RELATING THE QCD POMERON TO AN EFFECTIVE GLUON MASS

We construct the Pomeron as an exchange of two nonperturbative gluons, where the nonperturbative gluon propagator is described by an approximate solution of the Schwinger-Dyson equation which contains a dynamically generated gluon mass. We compute the total and elastic differential (dsigma/dt) cross sections for pp scattering, obtaining agreement with the experimental data for a gluon mass m = 370 MeV for LAMBDA(QCD) = 300 MeV. In particular, the Pomeron effectively behaves like a photon-exchange diagram with a coupling determined by the glucon mass.

Thermally enhanced cooperative energy-transfer frequency upconversion in terbium and ytterbium doped tellurite glass

Infrared-to-visible frequency upconversion through cooperative energy-transfer and thermal effects in Tb3+/Yb3+-codoped tellurite glasses excited at 1.064 mum is investigated. Bright luminescence emission around 485, 550, 590, 625 and 65 nm, identified as due to the D-5(4) --> F-7(J) (J= 6, 5, 4, 3, and 2) transitions of the terbium ions, respectively, was recorded. The excitation of the D-5(4) emitting level of the Tb3+ ions is assigned to cooperative energy-transfer from pairs of ytterbium ions.. The effect of temperature on the upconversion process was examined and the results revealed a fourfold upconversion enhancement in the 300-500 K interval. The enhancement of the upconversion process is due to the temperature dependence of the Yb3+-sensitizer absorption cross-section under anti-Stokes excitation. A rate-equation. model using multiphonon-assisted absorption for the ytterbium excitation combined with the energy migration effect between Yb-Yb pair, and Tb3+ ground-state depopulation via multiphonon excitation of the F-7(J) excited states describes quite well the experimental results. (C) 2003 Elsevier B.V. B.V. All rights reserved.

Positronium scattering from ions, atoms and molecules

Elastic and inelastic scattering of positronium (Ps) by H, He, He(+) and H(2) have been studied using coupled-channel calculations with a regularised nonlocal model exchange potential. Suitability and reliability of the theoretical scheme have been demonstrated by studying scattering cross-sections in various systems and also by addressing critical dynamic features like binding and resonances of Ps to open-shell-atoms. Results are found to be in good agreement with Variational predictions and experimental observations. (C) 2000 Elsevier B.V. Ltd. All rights reserved.

Spectroscopic properties of lead fluoroborate glasses doped with ytterbium

A new lead fluoroborate glass (PbO-PbF2-B2O3) doped with ytterbium (Yb:PbFB) is presented. Samples with different concentrations of Yb3+ were produced and had their emission cross-sections, fluorescence lifetimes and minimum pump intensities determined. They have high refractive index of 2.2 and a density of 4.4 g/cm(3). For a doping level of 1.153x10(20) ions/cm(3), the fluorescence lifetime, after excitation at 968 nm, is 0.81 ms, which is comparable to Yb:tellurite laser glass. Also, an emission band at 1022 nm is measured with emission cross-section of approximately 1.07x10(-20) cm(2) and fluorescence effective linewidth of 60 nm, which is comparable to Yb:phosphate laser glass. (C) 2001 Optical Society of America.

Influence of ambient gas on the growth and properties of porous tin-doped indium oxide thin films made by pulsed laser deposition

The laser ablation method was used for depositing porous nanocrystalline indium-tin oxide thin films for gas sensing applications. Samples were prepared at different pressures using three gases (O-2, 0.8N(2):0.2O(2), N-2) and heat-treated in the same atmosphere used for the ablation process. X-ray diffraction results show that the films are not oriented and the grain sizes are in the range between 15 and 40 nm. The grains are round shaped for all samples and the porosity of the films increases with the deposition pressure. The degree of sintering after heat treatment increases for lower oxygen concentrations, generating fractures on the surface of the samples. Film thicknesses are in the range of I pm for all gases as determined from scanning electron microscopy cross-sections. Electrical resistance varies between 36.3 ohm for the film made at 10 Pa pressure in N-2 until 9.35 x 10(7) ohm for the film made at 100 Pa in O-2. (C) 2007 Elsevier B.V. All rights reserved.

