Data from a new muon-nucleon scattering experiment includes 513 events, of which 449 are dimuons and 64 are trimuons. Conventional hadronic and electromagnetic processes account for less than 15% of these events. Model calculations suggest that the majority of the dimuons result from associated charm production with a total cross section of about 3 nb.

This Letter presents a measurement of the decay ${\tau }^{-}\to {\rho }^{-}{\nu }_{\tau }$ using data obtained with the Mark II detector at SPEAR. In the center-of-mass energy region $4.5<~E_{\mathrm{c}.\mathrm{m}.\mathrm{}}<~6.0$ GeV 85 events are observed in which a charged $\rho$ was found in coincidence with either an electron or a muon. It was determined that $B({\tau }^{-}\to {\rho }^{-}{\nu }_{\tau })=(20.5\mathrm{}\pm 4.1)\%$ and the ratio $\frac{B({\tau }^{-}\to {\rho }^{-}{\nu }_{\tau })}{B({\tau }^{-}\to e^{-}{\nu }_{\tau }{\overline{\nu }}_e)}=1.11\mathrm{}\pm 0.23$.

New calculations of charge-symmetry breaking in the $\mathrm{NN}$ interaction lead to radically altered predictions for the difference between $\mathrm{nn}$ and $\mathrm{pp}$ scattering lengths. Inclusion of the ${\eta }^{\prime }$ meson and use of realistic magnetic couplings for the vector meson changes the sign as well as the magnitude of earlier results. This sensitivity arises in part from cancellations within the pseudoscalar and vector contributions separately, making precise calculations difficult. Meson mixing is determined from quark mass differences, and the finite width of the $\rho$ meson is included.

For renormalizable theories without spontaneous symmetry breaking, a formalism is developed to systematically extract and evaluate the effects of heavy particles in low-energy physics through factorized local operators and the renormalization group. As an application, quantum electrodynamics with electrons and muons is considered and the effects of muons on the electron anomalous magnetic moment are assessed.

A number of properties of possible baryon- and lepton-nonconserving processes are shown to follow under very general assumptions. Attention is drawn to the importance of measuring ${\mu }^{+}$ polarizations and $\frac{{\overline{\nu }}_e}{e^{+}}$ ratios in nucleon decay as a means of discriminating among specific models.

The operator structure of the effective Hamiltonian mediating nucleon decay is analyzed. Selection rules diagnostic of the underlying superunified theories are derived. A kinship hypothesis important in analyzing nucleon decay is formulated and tests of it are proposed.

The first angular distributions for pion double charge exchange to discrete nuclear states are reported. The analog and nonanalog states are found to be equally strongly populated. The direct, surface-dominated nature of the reaction is clearly demonstrated by the angular distribution shapes. It is shown that in order to produce detailed agreement with the data, the present ideas for the pion charge-exchange form factors may need to be drastically revised.

$\pi$-${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ elastic and inelastic scattering was measured for both ${\pi }^{+}$ and ${\pi }^{-}$ at 180 MeV. The $\frac{{\pi }^{-}}{{\pi }^{+}}$ ratio of inelastic cross sections approaches 1 in scattering to collective states, whereas this ratio differs significantly from 1 for other excited states. In particular, a value close to the one for pion scattering from free nucleons at the (3,3) resonance was determined for the 9.5-MeV state.

The effects of final-state $\pi$-nucleus electromagnetic interactions in heavy-ion collisions are investigated in a covariant classical formulation. Experimentally observed mid-rapidity peaks in ${\pi }^{+}$ spectra are reproduced by a simple model and are shown to be sensitive to the gross features of the time-dependent nuclear charge distribution.

A multistate molecular approach to the proton-hydrogen-atom collision is formulated. Spurious long-range couplings are avoided and Galilean invariance enforced through variationally determined momentum translation factors. Well-defined radial and rotational couplings are employed in the 1-7-keV energy range in a ten-state calculation. Good agreement with Bayfield is obtained for the $2s$ charge-exchange probability. Concerning polarization of Lyman-$\alpha$ radiation, comparison with Kauppila et al. is made.

