 and the S _z = 1 configurations, is important. In the ground state of ¹²Be, the calculated energy exhibits similar characteristics, that the remarkable α clustering and the contribution of the spin-orbit interaction make the binding of the state with (3/2^- _π)²(1/2⁺ _σ)² configuration properly stronger in comparison with the closed p-shell (3/2^- _π)²(1/2^- _π)² configuration. This is related to the breaking of the N = 8 (closed p-shell) neutron magic number. Also, the molecule-like structure of the C isotopes is investigated using a microscopic α+α+α+n+n+ ^{. . .} model. The combination of the valence neutrons in the π- and the σ-orbit is promising to stabilize the linear-chain state against the breathing and bending modes, and it is found that the excited states of ¹⁶C are the most promising candidates for such structure. Received: 1 May 2001 / Accepted: 4 December 2001     

A UV–Visible–NIR fluorescence lifetime imaging microscope for laser-based biological sensing with picosecond resolution

This article describes the design and characterization of a wide-field, time-domain fluorescence lifetime imaging microscopy (FLIM) system developed for picosecond time-resolved biological imaging. The system consists of a nitrogen-pumped dye laser for UV–visible–NIR excitation (337.1–960 nm), an epi-illuminated microscope with UV compatible optics, and a time-gated intensified CCD camera with an adjustable gate width (200 ps-10^-3 s) for temporally resolved, single-photon detection of fluorescence decays with 9.6-bit intensity resolution and 1.4-μm spatial resolution. Intensity measurements used for fluorescence decay calculations are reproducible to within 2%, achieved by synchronizing the ICCD gate delay to the excitation laser pulse via a constant fraction optical discriminator and picosecond delay card. A self-consistent FLIM system response model is presented, allowing for fluorescence lifetimes (0.6 ns) significantly smaller than the FLIM system response (1.14 ns) to be determined to 3% of independently determined values. The FLIM system was able to discriminate fluorescence lifetime differences of at least 50 ps. The spectral tunability and large temporal dynamic range of the system are demonstrated by imaging in living human cells: UV-excited endogenous fluorescence from metabolic cofactors (lifetime ∼1.4 ns); and 460-nm excited fluorescence from an exogenous oxygen-quenched ruthenium dye (lifetime ∼400 ns). Received: 23 February 2003 / Published online: 22 May 2003 RID="*" ID="*"Corresponding author. Fax: +1-734/9361-905, E-mail: mycek@umich.edu     

The σK coupling in the chiral unitary approach and the isoscalar ¯N , ¯A interaction

We evaluate the “σ " exchange contribution to the ˉN → ˉN scattering within a chiral unitary approach. We show that the chiral transition potentials for ππ → Kˉ in the t -channel lead to a “σ " contribution that vanishes in the ˉ forward direction and, hence, would produce a null “σ " exchange contribution to the K^- optical potential in nuclear matter in a simple impulse approximation. This is a consequence of the fact that the leading-order chiral Lagrangian gives an I = 0 ππ → Kˉ amplitude proportional to the squared momentum transfer, q². This finding poses questions on the meaning or the origin of “σ " exchange potentials used in relativistic mean-field approaches to the K^- nuclear self-energy. This elementary “σ ” exchange potential in ˉN → ˉN is compared to the Weinberg-Tomozawa term and is found to be smaller than the present theoretical uncertainties but will be relevant in the future when aiming at fitting increasingly more accurate data.     

A note on inequalities for the masses of the lightest ππ resonances in multicoloured QCD

We derive and analyse inequalities relating masses of the lightest ππ resonances (ρ and σ) to coupling constants of the effective chiral Lagrangian in the limit of a large number of colours. Received: 15 July 2001 / Accepted: 23 October 2001     

Search for a bound trineutron with the 3He(π−pgr;+)nnn reaction

A search for the production of a bound trineutron state has been performed using the reaction ³He(π⁻,π⁺)nnn at incident pion energies of 65, 75, and 120 MeV. No evidence for the existence of the ³n was found, and an upper limit for the production cross section of approximately 30 nb/sr (2σ confidence level) was obtained. Received: 22 October 1998     

Amplification of near-infrared light in a photorefractive ring resonator

We have demonstrated efficient amplification of near-infrared, 0.83-μm and 1.06-μm light, in a photorefractive ring resonator using Rh:BaTiO₃. The optical power oscillating inside the ring exceeded the pump power by a factor of up to 2.34. The sensitivity of a ring resonator to nanometer changes in its length was characterised using a piezo-mirror. Received: 30 November 2000 / Revised version: 24 January 2001 / Published online: 20 April 2001     

