Entropy, area, and black hole pairs.
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: 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: 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: 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: 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: Relativistic nuclear structure effects in quasielastic neutrino scattering.
abstract: Charged-current cross sections are calculated for quasielastic neutrino and antineutrino scattering using a relativistic meson-nucleon model. We examine how nuclear-structure effects, such as relativistic random-phase-approximation (RPA) corrections and momentum-dependent nucleon self-energies, influence the extraction of the axial form factor of the nucleon. RPA corrections are important only at low-momentum transfers. In contrast, the momentum dependence of the relativistic self-energies changes appreciably the value of the axial-mass parameter {ital M}{sub {ital A}} extracted from dipole fits to the axial form factor. Using Brookhaven`s experimental neutrino spectrum we estimate the sensitivity of {ital M}{sub {ital A}} to various relativistic nuclear-structure effects.

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

title: Relativistic and nuclear structure effects in parity-violating quasielastic electron scattering.
abstract: The parity-violating longitudinal asymmetry [ital scrA] is calculated for quasielastic electron scattering. We use a varietry of relativistic mean field models for the response of nuclear matter and [sup 12]C at a momentum transfer of [ital q]=550 MeV/[ital c]. Relativistic effects from a reduced nucleon mass, RPA correlations, and vacuum polarization can all change [ital scrA] by a relatively large amount. These large nuclear structure corrections could make it impossible to extract strange quark matrix elements or radiative corrections to weak axial currents.

title: Hadron structure in a simple model of quark/nuclear matter.
abstract: We study a simple model for one-dimensional hadron matter with many of the essential features needed for examining the transition from nuclear to quark matter and the limitations of models based upon hadron rather than quark degrees of freedom. The dynamics are generated entirely by the quark confining force and exchange symmetry. Using Monte Carlo techniques, the ground-state energy, single-quark momentum distribution, and quark correlation function are calculated for uniform matter as a function of density. The quark confinement scale in the medium increases substantially with increasing density. This change is evident in the correlation function and momentum distribution, in qualitative agreement with the changes observed in deep-inelastic lepton scattering. Nevertheless, the ground-state energy is smooth throughout the transition to quark matter and is described remarkably well by an effective hadron theory based on a phenomenological hadron-hadron potential.