This Letter reports, for the first time, an experimental measurement of the collision-less dephasing time of the ${\nu }_3=2\mathrm{}$ level in S${\mathrm{F}}_6$ using Doppler-free two-photon degenerate four-wave mixing. The measurement is performed in the time domain by use of a short-pulse C${\mathrm{O}}_2$ laser to establish a molecular two-photon coherent state. The coherent state is then probed as a function of time to yield a dephasing time of 1.8 nsec which is related to intramolecular dephasing rates.

Saturation of Brillouin backscatter is observed from underdense plasma targets with C${\mathrm{O}}_2$ laser intensities as low as 2×${10}^{11}$ W/${\mathrm{cm}}^2$. Maximum reflectivity is about 5%, implying an ion-wave amplitude of 1%. The most likely limiting mechanism is found to be ion trapping, which is effective at our low $\frac{T_e}{T_i}$ ratio.

This Letter presents measurements of the mobile dislocation density in lead-5%-indium crystals, in which we measure the change in flux accompanying the motion of dislocations. In particular, it is shown that the present technique allows us to establish how the mobile dislocation density varies for crystals undergoing vastly different types of work hardening. In addition, we use the present technique to show how the mobile dislocation density varies with applied strain rate.

Molecular-dynamics calculations of shock waves in perfect three-dimensional solids at nonzero initial temperatures reveal a transition in the nature of the asymptotic shockwave structure as a function of shock strength. The key to this transition from nonsteady to steady waves where the Rankine-Hugoniot relations are obeyed is the partial relaxation of compressive shear stress behind the shock front which accompanies small, but permanent, transverse strains in atomic positions.

A Landau expansion of the normal-mode amplitude shows that the instability of helical textures in a magnetic field parallel to the superflow always occurs as an inverted bifurcation that does not represent a transition to a nearby steady configuration. Above a critical magnetic field, for example, physical arguments show that a persistent current in a torus will be partially quenched, leading to a stable wide-angle helix, but that the corresponding texture in an applied heat flow will oscillate anharmonically.

Correlation functions and the Cotton-Mouton coefficient are calculated for liquid crystals beyond the mean-field approximation. My results in the context of a first-order transition are compared with the recent experiments of Keyes and Shane for N-[$p$-methoxybenzylidine]-$p$-butylaniline (MBBA) connecting with the possible tricritical nature of the nematic-isotropic transition.

Neutron scattering measurements on single crystals of hexagonal tungsten bronzes $M_{0.33}$ W${\mathrm{O}}_3$ reveal low-frequency, relatively dispersionless phonon branches which can be associated with the vibrations of the metal atoms $M$ in the large open channels they occupy in the crystal structure. An analysis based on Eliashberg theory shows that the strong dependence of the superconducting transition temperature on the species $M$ arises from these special phonons.

Optically detected magnetic resonance in oxygen-doped GaP shows that the infrared emission at 0.84 eV and its phonon replicas are due to a spin-triplet to -singlet transition of the two-electron state of oxygen. The resonance data also show that the two-electron center has axial symmetry along the [110] axis, indicating that after capturing the second electron a strong lattice relaxation takes place.

By recursive methods, numerically exact free energies are calculated for $L\times L$ Ising lattices with bonds of randomly chosen sign, with $6<~L<~18$. Ground states of these systems are identified, and the response to ordering fields is studied. By performing Monte Carlo simulations for precisely the same systems we are able to unambiguously distinguish nonequilibrium phenomena from equilibrium properties. The $L$ dependence of our results suggests that there is no nonzero spin-glass order parameter for $L\to \infty$.

We have calculated the ground-state properties of the $s=\frac{1}{2}$ $\mathrm{XY}$ and Heisenberg models on finite triangular and square lattices with competing interactions which would lead to frustrated classical spin models. The evidence favors the hypothesis that quantum spin models on infinite two-dimensional lattices experience no frustration.