78,062 research outputs found
The low energy expansion of the one-loop type II superstring amplitude
The one-loop four-graviton amplitude in either of the type II superstring
theories is expanded in powers of the external momenta up to and including
terms of order s^4 log s R^4, where R^4 denotes a specific contraction of four
linearized Weyl tensors and s is a Mandelstam invariant. Terms in this series
are obtained by integrating powers of the two-dimensional scalar field theory
propagator over the toroidal world-sheet as well as the moduli of the torus.
The values of these coefficients match expectations based on duality relations
between string theory and eleven-dimensional supergravity.Comment: harvmac (b), 25 pages, 3 eps figures. v2: Factors of 2 corrected.
Conclusion unchange
A GAS OF D-INSTANTONS
A D-instanton is a space-time event associated with world-sheet boundaries
that contributes non-perturbative effects of order to
closed-string amplitudes. Some properties of a gas of D-instantons are
discussed in this paper.Comment: 11 Pages and 3 figures in uuencoded fil
Boundary Effects in String Theory
Some of the properties of string theory defined on world-sheets with
boundaries are reviewed. Particular emphasis is put on the possibility of
identifying string configurations (\lq D-instantons' and \lq D-branes') that
give rise to stringy non-perturbative effects.Comment: Talk presented at the Strings '95 Conference, USC (March 1995).
Latex, 10 page
The Application of Laser Intracavity Absorption Detector to Gas Chromatography of Trace Organic Pollutants in Water
A helium-neon (HeNe) laser operating simultaneously at 3.39 um (infrared) and 0.63 um (visible) has been used as a selective detector for hydrocarbons in the effluent of a gas chromatograph. The infrared and visible laser transitions originate at the same energy level and are competitive. When a hydrocarbon enters the laser\u27s resonant cavity, the 3.39 um energy is absorbed due to the C-H stretching vibration and the visible emission is enhanced. The visible laser emission is monitored with a photodiode as a quantitative measure of the concentration of the absorbing molecule. The minimum detectable concentration for propane using the double-beam configuration is 20 pg/mL, which is 25 times lower than the best value reported for a thermal conductivity detector. In practice, the detector\u27s selectivity for hydrocarbons is modified by various substituents. The detector responds to aliphatic and aromatic hydrocarbons with aliphatic side chains, except for those substituted with halogens. The HeNe laser intracavity absorption detector may be used without prior separation in some cases (e.g., methane in coal mines). This detector operates with nitrogen carrier gas without sacrifice of sensitivity and should be useful for monitoring organic pollutants since it does not respond to water or carbon dioxide. Also, it should be possible to manufacture this detector at competitive prices
Bifurcation analysis of frequency locking in a semiconductor laser with phase-conjugate feedback
We present a detailed study of the external-cavity modes (ECMs) of a semiconductor laser with phase-conjugate feedback. Mathematically, lasers with feedback are modeled by delay differential equations (DDEs) with an infinite-dimensional phase space. We employ new numerical bifurcation tools for DDEs to continue steady states and periodic orbits, irrespective of their stability. In this way, we show that the periodic orbits corresponding to the ECMs are connected to the steady state solution associated with the locking range of the laser. We also identify symmetric and nonsymmetric homoclinic orbits and hysteresis in the system
Vacuum Values for Auxiliary String Fields
Auxiliary string fields are introduced in light-cone gauge string field
theory in order to express contact interactions as contractions of cubic
vertices. The auxiliary field in the purely closed-string bosonic theory may be
given a non-zero expectation value, leading to a phase in which world-sheets
have boundaries.Comment: 13 pages, DAMTP/94-2
Summing Over World-Sheet Boundaries
The moduli associated with boundaries in a Riemann surface are parametrized
by the positions and strengths of electric charges. This suggests a method for
summing over orientable Riemann surfaces with Dirichlet boundary conditions on
the embedding coordinates. A light-cone parameterization of such boundaries is
also discussed.Comment: 10 page
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