32193057
High-energy symmetries of string theory.
By means of a recent analysis of the high-energy limit of string scattering, linear relations between string scattering amplitudes are derived. These are shown to hold order by order in perturbation theory. If one assumes that they hold for the full theory, they suggest the existence of an enormous string-broken symmetry which is restored at high energies. Some speculations as to the nature of this symmetry are presented.
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.
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.
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.
7846415
Low-energy theorem for scalar and vector interactions of a composite spin-1/2 system.
Scalar and vector interactions, with the scalar interaction coupled to a composite spin-1/2 system so as to cause a shift of its mass, are shown to obey a low-energy theorem which guarantees that the second order interaction due to z-graphs is the same as for a point Dirac particle. Off-shell and contact interactions appropriate to the composite system cancel and this is verified in a model of a composite fermion. The result provides a justification for the use of the Dirac equation as it has been in relativistic nuclear scattering and mean field theories.
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.
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.
32532095
Electromagnetic properties of the pion as a composite Nambu-Goldstone boson.
Motivated by the Nambu-Jona-Lasinio model of light mesons, we introduce a covariant separable interaction to model the structure of relativistic quark-antiquark systems.The Schwinger-Dyson equation for the quark self-energy is solved analytically, generating a dynamical quark mass through spontaneous breaking of chiral symmetry, and yielding a pion which has zero mass in the chiral limit.The Bethe-Salpeter vertex function for this q{vec}q pion, which has a momentum distribution and composite structure associated with the interaction, is obtained analytically.Using this vertex function, and a similar one for the p meson, we calculate the electromagnetic observables of this composite Nambu-Goldstone boson, including effects from p-meson dominance processes.Our calculatoin takes the composite structure of the mesons into account.The p meson effects are found to be very small in the pion charge form factor, but substantial in the charge radius.Using the model, predictions are m