40501364
Nonperturbative two-dimensional quantum gravity.
We propose a nonperturbative definition of two-dimensional quantum gravity, based on a double-scaling limit of the random-matrix model. We derive an exact differential equation for the partition function of two-dimensional gravity coupled to conformal matter as a function of the string coupling constant that governs the genus expansion of two-dimensional surfaces, and discuss its properties and consequences. We also construct and discuss the correlation functions of an infinite set of local operators for spherical topology.
121273546
A Mechanism for Quark Confinement
I shall describe in this lecture a new approach to the dynamics of quarks. It is widely believed that the strong interactions are described by QCD, a nonabelian (SU(3)) gauge theory of colored quarks and gluons, permanently confined in color singlet hadronic bound states. Our knowledge of the nature of the constituents of hadrons derives from their observed symmetries, the success of various phenomenological models such as the quark model and most important from the observed short distance behavior of hadronic currents. It is this free-field theory behavior at large momentum (asymptotic freedom) that singles out nonabelian gauge interactions as the source of the strong interactions, allows us to directly observe the quantum numbers of the quarks and to derive quantative predictions that can test the validity of the theory.
170218458
The discovery of asymptotic freedom and the emergence of QCD
T he progress of science is much more muddled than is depicted in most history books. This is especially true of theoretical physics, partly because history is written by the victorious. Consequently, historians of science often ignore the many alternate paths that people wandered down, the many false clues they followed, the many misconceptions they had. These alternate points of view are less clearly developed than the final theories, harder to understand and easier to forget, especially as these are viewed years later, when it all really does make sense. Thus, reading history one rarely gets the feeling of the true nature of scientific development, in which the element of farce is as great as the element of triumph.

The emergence of QCD is a wonderful example of the evolution from farce to triumph. During a very short period, a transition occurred from experimental discovery and theoretical confusion to theoretical triumph and experimental confirmation. In this Nobel lecture, I shall describe the turn of events that led to the discovery of asymptotic freedom, which in turn led to the formulation of QCD, the final element of the remarkably comprehensive theory of elementary particle physics—the Standard Model. I shall then briefly describe the experimental tests of the theory and the implications of asymptotic freedom.

The early 1960s, when I started my graduate studies at UC Berkeley, were a period of experimental supremacy and theoretical impotence. The construction and utilization of major accelerators were proceeding at full steam. Experimental discoveries and surprises appeared every few months. There was hardly any theory to speak of. The emphasis was on phenomenology, and there were only small islands of theoretical advances here and there. Field theory was in disgrace; S-Matrix theory was in full bloom. Symmetries were all of the rage. Of the four forces …
124543277
Are instantons found
It is argued that the recent prediction of the a dependence of the coupling, g (a), of an effective lattice gauge theory for quarkless qauntum chromodynamics is confirmed by a recent numerical evaluation of g (a) for a latticized SU(2) gauge theory, thus providing strong evidence for the dynamical role of instantons. Additional predictions, testing the validity of the quantum chromodynamics bag model, are presented.
120775506
Density of states in the presence of a strong magnetic field and random impurities
Abstract A supersymmetric formalism is used to rederive a recent beautiful result of Wegner concerning the density of state of an electron gas in the presence of a strong magnetic field and impurities. The derivation is simple and applicable to any (local) distribution of scatterers. Explicit formulae are given in two dimensions. A Poisson distribution of scatterers leads to an interesting condensation phenomenon when the magnetic field is increased.
36890636
The Triumph and limitations of quantum field theory
Talk presented at the conference ``Historical and Philosophical Reflections on the Foundations of Quantum Field Theory,'' at Boston University, March 1996. It will be published in the proceedings of this conference.
26437398
Isoscalar meson exchange currents and the deuteron form factors.
The electromagnetic form factors for the RhoPiGamma and wSigmaGamma vertices are calculated from quark loop diagrams which take the qq structure of the Pi, Sigma, Rho, w mesons into account.The resulting form factors decrease with increasing Q 2 (the square of the four-momentum of the off-shell photon) considerably more rapidly than the monopole form factors obtained from vector meson dominance.The implications of this behavior, which has a significant effect on the elastic electromagnetic form factors of deuteron, is discussed.
33923932
Current conservation and interaction currents in relativistic meson theories.
The relation between the nucleon-nucleon interaction and exchange currents needed for current conservation are derived for the Bethe-Salpeter formalism, and for the approach in which the spectator particle is restricted to its mass shell. For both approaches, it is shown how to achieve current conservation for a completely general isospin dependent, energy dependent interaction with arbitrary phenomenological electromagnetic form factors for the nucleon and mesons, and with strong form factors at the meson-nucleon vertices. Contrary to what has often been stated in the literature, the development shows that current conservation places no restrictions on the use of different electromagnetic form factors for mesons and nucleons, and that phenomenological meson-nucleon form factors can be introduced in a way which is consistent with current conservation. The longitudinal part of the exchange current is uniquely determined by current conservation, and for the common case of an interaction that only depends on the invariant momentum transfer variable an explicit expression for this longitudinal exchange current is given. The transverse part, which contains all electromagnetic form factors, is unconstrained by current conservation.
10889898
Unitary, gauge invariant, relativistic resonance model for pion photoproduction.
Pion photoproduction up to 770 MeV photon laboratory energy is described by a manifestly covariant wave equation, which includes a treatment of the final state {pi}N interactions consistent with the covariant, unitary, resonance model of pi-N scattering previously developed. The kernel of the equation includes nucleon (N), Roper (N*), Delta, and D{sub 13} poles and their crossed poles, as well as {pi}, {mu}, and w exchange terms. The Kroll-Ruderman term and other interaction currents insure that the model is exactly gauge invariant to all orders in the strong coupling, g{sub {pi}NN}, and that the low energy theorem is satisfied. Unitarity is maintained up to first order in the charge e (Watson theorem). The complete development of this model, which gives a good fit to all the data up to 770 MeV, is presented.
6742150
Gauge invariance and Compton scattering from relativistic composite systems.
Using the Ward-Takahashi (WT) identity and the Bethe-Salpeter wave equation, we investigate the dynamical requirements imposed by electromagnetic gauge invariance on Compton scattering from relativistic composite systems. The importance of off-shell rescattering in intermediate states, which is equivalent to final state interactions in inclusive processes, is clarified in the context of current conservation. It is shown that, if the nuclear force is nonlocal, there will be both two-photon interaction currents and rescattering contributions to terms involving one-photon interaction currents. We derive the two-body WT identity for the two-photon interaction currents, and obtain explicit forms for the interaction current operators for three illustrative models of nuclear forces: (a) two-pion exchange forces with baryon resonances, (b) covariant separable forces, and (c) charged one-pion exchange.