Isoscalar-isovector interferences in πN↦Ne+e- reactions as a probe of baryon resonance dynamics

The isoscalar-isovector (ρ-ω) interferences in the exclusive reactions π^- p↦ne ⁺ e ^- and π⁺ n↦pe ⁺ e ^- near the ω threshold leads to a distinct difference of the dielectron invariant-mass distributions depending on beam energy. The strength of this effect is determined by the coupling of resonances to the nucleon vector-meson channels and other resonance properties. Therefore, a combined analysis of these reactions can be used as a tool for determining the baryon resonance dynamics. Received: 14 August 2001 / Accepted: 3 October 2001     

3D DuMond diagrams of multi-crystal Bragg-case synchrotron topography. I. Flat sample

The 3D representation of the DuMond diagram is used to explain the dimensional features of X-ray topographs obtained by multi-crystal configuration with a synchrotron beam. Symmetric Bragg-case reflections are considered for a flat double-crystal monochromator and a flat sample. Two ways of sample alignment are taken into account. They are referred to as σ–σ and σ–π geometries, where the diffraction plane of the sample is parallel and perpendicular, respectively, to the vertical diffraction plane of the monochromator (σ polarization). It is shown that the shape of the sample image is closely connected to the shape the diffraction domain common to monochromator and sample assumes in the 3D DuMond diagram. An experiment is reported for the less commonly used σ–π topography, showing how the lattice mismatch and its lateral homogeneity are determined in samples made by epilayer and substrate. Received: 31 July 2000 / Accepted: 25 January 2001 / Published online: 3 May 2001     

ωN final state interactions and ω-meson production from heavy-ion collisions

We calculate the elastic and inelastic ωN→ωN, →πN, →ρN, →ρπN, →ππN, →σN reactions within a boson exchange approximation where the ωρπ coupling constant and form factor are fixed by the reaction πN→ωN in comparison to the experimental data. We find rather large ωN cross sections at low relative momenta of the ω-meson which leads to a substantial broadening of the ω-meson width in nuclear matter. The implications of the ωN final state interactions are studied for ω production in ¹²C +¹²C, ⁴⁰Ca +⁴⁰Ca and ⁵⁸Ni +⁵⁸Ni reactions at about 2 · A GeV within the HSD transport approach; the drastic changes of the transverse mass spectra relative to a general m _T-scaling (for π⁰ and η mesons) might be controlled experimentally by the TAPS Collaboration. Received: 28 April 1999 / Revised version: 7 June 1999     

Determination of nonlinear σ-ω-ρ model parameters in the relativistic mean-field theory by nuclear-matter properties

The parameters of the σ-ω-ρ model in the relativistic mean-field theory with nonlinear σ-meson self-interaction are determined by nuclear-matter properties, which are taken as those extracted by fits to data based on nonrelativistic nuclear models. The values of the relevant parameters are C _σ ²∼ 94, C _ω ²∼ 32, C _ρ ²∼ 26, b∼ - 0.09, c∼ 1, and the σ-meson mass m _σ∼ 370 MeV, while the value of the calculated nuclear- surface thickness is t∼ 1.4 fm. The field system is shown to be stable, since the σ-meson self-interaction energy is a lower bound in this whole parameter region with positive c. On the other hand, the effective nucleon mass M^* is larger than 0.73M, if the symmetry incompressibility K_s is assumed to be negative and the nuclear-matter incompressibility K₀ is kept less than 300 MeV. Received: 27 June 2001 / Accepted: 5 October 2001     

Optical surface absorption by surface-adsorbate charge-transfer excitations

Metal-insulator-silver junctions are well suited to measure genuine surface optical excitations, especially electronic transitions to the lowest unoccupied molecular orbitals (LUMO) of adsorbates on silver. This is demonstrated by the increase of internal photoemission current after monolayer coverage with C₂H₄ (which has π^*-LUMO) by a factor of about 2.5, but a missing change in the case of C₂H₆ (which has no π^*-LUMO ). Received: 26 November 1999 / Published online: 8 March 2000     

Excitation of the Roper resonance in single- and double-pion production in nucleon-nucleon collisions⋆

In most investigations the Roper resonance is sensed only very indirectly via complex partial-wave analyses. We find indications for its excitation in the invariant nπ⁺ mass spectrum of the pp → npπ⁺ reaction at M ≈ 1360 MeV with a width of ≈ 150 MeV . The values fit very favorably to the most recent phase shift results as well as to the observations at BES. In the near-threshold two-pion production pp → ppπ⁰π⁰ , where the Roper excitation and its subsequent decays via the routes N ^* → Δπ → Nππ and N ^* → Nσ are the only dominant processes, we find its direct decay into the Nσ channel to be the by far dominating decay process --in favor of a monopole excitation of the Roper resonance.     

