Strings in strong gravitational fields.
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: Photoemission “experiments” on holographic superconductors
abstract: We study the effects of a superconducting condensate on holographic Fermi surfaces. With a suitable coupling between the fermion and the condensate, there are stable quasiparticles with a gap. We find some similarities with the phenomenology of the cuprates: in systems whose normal state is a non-Fermi liquid with no stable quasiparticles, a stable quasiparticle peak appears in the condensed phase.

title: Microstates of a neutral black hole in M theory.
abstract: We consider vacuum solutions in M theory of the form of a five-dimensional Kaluza-Klein black hole cross T6. In a certain limit, these include the five-dimensional neutral rotating black hole (cross T6). From a type-IIA standpoint, these solutions carry D0 and D6 charges. We show that there is a simple D-brane description which precisely reproduces the Hawking-Bekenstein entropy in the extremal limit, even though supersymmetry is completely broken.

title: Where is the information stored in black holes
abstract: It is shown that many modes of the gravitational field exist only inside the horizon of an extreme black hole in string theory. At least in certain cases, the number of such modes is sufficient to account for the Bekenstein-Hawking entropy. These modes are associated with sources which carry Ramond-Ramond charge, and so may be viewed as the strong coupling limit of D-branes. Although these sources naturally live at the singularity, they are well defined and generate modes which extend out to the horizon. This suggests that the information in an extreme black hole is not localized near the singularity or the horizon, but extends between them.

title: Pair creation of extremal black holes and Kaluza-Klein monopoles.
abstract: Classical solutions describing charged dilaton black holes accelerating in a background magnetic field have recently been found. They include the Ernst metric of the Einstein-Maxwell theory as a special case. We study the extremal limit of these solutions in detail, both at the classical and quantum levels. It is shown that near the event horizon the extremal solutions reduce precisely to the static extremal black hole solutions. For a particular value of the dilaton coupling, these extremal black holes are five-dimensional Kaluza-Klein monopoles. The Euclidean sections of these solutions can be interpreted as instantons describing the pair creation of extremal black holes and/or Kaluza-Klein monopoles in a magnetic field. The action of these instantons is calculated and found to agree with the Schwinger result in the weak-field limit. For the Euclidean Ernst solution, the action for the extremal solution differs from that of the previously discussed wormhole instanton by the Bekenstein-Hawking entropy. However, in many cases quantum corrections become large in the vicinity of the black hole, and the precise description of the creation process is unknown.

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: Detection of atmospheric neutrinos and relativistic nuclear structure effects.
abstract: Neutrino-nucleus cross sections for the detection of atmospheric neutrinos are calculated in relativistic impulse and random phase approximations. Pion production is estimated via inclusive {Delta}-hole nuclear excitations. The number of pion events can confuse the identification of the neutrino flavor. As a further source of pions we calculate coherent pion production (where the nucleus remains in the ground state). We examine how these nuclear structure effects influence atmospheric neutrino experiments and study possible improvements in the present detector simulations.

title: Density dependence of nuclear neutrino-pair production.
abstract: Nuclear decay via neutrino-pair production is found to have a large density dependence because of intermediate electromagnetic couplings to electrons. These are calculated in a relativistic random phase approximation. The decay rate for vector (Fermi) transitions producing electron type neutrino pairs can be enhanced by 10{sup 5} or more at densities relevant for stellar collapse. Axial-vector (Gamow-Teller) transitions are not enhanced.

title: Quasielastic proton-nucleus scattering in a relativistic plane-wave impulse approximation.
abstract: Proton-nucleus quasielastic scattering cross sections and spin observables are calculated in a relativistic plane-wave impulse approximation for nuclei from $^{12}\mathrm{C}$ to $^{208}\mathrm{Pb}$ at energies of 300 to 800 MeV. Relativistic effects are incorporated by using Dirac spinors with enhanced lower components characterized by an effective mass from 0.8 to 0.9M. The analyzing power is found to decrease significantly from free NN values when relativistic spinors are used. This reduction is in good agreement with data. Little change in polarization transfer coefficients ${D}_{i\mathcal{'}j}$ is found at 500 MeV. However, predictions of enhanced ${D}_{s\mathcal{'}s}$ and ${D}_{l\mathcal{'}l}$ are made at lower energies. Calculations are also presented for (p,n) quasifree charge exchange reactions. Here the analyzing power is almost unchanged but large changes in cross section and polarization transfer coefficients are possible. These changes depend on how pion contributions to the NN amplitudes are treated. Suggestions for future theoretical and experimental work are made.

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: Nuclear to quark-matter transition in the string-flip model.
abstract: We have calculated ground-state properties of nuclear matter in a simple string-flip quark model in one dimension. Special emphasis was placed on the transition from hadron to quark matter and to the response of the system to color fields. Numerical simulations involving a large number of quarks were carried out for two- and three-quark clusters. This in turn enabled us to investigate the hadron-to-quark transition through the study of finite-size effects. The concept of a sudden-quark potential was introduced as an example of a generic response of the medium to color fields. We have argued that this response might be relevant to the propagation of a rapidly moving quark-antiquark pair (e.g., J/ψ meson) through the medium