We describe a simple computation of the worm-like chain model and obtain the corresponding force-versus-extension curve. We propose an improvement to the Marko and Siggia interpolation formula of Bustamante et al (Science 1994, 265:1599-1600) that is useful for fitting experimental data. We apply it to the experimental elasticity curve of single DNA molecules. Finally, we present a tool to study the agreement between the worm-like chain model and experiments.

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Optical experiments have demonstrated cases in which mirror symmetry in stable atoms is broken during the absorption or emission of light. Such results, which are in conflict with quantum electrodynamics, support the theory of unification of the electromagnetic and weak interactions. The interpretation of the experimental results is based on exchanges of weak neutral Z 0 bosons between the electrons and the nucleus of the atom. A concise review of these phenomena in atomic physics is presented. The role of precise caesium parity-violation experiments, as a source of valuable information about electroweak physics, is illustrated by examples pertaining to experimental conditions which, in some cases, are not accessible to accelerator experiments. We give the basic principles of experiments, some under way and others completed, where a quantitative determination of the nuclear weak charge, Q W , which plays for the Z 0 exchange the same role as the electric charge for the Coulomb interaction is to be, or has been achieved. In the most recent and most precise experiment the accuracy on Q W is limited to 1% by the uncertainty due to atomic physics calculations. Such a result challenges specialists in atomic theory and nuclear structure, since a more accurate determination of Q W would mean more stringent constraints upon possible extensions of the standard model. Moreover, clear evidence has recently been obtained for the existence of the nuclear anapole moment, which describes the valence electron interaction with a chiral nuclear-magnetization component induced by the parity-violating nuclear forces. In writing this review, our hope was to make clear that any improvement in atomic parity-violation measurements will allow the exploration of new areas of electroweak physics.

Within a simple elastic theory, we study the elongation versus force characteristics of a supercoiled DNA molecule at thermal equilibrium in the regime of small supercoiling. The partition function is mapped to the path integral representation for a quantum charged particle in the field of a magnetic monopole with unquantized charge. We show that the theory is singular in the continuum limit and must be regularised at an intermediate length scale. We find good agreement with existing experimental data, and point out how to measure the twist rigidity accurately. LPTENS 97/28.

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