5840401
Evanescent black holes.
A renormalizable theory of quantum gravity coupled to a dilaton and conformal matter in two spacetime dimensions is analyzed. The theory is shown to be exactly solvable classically. Included among the exact classical solutions are configurations describing the formation of a black hole by collapsing matter. The problem of Hawking radiation and back reaction of the metric is analyzed to leading order in a $\frac{1}{N}$ expansion, where $N$ is the number of matter fields. The results suggest that the collapsing matter radiates away all of its energy before an event horizon has a chance to form, and black holes thereby disappear from the quantum-mechanical spectrum. It is argued that the matter asymptotically approaches a zero-energy "bound state" which can carry global quantum numbers and that a unitary $S$ matrix including such states should exist.
42736184
Nonrenormalization of the superstring tension.
It is argued that the superstring tension is not renormalized in perturbation theory for vacua which preserve N= 1 spacetime supersymmetry. Some implications of this result for macroscopic superstrings are discussed, as well as some analogies between macroscopic superstrings and solitons in supersymmetric theories.
12124707
Current directions in string theory and abstract supersymmetry
I discuss some current thoughts on how low-energy measurements and consistency constrain theories at high energies with emphasis on string theory. I also discuss some recent work on the dynamics of supersymmetric gauge theories.
120980643
Open strings on orbifolds
Abstract We consider propagation of type I SO(32) superstrings on orbifolds. It is shown that anomaly cancellation requires the existence of “twisted” open strings and we determine the Chan-Paton factors for these strings in a simple example.
117239200
Conformal Field Theory and String Compactification
8898134
Localized tachyons and RG flows
We study condensation of closed string tachyons living on defects, such as orbifold fixed planes and Neveu-Schwarz fivebranes. We argue that the high energy density of localized states decreases in the process of condensation of such tachyons. In some cases this means that $c_{eff}$ decreases along the flow; in others, $c_{eff}$ remains constant and the decreasing quantity is a closed string analog, $g_{cl}$, of the ``boundary entropy'' of D-branes. We discuss the non-supersymmetric orbifolds $C/Z_n$ and $C^2/Z_n$. In the first case tachyon condensation decreases $n$ and in some cases connects type II and type 0 vacua. In the second case non-singular orbifolds are related by tachyon condensation to both singular and non-singular ones. We verify that $g_{cl}$ decreases in flows between non-singular orbifolds. The main tools in the analysis are the structure of the chiral ring of the perturbed theory, the geometry of the resolved orbifold singularities, and the throat description of singular conformal field theories.
206324920
New measurement of the charge radius of the neutron.
The neutron transmission through a thorogenic liquid {sup 208}Pb sample 2 in. thick has been measured in the neutron energy range between 0.1 and 360 eV at the ORNL neutron source ORELA. Analyzing the shape of the transmission spectra as a function of neutron energy, agreement was found with the predictions by the atomic form factor. With a sensitivity for the mean squared charge radius of the neutron {l_angle}{ital r}{sup 2}{sub {ital n}}{r_angle} as high as 3%, a very reliable and also accurate result of {l_angle}{ital r}{sup 2}{sub {ital n}}{r_angle}={minus}0.113{plus_minus}0.003{plus_minus}0.004 fm{sup 2} was extracted. For the neutron-electron scattering length we obtained {ital b}{sub {ital ne}}=({minus}1.31{plus_minus}0.03{plus_minus}0.04){times}10{sup {minus}3} fm.
1166257
Peripheral reactions induced by a 2.1-GeV/nucleon 12C beam on a C target using a simple combinatorial model.
Recently acquired data on peripheral reactions induced by a /sup 12/C beam at 2.1 GeV/nucleon on a C target have been used to create a distribution of exclusive yields (''mass patterns'') in which the charge of detected fragments is ignored and the data supplemented by (presumed) missing nucleons. A simple multinomial expression using probabilities obtained from inclusive mass yields gives a satisfactory account of the revised data.
37326897
Momentum distribution of projectile fragments and nuclear shape.
When one or two neutrons are lost in a collision between a projectile with a diffuse neutron-rich surface and a target nucleus, the transverse momentum distribution of the fragments thus produced can be fit with a double Gaussian curve in which a narrow component is superimposed on a much broader one. It is shown that this phenomenon is also observed for fragments from the normal'' projectile [sup 12]C. A density-dependent nucleon Fermi momentum and fragment recoil with the momentum of the lost nucleon give good agreement with experiment. The two Gaussian components do not require the presence near the nuclear surface of nucleons with two widely different binding energies, as often supposed.
12086987
Geodesics in Kasner universes.
Recently, Kasner model universes of various dimensionalities have been explored to see if they could be used in the manner of Kaluza-Klein models to provide the additional unobservable dimensions called for in current elementary-particle models. These discussions have focused narrowly on the number of dimensions and the manner in which those dimensions in excess of the observable four of spacetime behave. This paper discusses some features of Kasner models which ultimately should be considered if these models are to provide a useful arena for studying elementary particles. In particular, geodesics are examined in detail. It is found that there may be a limit to the distance traveled by timelike particles, that spacelike geodesics are limited in their temporal extent, and that for null geodesics in expanding directions the coordinate speed approaches zero.
10069365
58Ni+n transmission, differential elastic scattering, and capture measurements and analysis up to 813 keV.
High-resolution neutron cross section measurements for $^{58}\mathrm{enriched}$ targets were made at the Oak Ridge Electron Linear Accelerator of transmission below 20 MeV, of differential elastic scattering from 10 keV to 5 MeV, and of capture from 2.6 keV to 2.5 MeV. The transmission data were analyzed from 0.1 to 813 keV with a multilevel R-matrix code which uses Bayes' theorem for the fitting process. This code provides energies and neutron widths of resonances within the analyzed region, as well as a possible parametrization for resonances external to that region, as a way of describing the smooth cross section over the entire energy range. The differential elastic data at different scattering angles were compared to theoretical calculations from 30 to 813 keV using an R-function code which is based on the Blatt-Biedenharn formalism. Various combinations of spin and parity were tested to predict cross sections for the well-defined lg0 resonances, and comparison with the data then provided spin and parity assignments for most of these resonances.