A study of e+e− pairs in the mass range 11 to 25 GeV/c2 at the CERN intersecting storage rings

A sample of 58e⁺e^? events with an invariant mass greater than 11 GeV/c² produced in pp collisions at a centre-of-mass energy of 62.3 GeV is discussed. The cross sections are presented as a function of the mass and transverse momentum. The electron pairs produced with a mean transverse momentum of 2.50 ± 0.25 GeV/c.     

Symmetry breaking by instantons in supergravity

The non-zero modes of different spin operators on the background of a self-dual gravitational instanton are all related by global supersymmetry transformations and completely cancel in the one-loop term, which is determined entirely by the zero modes. We derive the number of zero modes of each spin. In an asymptotically locally Euclidean self-dual instanton there are 2τ $\text{spin}\text{-}\text{3}\text{2}$ zero modes and 3τ spin-2 zero modes, where τ is the Hirzebruch signature. Up to 3 of the spin-2 zero modes (depending on boundary conditions) may correspond to global rotations. The $\text{spin}\text{-}\text{3}\text{2}$ zero modes break the U(1) chiral symmetry and give rise to helicity-changing amplitudes. Together with the spin-2 zero modes they determine the trace anomaly or scaling behaviour. We can compare our results with the perturbation theory predictions for the axial vector current and trace anomalies in K3, the unique compact self-dual gravitational instanton, because in this case there are no boundary terms. We obtain agreement.     

A measurement of the production of massive e+e− pairs in proton-proton collisions at √s = 62.4 GeV

An apparatus consisting of a superconducting solenoid magnet, cylindrical drift-chambers, and two arrays of lead-glass ?erenkov counters has been used at the CERN ISR to study the production of e⁺e^? pairs of invariant mass above 6.5 GeV/c². Cross sections for the continuum and the ? family of resonances are presented, as well as the mean transverse momentum 〈p_T〉 of the electron-positron pairs in the continuum and resonance region.     

Interplay of light antenna and excitation "energy reservoir" effects in a bichromophoric system based on ruthenium-polypyridine and pyrene units linked by a long and flexible poly(ethylene glycol) chain

Steady-state and time-resolved spectroscopic properties of bichromophoric species containing [Ru(bpy)(3)](2+) and pyrene (pyr) units linked together by flexible poly(ethylene glycol) chains of variable length, [Ru(bpy)(2)(bpy-pyr)](PF(6))(2) (1) and [Ru(bpy)(2)(bpy-O6-pyr)](PF(6))(2) (2), have been investigated in acetonitrile solvent. The complexes were designed with the aim of examining the intercomponent energy-transfer processes taking place after light absorption at the two chromophores and the influence of the distance separation between them; in the case of complex 2, the linking chain in the extended conformation is as long as 21 A. Direct excitation of the pyrene unit (lambda(exc) = 410 nm) results in singlet-to-singlet energy transfer (an antenna effect) to the Ru-based component, (1)pyr --> (1)MLCT, which we analyze in terms of the F?rster mechanism taking place with unit efficiency. Analysis of the time-resolved pyrene fluorescence reveals that the actual center-to-center distance separation (d(cc)) between the photoactive centers changes according to a Gaussian distribution, with an average d(cc) = 13.6 A (distribution width, a = 2.8 A) and 12 A (a = 10.2 A), for 1 and 2, respectively; this is ascribed to folding of the poly(ethylene glycol) linking chain. In O(2)-free solvent at room temperature, after population of the (1)MLCT level (which takes place either because of direct excitation by using lambda(exc) > 355 nm or via the "antenna" effect) and subsequent intersystem crossing localized at the Ru center, (1)MLCT --> (3)MLCT, a triplet-triplet thermal equilibration is established which involves the physically separated centers, (3)MLCT <--> (3)pyr, with K(eq) = 11 (the energy gap between the two levels is 480 cm(-1), as determined from luminescence data obtained at 77 K). As a consequence of this equilibrium, the (3)MLCT luminescence lifetime becomes tau(Ru) approximately 9 micros both in 1 and 2, i.e., 1 order of magnitude longer than for the unsubstituted [Ru(bpy)(3)](2+) luminophore. In air-equilibrated solvent, diffusional quenching by O(2) effectively depletes the (3)pyr level and only the forward (3)MLCT --> (3)pyr energy transfer step is observed with k(en) = 4 x 10(8) and 2 x 10(8) s(-1) for 1 and 2, respectively. As briefly discussed, reasons for the high rate constants observed for the various triplet-triplet steps may be traced back to the folding properties of the linking chains.     

