1,720 research outputs found
Connections between M-theory and superstrings
This article reviews the non-perturbative structure of certain higher
derivative terms in the type II string theory effective action and their
connection to one-loop effects in eleven-dimensional supergravity compactified
on a torus. New material is also included that was not presented in the talks.Comment: 24 pages LaTeX. Based on talks given at various conferences. To be
published in the proceedings of the 1997 Advanced Study Institute on Strings,
Branes and Dualities, Cargese, Franc
Reactions of polycyclic aromatic hydrocarbon radical cations with model biological nucleophiles
The determination of gas-phase reactivity of a series of polycyclic aromatic hydrocarbons (PAHs) with nucleophiles is directed at achieving isomer differentiation through ion-molecule reactions and collisionally activated decomposition spectra. A series of PAH isomers form gas-phase [adduci — H]+ ions with the reagent nucleophiles pyridine and N-methylimidazole. Collisionally activated decomposition spectra of the [adduct — H]+ ions of the pyridine/PAH systems are dominated by products formed by losses of C5H4N, C5H5N (presumably neutral pyridine), and C5H6N. Collisional activation of PAH/N-methylimidazole [adduct — H]+ ions causes analogous losses of C4H5N2, C4H6N2 (presumably neutral N-methylimidazole), and C4H7N2. The relative abundances of the ions that result from these losses are highly isomer specific for N-methylimidazole but less so for pyridine. Furthermore, PAH/N-methylimidazole [adduct — H]+ ions undergo a series of metastableion decompositions that also provide highly isomer-specific information. The C4H7N2 (from PAH/N-methylimidazole product ions) and C5H6N (from PAH/pyridine product ions) losses tend to increase with the ΔHf of the PAH radical cation. In addition, it is shown that the fragmentation patterns of these gas-phase PAH/nucleophile adducts are similar to fragmentation patterns of PAH/nucleoside adducts generated in solution, which suggests that the structures of products formed in gas-phase reactions are similar to those produced in solution
Effects of D-instantons
Scattering of fundamental states of type IIB supergravity and superstring
theory is discussed at low orders in perturbation theory in the background of a
D-instanton. The integration over fermionic zero modes in both the low energy
supergravity and in the string theory leads to explicit nonperturbative terms
in the effective action. These include a single instanton correction to the
known tree-level and one-loop interactions. The `spectrum' of
multiply-charged D-instantons is deduced by T-duality in nine dimensions from
multiply-wound world-lines of marginally-bound D-particles. This, and other
clues, lead to a conjectured SL(2,Z) completion of the terms which
suggests that they are not renormalized by perturbative corrections in the
zero-instanton sector beyond one loop. The string theory unit-charged
D-instanton gives rise to point-like effects in fixed-angle scattering, raising
unresolved issues concerning distance scales in superstring theory.Comment: 31 pages, 6 figures, Latex, Reference added, corrected coefficients
in expansion of generalized Eisenstein series in equation 66 now agree with
hep-th/970414
Microcanonical entropy inflection points: Key to systematic understanding of transitions in finite systems
We introduce a systematic classification method for the analogs of phase
transitions in finite systems. This completely general analysis, which is
applicable to any physical system and extends towards the thermodynamic limit,
is based on the microcanonical entropy and its energetic derivative, the
inverse caloric temperature. Inflection points of this quantity signal
cooperative activity and thus serve as distinct indicators of transitions. We
demonstrate the power of this method through application to the long-standing
problem of liquid-solid transitions in elastic, flexible homopolymers.Comment: 4 pages, 3 figure
An all-electrical torque differential magnetometer operating under ambient conditions
An all-electrical torque differential magnetometry (also known as cantilever
magnetometry) setup employing piezoelectric quartz tuning forks is
demonstrated. The magnetometer can be operated under ambient conditions as well
as low temperatures and pressures. It extends the allowed specimen mass range
up to several 10 g without any significant reduction in the sensitivity.
Operation under ambient conditions and a simple all-electrical design of the
magnetometer should allow for an easy integration with other experimental
setups. The uniaxial magnetic anisotropy of a 25 m diameter iron wire,
measured under ambient conditions with a high signal to noise ratio, was found
to be in good agreement with its literature value. Further applications of the
technique are discussed.Comment: 9 two-column pages, 9 figure
Evaluation of the Free Energy of Two-Dimensional Yang-Mills Theory
The free energy in the weak-coupling phase of two-dimensional Yang-Mills
theory on a sphere for SO(N) and Sp(N) is evaluated in the 1/N expansion using
the techniques of Gross and Matytsin. Many features of Yang-Mills theory are
universal among different gauge groups in the large N limit, but significant
differences arise in subleading order in 1/N.Comment: 10 pages; no figures; LaTe
Covariant equations for the three-body bound state
The covariant spectator (or Gross) equations for the bound state of three
identical spin 1/2 particles, in which two of the three interacting particles
are always on shell, are developed and reduced to a form suitable for numerical
solution. The equations are first written in operator form and compared to the
Bethe-Salpeter equation, then expanded into plane wave momentum states, and
finally expanded into partial waves using the three-body helicity formalism
first introduced by Wick. In order to solve the equations, the two-body
scattering amplitudes must be boosted from the overall three-body rest frame to
their individual two-body rest frames, and all effects which arise from these
boosts, including the Wigner rotations and rho-spin decomposition of the
off-shell particle, are treated exactly. In their final form, the equations
reduce to a coupled set of Faddeev-like double integral equations with
additional channels arising from the negative rho-spin states of the off-shell
particle.Comment: 57 pages, RevTeX, 6 figures, uses epsf.st
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