Dynamical color correlations in a SU(2)c quark exchange model of nuclear matter.
The quark exchange model is a simple realization of an adiabatic approximation to the strong-coupling limit of quantum chromodynamics: the quarks always coalesce into the lowest energy set of flux tubes. Nuclear matter is thus modeled in terms of its quarks. We wish to study the correlations imposed by total wave-function antisymmetry when color degrees of freedom are included. To begin with, we have considered one-dimensional matter with a SU(2) color internal degree of freedom only. We proceed by constructing a totally antisymmetric, color singlet [ital Ansatz] characterized by a variational parameter [lambda] (which describes the length scale over which two quarks in the system are clustered into hadrons) and by performing a variational Monte Carlo calculation of the energy to optimize [lambda] for a fixed density. We calculate the [ital q]-[ital q] correlation function as well, and discuss the qualitative differences between the system at low and high density.

title: Semiclassical quark model of the nuclear matter response
abstract: The longitudinal response function R(q,w) of nuclear matter is calculated in a semiclassical quark model. The model has a many‐body string‐flip potential that confines quarks into hadrons and avoids color van der Waals forces. Molecular dynamics simulations are used to calculate R(q,w) in one space dimension for a variety of momentum transfers q and excitation energies w. The response function decreases with density (compared to a free hadron response) because of quark exchange between hadrons.

title: Equation of state of dense matter from a density dependent relativistic mean field model
abstract: We calculate the equation of state (EOS) of dense matter using a relativistic mean field (RMF) model with a density dependent coupling that is a slightly modified form of the original NL3 interaction. For nonuniform nuclear matter we approximate the unit lattice as a spherical Wigner-Seitz cell, wherein the meson mean fields and nucleon Dirac wave functions are solved fully self-consistently. We also calculate uniform nuclear matter for a wide range of temperatures, densities, and proton fractions, and match them to nonuniform matter as the density decreases. The calculations took over 6000 CPU days in Indiana University's supercomputer clusters. We tabulate the resulting EOS at over 107,000 grid points in the proton fraction range Y{sub P}=0 to 0.56. For the temperature range T=0.16 to 15.8 MeV, we cover the density range n{sub B}=10{sup -4} to 1.6 fm{sup -3}; and for the higher temperature range T=15.8 to 80 MeV, we cover the larger density range n{sub B}=10{sup -8} to 1.6 fm{sup -3}. In the future we plan to study low density, low temperature (T<15.8 MeV), nuclear matter using a Virial expansion, and we will match the low-density and high-density results to generate a complete EOS table for use in astrophysical simulationsmore » of supernova and neutron star mergers.« less

title: Structure of accreted neutron star crust
abstract: Using molecular dynamics simulations, we determine the structure of neutron star crust made of rapid proton capture nucleosynthesis material. We find a regular body-centered cubic lattice, even with the large number of impurities that are present. Low-charge-Z impurities tend to occupy interstitial positions, while high-Z impurities tend to occupy substitutional lattice sites. We find strong attractive correlations between low-Z impurities that could significantly increase the rate of pycnonuclear (density driven) nuclear reactions. The thermal conductivity is significantly reduced by electron impurity scattering. Our results will be used in future work to study the effects of impurities on mechanical properties such as the shear modulus and breaking strain.

title: Role of heavy-meson exchange in pion production near threshold.
abstract: Recent calculations of s-wave pion production have severely underestimated the accurately known pp → pp� 0 total cross section near threshold. In these calculations, only the single-nucleon axial-charge operator is considered. We have calculated, in addition to the one-body term, the two-body contributions to this reaction that arise from the exchange of mesons. We find that the inclusion of the scalar �-meson exchange current (and lesser contributions from other mesons) increases the cross section by about a factor of five, and leads to excellent agreement with the data. The results are neither very sensitive to changes in the distorting potential that generates the NN wave function, nor to different choices for the meson-nucleon form factors. We argue that pp → pp� 0 data provide direct experimental evidence for meson

title: Breaking strain of neutron star crust and gravitational waves.
abstract: Mountains on rapidly rotating neutron stars efficiently radiate gravitational waves. The maximum possible size of these mountains depends on the breaking strain of the neutron star crust. With multimillion ion molecular dynamics simulations of Coulomb solids representing the crust, we show that the breaking strain of pure single crystals is very large and that impurities, defects, and grain boundaries only modestly reduce the breaking strain to around 0.1. Because of the collective behavior of the ions during failure found in our simulations, the neutron star crust is likely very strong and can support mountains large enough so that their gravitational wave radiation could limit the spin periods of some stars and might be detectable in large-scale interferometers. Furthermore, our microscopic modeling of neutron star crust material can help analyze mechanisms relevant in magnetar giant flares and microflares.

title: Entropy, area, and black hole pairs.
abstract: We clarify the relation between gravitational entropy and the area of horizons. We first show that the entropy of an extreme Reissner-Nordstr\"om black hole is zero, despite the fact that its horizon has nonzero area. Next, we consider the pair creation of extremal and nonextremal black holes. It is shown that the action which governs the rate of this pair creation is directly related to the area of the acceleration horizon and (in the nonextremal case) the area of the black hole event horizon. This provides a simple explanation of the result that the rate of pair creation of nonextreme black holes is enhanced by precisely the black hole entropy. Finally, we discuss black hole annihilation, and argue that Planck scale remnants are not sufficient to preserve unitarity in quantum gravity.

title: Origin of gauge invariance in string theory.
abstract: We present a first quantization of the space-time embedding X/sup ..mu../ and the world-sheet metric rho of the open bosonic string. The world-sheet metric rho decouples from S-matrix elements in 26 dimensions. This formulation of the theory naturally includes 26-dimensional gauge transformations. The gauge invariance of S-matrix elements is a direct consequence of the decoupling of rho. Second quantization leads to a string field Phichemically bondX/sup ..mu../,rhochemically bond with a gauge-covariant equation of motion.

title: An equivalence between momentum and charge in string theory.
abstract: It is shown that for a translationally invariant solution to string theory, spacetime duality interchanges the momentum in the symmetry direction and the axion charge per unit length. As one application, we show explicitly that charged black strings are equivalent to boosted (uncharged) black strings. The extremal black strings (which correspond to the field outside of a fundamental macroscopic string) are equivalent to plane-fronted waves describing strings moving at the speed of light.

title: Strings in strong gravitational fields.
abstract: String propagation in exact plane-wave solutions (with nonzero axion and dilaton fields) is analyzed. In these backgrounds, strings can undergo transitions from one state to another. Selection rules are derived which describe allowed and forbidden transitions of the string. It is shown that singular plane waves result in infinitely excited strings. An example is given of a solution whose singular properties are the opposite of an orbifold: it is geodesically complete, but still singular from the standpoint of string theory. Some implications of these results are discussed.

title: Exact solutions and singularities in string theory.
abstract: We construct two new classes of exact solutions to string theory which are not of the standard plane wave of gauged WZW type. Many of these solutions have curvature singularities. The first class includes the fundamental string solution, for which the string coupling vanishes near the singularity. This suggests that the singularity may not be removed by quantum corrections. The second class consists of hybrids of plane wave and gauged WZW solutions. We discuss a four-dimensional example in detail.