A comparative evaluation of "depth pulses" and related sequences for spatial discrimination in in-vivo NMR spectroscopy with surface coils

The performance of a range of "depth pulses" and related pulse sequences for achieving enhanced spatial resolution in surface coil magnetic resonance spectroscopy (MRS) has been compared both experimentally and by computer simulation. A detailed analysis of the spatial selectivity of these sequences both on- and off-resonance suggests that the problem of combining good spatial resolution with adequate bandwidth has yet to be fully resolved. These and related problems, such as the shape of the sensitive volume and its dependence on resonance offset, are likely to limit the use of depth pulses, particularly where alternative methods of spatial selection are available.     

Large cluster formation through multiple substitution with lanthanide cations (La, Ce, Nd, Sm, Eu, and Gd) of the polyoxoanion

Several new large polyoxotungstates have been synthesized by reaction of lanthanide cations with the well-known "As(4)W(40)" anion, [(B-alpha-AsO(3)W(9)O(30))(4)(WO(2))(4)](28-) (1). The heteropolyanions [(H(2)O)(11)Ln(III)(Ln(III)(2)OH)(B-alpha-AsO(3)W(9)O(30))(4)(WO(2))(4)](20)(-) (Ln = Ce, Nd, Sm, Gd) (2-4) (Ln(3)As(4)W(40)) and [M(m)()(H(2)O)(10)(Ln(III)(2)OH)(2)(B-alpha-AsO(3)W(9)O(30))(4)(WO(2))(4)]((18-m)(-)) (Ln = La, Ce, Gd and M = Ba, K, none) (5-7) (Ln(4)As(4)W(40)) have been isolated as alkali metal and ammonium salts, respectively, and characterized by single-crystal X-ray analysis, elemental analysis, and IR and (183)W-NMR spectroscopy. The X-ray analyses revealed interanionic W-O-Ln bonds between adjacent Ln(x)()As(4)W(40) units forming a "dimer" for x = 3 and chains for x = 4. Upon dissolving in water these bonds hydrolyze and the monomeric species form. The straightforward syntheses which require the use of concentrated NaCl solutions (1-4 M) and the addition of stoichiometric amounts of Ba(2+) or K(+) reemphasize the importance of the presence of appropriate countercations for the assembly of large polyoxometalate structures.     

The structure of the toxin from helminthosporium carbonum

Using Fast Atom Bombardment Mass Spectrometry and Mass Spectrometry/Mass Spectrometry, the structure of HC-toxin a metabolite of $\text{Helminthosporium}$$\text{carbonum}$, is postulated to be structure 3.     

Characterization of the reactive and dissociative behavior of transition metal oxide cluster ions in the gas phase

The reactive and dissociative behavior of molybdenum and tungsten oxide cluster ions has been studied in the gas phase using a triple quadrupole mass spectrometer. Cluster ions (MO₃) _n ^? were formed via a simple thermal desorption/electron capture negative ionization method, and their structures were characterized by collision-activated dissociation (CAD). Typically, the clusters fragment by losses of neutral (MO₃) units. Reactions of the oxide cluster ions with ethylene oxide, cyclohexene oxide, ethylene sulfide cyclohexene sulfide, 2,3-butanedione, and 2,4-pentanedione were examined, and product ions were characterized by CAD. The clusters react with ethylene oxide by addition of ethylene oxide or net addition of oxygen, whereas the clusters react with ethylene sulfide via net addition of one or two sulfur atoms. Reactions of the clusters with the diones result in addition of one or two dione units, in some cases with dehydration.     

Re(I) sensitised near-infrared lanthanide luminescence from a hetero-trinuclear Re2Ln array

The trinuclear complexes Re2Ln (Ln = Nd, Yb or Er) contain two Re(I) tricarbonyl units linked to a DTPA binding site via 2,2'-bipyridyl ligands; Ln(III)-centred emission is sensitised by the Re(I) MLCT excited states.