THE EFFECT OF POSITRONIUM FORMATION IN POSITRON ALKALI-ATOM SCATTERING

We study e+-Na, e+-K, and e+-Rb scattering using the close coupling approach in the static and coupled static expansion schemes. We calculate partial wave elastic scattering phase shifts and total elastic and Ps formation cross sections up to an incident positron energy of 100 eV. The effect of the positronium formation channel on the elastic channel is found to be strong in all cases up to an incident positron energy of 10 eV. We also make an estimate of the total cross section which exhibits a minimum as a function of energy at low energies.

Ps-H scattering using the three-state positronium close-coupling approximation

Positronium scattering off a hydrogen target has been studied employing a three-state positronium model close-coupling approximation (CCA) with and without electron exchange. Elastic, excitation and quenching cross sections are reported at low and medium energies. The effect of electron exchange is found to be significant at low energies. The ratio of quenching to the total cross section (the conversion ratio) approaches the value of 0.25 with increase of energy, as expected.

Raman scattering, differential scanning calorimetry and Nd3+ spectroscopy in alkali niobium tellurite glasses

Alkali niobium tellurite glasses have been prepared and some of their properties measured by differential scanning calorimetry and Raman scattering. The vitreous domain was established in the pseudo ternary phases diagram for the system TeO2-Nb2O5-(0.5K(2)O-0.5Li(2)O). Raman scattering shows that for samples in the TeO2 rich part of the phase diagram the vitreous structure is composed essentially of (TeO4) units connected by the vertices, as in the alpha-TeO2 crystal. The addition of alkali and niobium oxides causes depolymerization to occur with structures composed essentially of (TeO3) and (NbO6) units. Samples with the composition (mol%) 80TeO(2)-10Nb(2)O(5)-5K(2)O-5Li(2)O, stable against crystallization, were prepared containing up to 10% mol Nd3+. The addition of this oxide increases the rigidity of the vitreous network shifting characteristic temperatures to higher temperatures. For the 10% Nd3+ sample amorphous phase separation is assumed to exist from the observation of two glass transition temperatures. Spectroscopic properties such as Judd-Ofelt Omega(lambda) intensity parameters, radiative emission probabilities, and induced emission cross sections were calculated. From these results and also from the emission quenching observed as a function of Nd3+ concentration, we suggest that these glasses could be utilized in optical amplifying devices. (C) 1999 Elsevier B.V. B.V. All rights reserved.

Elastic scattering and the proton form factor

We use the QCD pomeron model proposed by Landshoff and Nachtmann to compute the differential and the total cross-sections for pp scattering in order to discuss a QCD-based approach to the proton form factor. This model is quite dependent on the experimental electromagnetic form factor, and it is not totally clear why this form factor gives good results even at moderate transferred momentum. We exchange the electromagnetic form factor by the asymptotic QCD proton form factor determined by Brodsky and Lepage (BL) plus a prescription for its low energy behavior dictated by the existence of a dynamically generated gluon mass. We fit the data with this QCD inspired form factor and a value for the dynamical gluon mass consistent with the ones determined in the literature. Our results also provide a determination of the proton wave function at the origin, which appears in the BL form factor.

Spectroscopic properties of Yb3+ doped PbO-Bi2O3-Ga2O3 glasses for IR laser applications

PbO-Bi2O3-Ga2O3 glasses doped with different concentrations of Yb3+ are presented. The spectroscopic properties and laser parameters are calculated and a comparison between different results obtained when calculating the Yb3+ emission cross-section with the reciprocity method and with the Fuchtbauer-Ladenburg formula is presented. The behavior of the near-infrared luminescence is described theoretically by a rate equation and compared with the experimental results. This host doped with Yb3+ is a promising material for laser action at 1019 nm, with properties similar to other known glasses used as active laser media; the emission cross-section of 1.1 x 10(-20) cm(2), the high absorption cross-section (of 2.0 x 10(-20) cm(2)) and a minimum pump intensity of 2.4 kW/cm(2) are interesting properties for short pulse generation. (C) 2005 Elsevier B.V. All rights reserved.