Magnetic moment and radius of the nucleon in a nonlocal model of meson-nucleon interaction

Magnetic moment and radius of the nucleon are calculated in a nonlocal extension of the chiral linear σ-model. Properties of the nonlocal model under the vector and axial transformations are considered. The conserved electromagnetic and vector currents, and partially conserved axial vector current are obtained. In the calculation of the nucleon electromagnetic vertex the π- and σ-loop diagrams are included. Contribution from vector mesons is added in the vector meson dominance model with a gauge-invariant photon-meson coupling. The nonlocality parameter associated with the πN interaction is fixed from the experimental magnetic moment of the neutron. Other parameters (nonlocality parameter for the σN interaction and the mass of the σ-meson) are constrained by the magnetic moment of the proton. The calculated electric and magnetic mean-square radii of the proton and neutron are in satisfactory agreement with experiment. Received: 12 February 2001 / Accepted: 4 September 2001     

3D DuMond diagrams of multi-crystal Bragg-case synchrotron topography. II. Curved sample

The dimensional features of multi-crystal synchrotron X-ray topographs are explained by 3D DuMond diagrams for a flat double-crystal monochromator and a curved sample. Symmetric Bragg-case reflections are assumed for all crystals. σ–σ and σ–π geometries are considered, where the diffraction plane of the sample is parallel and perpendicular, respectively, to the vertical diffraction plane of the monochromator (σ polarization). It is shown that the shape of the sample image is closely connected to the shape of the volume shared by the diffraction domains of monochromator and sample in the 3D DuMond diagram. In particular, for the σ–π set-up, the image shape depends on the curvature value and sign. An experiment is reported for this latter crystal geometry to determine lattice mismatch, its lateral homogeneity and curvature value and sign in a sample made of epilayer and substrate. Received: 31 July 2000 / Accepted: 25 January 2001 / Published online: 26 April 2001     

ππ S-wave phase shifts and non-perturbative chiral approach

We extend a recent non-perturbative chiral approach to ππ S-wave scattering of Oller and Oset by including the couplings to the ηη-channel. We find that the isospin-zero and isospin-two ππ S-wave phase shifts of the model deviate considerably from a recent solution of the Roy-equations. Including the couplings with the ηη-channel does not improve the situation. In particular, no σ-meson like enhancement structure shows up in the Roy-equation solution. We also consider the ππ-scattering lengths in this approach. Received: 17 August 1998 / Revised version: 21 September 1998     

Holographic recording by excitation of metastable electronic states in Na2[Fe(CN)5NO]·2H2O: a new photorefractive effect

Elementary holographic phase gratings can be written in single crystals of Na₂[Fe(CN)₅NO]·2H₂O, sodiumnitroprusside, by excitation of metastable electronic states in the blue–green spectral range. For light polarized parallel to the crystallographic a and b axes of the orthorhombic crystal the light-induced modulation of the refractive index reaches Δn≈2×10^-3 at λ=514.5 nm. Although the largest population of the metastable states is reached for light polarized parallel to the crystallographic c axis, a photorefractive response is not observed. In contrast to electro-optic photorefractive materials the photorefractive effect depends mono-exponentially on the exposure and on the modulation of the incident light interference pattern. Beam-coupling experiments demonstrate that written gratings are in phase with the interference pattern in correspondence with the fact that the excitation of the metastable electronic states is local. The width of the rocking curve shows that the holographic gratings are written completely over the volume of the crystal. Variations of the wavelengths within the excitation range as well as of the crystal thickness do not influence the maximum photorefractive response. Investigations on the grating vector of the written gratings show unambiguously that charge migration is not responsible for the photorefractive effect. Received: 18 November 1998 / Revised version: 26 January 1999 / Published online: 12 April 1999     

Dilepton production from ρ-mesons in a quark-gluon plasma

Assuming that ρ-mesons exist in a quark-gluon plasma at temperatures close to the QCD phase transition, we calculate the dilepton production rate from q-ˉq annihilation via a ρ-meson state using Vector Meson Dominance. The result is compared to the rates from direct q-ˉq annihilation and from π⁺-π⁻ annihilation. Furthermore we discuss the suppression of low mass dileptons if the quarks assume an effective mass in the quark-gluon plasma. Received: 8 September 1999 / Revised version: 22 October 1999