The Gerasimov-Drell-Hearn sum rule and the infinite-momentum limit

We study the current-algebra approach to the Gerasimov-Drell-Hearn sum rule, paying particular attention to the infinite-momentum limit. Employing the order-alpha^2 Weinberg-Salam model of weak interactions as a testing ground, we find that the legitimacy of the infinite-momentum limit is intimately connected with the validity of the naive equal-times algebra of electric charge densities. Our results considerably reduce the reliability of a recently proposed modification of the Gerasimov-Drell-Hearn sum rule, originating from an anomalous charge-density algebra.Comment: 12 pages; 6 figures; LaTeX; submitted to Z.Phys.

Handling the Handbag Diagram in Compton Scattering on the Proton

Poincare invariance, gauge invariance, conservation of parity and time reversal invariance are respected in an impulse approximation evaluation of the handbag diagram. Proton wave functions, previously constrained by comparison with measured form factors, that incorporate the influence of quark transverse and orbital angular momentum (and the corresponding violation of proton helicity conservation) are used. Computed cross sections are found to be in reasonably good agreement with early measurements. The helicity correlation between the incident photon and outgoing proton, $K_{LL}$, is both large and positive at back angles. For photon laboratory energies of $\le$ 6 GeV, we find that $K_{LL}\ne A_{LL}$, $D_{LL}\ne1$, and that the polarization $P$ can be large.Comment: 9 pages, 6 figures. Replacement fixes some typos, improves references and figures. An error in Fig. 6 was corrected and related comments in the text change

Robustness and epistasis in mutation-selection models

We investigate the fitness advantage associated with the robustness of a phenotype against deleterious mutations using deterministic mutation-selection models of quasispecies type equipped with a mesa shaped fitness landscape. We obtain analytic results for the robustness effect which become exact in the limit of infinite sequence length. Thereby, we are able to clarify a seeming contradiction between recent rigorous work and an earlier heuristic treatment based on a mapping to a Schr\"odinger equation. We exploit the quantum mechanical analogy to calculate a correction term for finite sequence lengths and verify our analytic results by numerical studies. In addition, we investigate the occurrence of an error threshold for a general class of epistatic landscape and show that diminishing epistasis is a necessary but not sufficient condition for error threshold behavior.Comment: 20 pages, 14 figure

Von den Quarks zur neuen Kosmologie: Eindrücke von der XXIV. Internationalen Konferenz über Hochenergiephysik Rochester-Konferenz, München, 4.-10. August 1988

Das in jedem zweiten Jahr stattfindende große internationale Treffen der Elementarteilchenphysiker, auch unter dem Namen „Rochester-Konferenz” bekannt (vgl. Phys. Bl. 42(1986) Nr. 11, S. 388–389), führte in diesem Jahr ca. 1000 Physiker aus allen Ländern der Erde nach München. Das lokale Organisationskomitee unter Leitung der Professoren G. Buschhorn und K. Pretzl vom MPI für Physik und Astrophysik, Werner-Heisenberg-Institut, hatte die Tagung optimal vorbereitet, so daß sich die Teilnehmer ohne Reibungsverluste dem wissenschaftlichen Programm widmen konnten. Für die deutschen Physiker war die XXIV. Rochester-Konferenz ein Ereignis von besonderer Bedeutung, denn zum ersten Mal fand dieses herausragende Treffen der Hochenergiephysiker auf bundesdeutschem Boden statt. Im folgenden geben ein Theoretiker (Prof. H. Rollnik, Bonn) und ein Experimentalphysiker (Prof. D. Wegener, Dortmund) ihre Eindrücke von der Konferenz wieder