Pauli blocking and medium effects in nucleon knockout reactions

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Quenching of para-H-2 with an ultracold antihydrogen atom (H)over-bar(1s)

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

NDACC/SAOZ UV-visible total ozone measurements: improved retrieval and comparison with correlative ground-based and satellite observations

Accurate long-term monitoring of total ozone is one of the most important requirements for identifying possible natural or anthropogenic changes in the composition of the stratosphere. For this purpose, the NDACC (Network for the Detection of Atmospheric Composition Change) UV-visible Working Group has made recommendations for improving and homogenizing the retrieval of total ozone columns from twilight zenith-sky visible spectrometers. These instruments, deployed all over the world in about 35 stations, allow measuring total ozone twice daily with limited sensitivity to stratospheric temperature and cloud cover. The NDACC recommendations address both the DOAS spectral parameters and the calculation of air mass factors (AMF) needed for the conversion of O-3 slant column densities into vertical column amounts. The most important improvement is the use of O-3 AMF look-up tables calculated using the TOMS V8 (TV8) O-3 profile climatology, that allows accounting for the dependence of the O-3 AMF on the seasonal and latitudinal variations of the O-3 vertical distribution. To investigate their impact on the retrieved ozone columns, the recommendations have been applied to measurements from the NDACC/SAOZ (Systeme d'Analyse par Observation Zenithale) network. The revised SAOZ ozone data from eight stations deployed at all latitudes have been compared to TOMS, GOMEGDP4, SCIAMACHY-TOSOMI, SCIAMACHY-OL3, OMI-TOMS, and OMI-DOAS satellite overpass observations, as well as to those of collocated Dobson and Brewer instruments at Observatoire de Haute Provence (44 degrees N, 5.5 degrees E) and Sodankyla (67 degrees N, 27 degrees E), respectively. A significantly better agreement is obtained between SAOZ and correlative reference ground-based measurements after applying the new O-3 AMFs. However, systematic seasonal differences between SAOZ and satellite instruments remain. These are shown to mainly originate from (i) a possible problem in the satellite retrieval algorithms in dealing with the temperature dependence of the ozone cross-sections in the UV and the solar zenith angle (SZA) dependence, (ii) zonal modulations and seasonal variations of tropospheric ozone columns not accounted for in the TV8 profile climatology, and (iii) uncertainty on the stratospheric ozone profiles at high latitude in the winter in the TV8 climatology. For those measurements mostly sensitive to stratospheric temperature like TOMS, OMI-TOMS, Dobson and Brewer, or to SZA like SCIAMACHY-TOSOMI, the application of temperature and SZA corrections results in the almost complete removal of the seasonal difference with SAOZ, improving significantly the consistency between all ground-based and satellite total ozone observations.

Probing SU(2) symmetry breaking in the nucleon sea

Investigation of invariant cross-sections for production of K*- and K*0, in the fragmentation region of the proton, in p - p and γ - p reactions, gives a direct and unambiguous probe to the symmetry breaking of the nucleon sea. Based on existing data, we clearly found a large asymmetry of the sea. Our result is in excellent agreement with NA51 measurement, signaling lack of any nuclear effect. The measurement can be carried out in a single experimental set up. The ratio K*-/K*0 is equivalent to ū/d̄, with easy access to the x-dependence of the asymmetry. The observed asymmetry from available experimental data is used to improve the valon-recombination model. © 1997 Elsevier Science B.V.

Gerasimov-Drell-Hearn sum rule and the single-pion photoproduction multipole E0+ close to threshold

The long-standing discrepancy between the Gerasimov-Drell-Hearn sum rule and the analysis of pion photoproduction multipoles is greatly diminished by use of s-wave multipoles that are in accord with the predictions of chiral perturbation theory and describe the experimental data in the threshold region. The remaining difference may be due to contributions of channels with more pions and/or heavier mesons whose contributions to the sum rule remain to be investigated by a direct measurement of the photoabsorption cross sections.